Chapter 3: Inheritance and Variation
In a quiet monastery garden in Moravia, a monk spent eight summers doing something that must have looked very odd to anyone watching. Gregor Mendel, born in 1822 to a peasant farming family, was opening pea flowers with a fine brush, dusting pollen from one plant onto another, tying little bags over the flowers so no stray pollen could interfere — and then counting. Counting seeds. Counting plants. Thousands of them.
The garden pea, Pisum sativum, was a shrewd choice. It grew fast, it could be self-pollinated or cross-pollinated at will, and it came in neat either-or forms: seeds round or wrinkled, flowers purple or white, plants tall or short. Nothing in between to argue about.
What Mendel did next was stranger still for a biologist of his time — he treated living organisms like a mathematics problem. He recorded how many offspring of each type appeared, and expressed his results as ratios. Out of that arithmetic came a startling idea: that inherited characters are not liquids that blend, but discrete particles, passed on unchanged from parent to offspring. He called them ‘factors’. We call them genes.
Mendel published his work in 1866. Almost nobody read it. Only in 1900, long after his death, did three scientists independently stumble upon the same rules — and the study of heredity finally began.
Diagram
Fig. 3.0 – Mendel in a monastery garden hand-pollinating a pea flower with a brush, with a row of pea plants and a notebook of tallies beside him; label the pea flower, brush, and pollen bag.
This chapter will answer each of these questions.
3.1 Chromosomes and Mechanism of Inheritance
Children often resemble their parents — in the shape of the nose, the colour of the eyes, sometimes even in the way they walk. Yet no child is an exact copy of either parent. Biology has to explain both of these facts at once: the passing on of likeness, and the appearance of differences.
The passing of genetic information from one generation to the next is called heredity or inheritance. The differences that still show up among individuals of the same kind are called variation. Together, heredity and variation are the two themes this chapter is built on.
A remarkable thing about the story of inheritance is its order. The mechanism of inheritance was worked out successfully before chromosomes had been properly observed and long before anyone knew what a gene was. The rules were discovered first; the physical machinery that obeys them was identified later.
Mendel’s Approach
Gregor Mendel, the son of a peasant farmer, was born in Moravia in 1822. He was the first person to give an accurate explanation of the mechanism of inheritance, and he did it using the hybridization technique — deliberately crossing two plants that differ in a chosen character, and then following what happens to that character in the offspring.
His material was the garden pea, Pisum sativum, in which contrasting characters can be easily recognised — a seed is either round or wrinkled, a plant is either tall or dwarf, with no confusing in-between forms to judge. This clarity is exactly what makes counting possible.
Mendel worked with seven traits of the pea plant. He studied them individually, one at a time, and also in combinations of two or three characters at a time. Crucially, he then processed his data mathematically and statistically — an approach no biologist of his time was using.
From ‘Factors’ to Genes
From his results Mendel postulated the principles of heredity, which later became the fundamental laws of heredity, as proposed by Correns (1900). His central insight was that traits as such are not inherited physically. Instead, something present inside the gametic cell is passed on, and that something then causes the trait to appear in the new individual.
To this ‘something’ Mendel gave the name factors — the units responsible for the expression of a particular trait or character. He proposed that factors are particulate in nature: they behave like separate, stable particles, not like liquids that mix.
The Mendelian factors are now called genes. These factors occur in pairs in the parents, and during gamete formation the two members of a pair separate — they segregate from each other — without blending or mixing. This is why a character that disappears in one generation can reappear, unchanged, in the next.
The steps of Mendel’s method are summarised in FC 3.1.
Flowchart
FC 3.1 – Mendel’s experimental method: select pure-breeding pea plants with contrasting characters → cross-pollinate them (hybridization) → observe the F1 offspring → allow F1 to self-pollinate → observe the F2 offspring → count each type → express the result as a ratio
Reasons for Mendel’s Success
Mendel succeeded where others had failed for two closely linked reasons:
- His experiments were carefully planned and involved large samples. A cross performed on a handful of plants can be misled by chance; a cross performed on hundreds of plants cannot.
- He carefully recorded the number of plants of each type and expressed his results in terms of ratios, rather than describing them in words. A ratio reveals a rule that a verbal description hides.
Where the Factors Sit
Each of the seven different characters in the pea plant is controlled by a single factor. These factors are located on separate chromosomes, and they are transmitted from generation to generation along with those chromosomes — a point that becomes the basis of the chromosomal theory of inheritance later in this chapter. The way a pair of factors sits on a pair of chromosomes, and separates when gametes are formed, is shown in Fig. 3.1a.
Diagram
Fig. 3.1a – A pair of homologous chromosomes in a cell, each carrying one factor of a pair at the same position, separating into two different gametes; label chromosome, factor (allele), gamete.
Mendel also introduced two ideas that run through the whole of genetics: dominance and recessiveness. When two contrasting factors are present together, one of them may express itself fully and hide the other. The one that shows itself is dominant; the one that is masked, but not destroyed, is recessive.
Before we follow Mendel into his actual crosses, we need the vocabulary he and later geneticists used. The next section sets out those terms and symbols.
3.2 Genetic Terminology
Genetics has a vocabulary of its own, and much of it was built around Mendel’s pea crosses. Before we follow his experiments, it is worth spending time on these terms and symbols. Almost every question in genetics — in the board exam and in NEET — is written in this language, so a term half-understood here becomes a mark lost later.
Character, Trait and Factor
A character is a specific feature of an organism, such as the height of the stem of a plant. Notice that a character names what is being looked at, not what its value is.
A trait is an inherited character together with its detectable variant — tall or dwarf, round or wrinkled. So “plant height” is the character, while “tall” and “dwarf” are two traits of that character.
A factor is a unit of heredity: a particle present in the organism that is responsible for the inheritance and expression of a character. It determines a genetical (biological) character of the organism, and it is passed from one generation to the next through the gametes.
Genes and Alleles
Mendel could only call his unit of heredity a ‘factor’, because in his day nobody knew what it was made of. Today we know its chemical identity.
A gene is a particular segment of DNA which is responsible for the inheritance and expression of a character.
Most genes exist in more than one version. Alleles, or allelomorphs, are the two or more alternative forms of a given gene (factor). They occupy identical loci — that is, identical positions — on homologous chromosomes. “Allele” is simply the short form of “allelomorph”.
This is why the gene for plant height in the pea can be written as \(T\) (the allele for tallness) or \(t\) (the allele for dwarfness). Both sit at the same address on the chromosome; they are alternatives for the same job, not two different genes. The position of an allele on a chromosome, and the fact that its partner sits opposite it, is shown in Fig. 3.2a.
Diagram
Fig. 3.2a – A pair of homologous chromosomes side by side, with alleles T and t marked at the same locus on the two partners; label locus, allele, homologous chromosomes.
Dominant and Recessive
A dominant allele expresses its trait even in the presence of an alternative allele — that is, even in the heterozygous condition. Put another way, the allele that expresses itself in the F1 generation is called dominant. It masks the expression of the other allele of the pair in F1.
A recessive allele is not expressed in the presence of an alternative allele, that is, in the heterozygous condition. It expresses itself only in the presence of another identical allele. It is the allele that does not appear in the F1 hybrid.
By convention the dominant allele is written with a capital letter and the recessive with the same letter in small case — \(T\) for tall, \(t\) for dwarf.
Phenotype and Genotype
The phenotype is the external appearance of an individual for any trait. It is observable, and it is determined by the different combinations of alleles the individual carries. In the pea, for the character of plant height, tall and dwarf are the two phenotypes; tall is produced by \(TT\) or \(Tt\), and dwarf by \(tt\).
The genotype is the genetic constitution, or genetic make-up, of an organism with respect to a particular trait. It is a representation of the genetic constitution of an individual with respect to a single character or a set of characters. So a tall pea plant can have the genotype \(TT\) or \(Tt\), while a dwarf plant must be \(tt\).
This table also contains an important asymmetry worth pausing on. A dwarf plant announces its genotype: only \(tt\) can be dwarf. A tall plant does not — it may be \(TT\) or \(Tt\), and looking at it will never tell you which. That single problem is what the test cross, later in this chapter, is designed to solve.
Homozygous, Heterozygous and Pure Lines
An individual possessing identical alleles for a particular trait is called homozygous, or pure, for that trait. A homozygote breeds true to the trait, and it produces only one type of gamete — for example, a tall plant with \(TT\) produces only \(T\) gametes, and a dwarf plant with \(tt\) produces only \(t\) gametes.
An individual possessing contrasting alleles for a particular trait is called heterozygous. A heterozygote does not breed true for that trait and produces two types of gametes; the F1 hybrids (\(Tt\)) are the standard example. A heterozygous individual is also called a hybrid.
A pure line is an individual, or a group of individuals (a population), which is homozygous or true-breeding for one or more traits — a plant which breeds true for a particular character. A pure line is a descendant of a single homozygous parent, produced after self-fertilization.
Pure lines are the essential starting material of a genetic experiment. Unless the parents are pure, the results of a cross cannot be interpreted, because you would not know what the parents themselves were carrying. This is why Mendel first grew his pea varieties for generations by self-pollination, keeping only those that bred true, before he crossed anything.
A monohybrid is an individual that is heterozygous for one trait, produced from a cross between two pure parents that differ in a single pair of contrasting characters — for example, the hybrid tall plant produced in a cross between a pure tall and a pure dwarf parent. It is a heterozygote for a single pair of alleles.
The Generations: F1 and F2
The F1 generation is the first filial generation. It consists of all the offspring produced from a parental cross; in other words, it is the first generation obtained from a mating between pure parents having contrasting characters.
The F2 generation is the second generation (progeny), produced by selfing (inbreeding) the F1 offspring. For example, the progeny produced from a cross between two F1 individuals (\(Tt \times Tt\)) form the second filial generation.
The Punnett Square
A Punnett square, or checker board, is a probability table representing the different permutations and combinations of fertilization between the gametes of the opposite mating types. In short, it is a diagrammatic representation of a particular cross, used to predict the progeny of that cross.
The square is worth understanding rather than memorising. Each row stands for one kind of female gamete, each column for one kind of male gamete, and each box for one fertilization event. Because fertilization is random, every box is equally likely — which is exactly why the boxes can be counted to give a ratio.
Homologous Chromosomes
Homologous chromosomes are the morphologically, genetically and structurally essentially identical chromosomes present in a diploid cell. Such chromosomes synapse — that is, pair closely — during meiosis.
One member of each homologous pair comes from the mother and one from the father. This single fact explains why alleles occur in pairs in a diploid individual, and why they must separate when gametes are formed.
Back Cross and Test Cross
A back cross is a cross of an F1 progeny with any of its parents — for example, F1 tall × pure tall, or F1 tall × pure dwarf; that is, \(Tt \times TT\) or \(Tt \times tt\).
A test cross is a cross of the F1 progeny with the homozygous recessive parent — F1 tall × pure dwarf, that is, \(Tt \times tt\). It is used to test the homozygous or heterozygous nature of a hybrid. A test cross is one kind of back cross.
Phenotypic and Genotypic Ratios
The phenotypic ratio is the ratio of the offspring produced in F2 and subsequent generations with respect to their physical appearance. For example, \(3\ \text{Tall} : 1\ \text{dwarf}\) is the F2 phenotypic ratio in a monohybrid cross.
The genotypic ratio is the ratio of the offspring produced in F2 and subsequent generations with respect to their genetic make-up. For example, \(1\ TT : 2\ Tt : 1\ tt\) is the F2 genotypic ratio in a monohybrid cross.
Monohybrid Cross
A cross between parents differing in only one heritable trait is called a monohybrid cross — for example, a cross of pure tall and pure dwarf plants. Mendel performed the monohybrid cross between two pea plants with only one pair of contrasting characters.
The whole sequence of a monohybrid cross is set out in FC 3.1a, and the F2 combinations are worked out in the Punnett square of Fig. 3.2b.
Flowchart
FC 3.1a – Monohybrid cross: Parents Tall (TT) × Dwarf (tt) → gametes T and t → F1 = Tt (all Tall) → selfing of F1 (Tt × Tt) → gametes T, t from each parent → F2 = 1 TT : 2 Tt : 1 tt → phenotypes 3 Tall : 1 dwarf
Diagram
Fig. 3.2b – A 2 × 2 Punnett square for the cross Tt × Tt, with male gametes T and t across the top and female gametes T and t down the side, and the four boxes filled with TT, Tt, Tt, tt; mark the phenotype under each box.
Dihybrid Cross
A cross between parents differing in two heritable traits is called a dihybrid cross — for example, a cross of a pure tall, round-seeded plant with a dwarf, wrinkled-seeded plant. Mendel also performed the dihybrid cross between pea plants that differed in two pairs of contrasting characters.
The standard dihybrid cross uses seed shape and seed colour. The parents are round, yellow-seeded (\(RRYY\)) and wrinkled, green-seeded (\(rryy\)). The first parent can form only \(RY\) gametes and the second only \(ry\) gametes, so the entire F1 is \(RrYy\) — round, yellow seeds.
When this F1 is selfed, each parent produces four kinds of gametes, \(RY\), \(Ry\), \(rY\) and \(ry\), because the two pairs of alleles separate independently of one another. Four kinds of male gametes combining with four kinds of female gametes give \(4 \times 4 = 16\) combinations in F2, which is why a dihybrid Punnett square is a \(4 \times 4\) checker board (Fig. 3.2c).
Diagram
Fig. 3.2c – A 4 × 4 Punnett square for the cross RrYy × RrYy, with male gametes RY, Ry, rY, ry across the top and female gametes RY, Ry, rY, ry down the side, and the sixteen boxes left blank for the student to fill.
With this vocabulary in place, we can now state the laws that Mendel drew from these very crosses.
3.3 Mendel’s Laws of Inheritance
Mendel did not begin with laws. He began with counted plants and ratios, and from these he proposed three basic postulates about how factors behave. On the basis of these postulates, three laws were formulated. They are described below.
1. Law of Dominance
In both monohybrid and dihybrid crosses, the phenotypic characters are controlled by discrete units called factors. In a dissimilar pair of factors, one member of the pair dominates over the other — the dominating member is the dominant factor, and the one that is suppressed is the recessive factor.
This law is what explains a result that looks puzzling at first sight: in a monohybrid cross, only one of the two parental characters appears in F1, while both parental characters appear again in F2. The law of dominance is used to explain the expression of only one of the parental characters of a monohybrid cross in F1, and the expression of both in F2.
Note the exact wording carefully. The law defines dominance in terms of what appears in F1 after crossing homozygous parents. It does not say that the recessive factor has vanished, and it does not say that dominance must be complete in every organism — a point that becomes important later, when we look at deviations from Mendel’s findings.
2. Law of Segregation (Law of Purity of Gametes)
This law is based on a simple observed fact: the alleles do not show any blending or mixing. Both the alleles (characters) are recovered as such in the F2 generation, even though one of them is not seen at the F1 stage.
Now ask what must be happening inside the F1 hybrid for this to be possible. During the formation of gametes, the two alleles (factors) present in the hybrid must obviously separate, or segregate, from each other — otherwise the recessive type could never appear in F2 at all. The reappearance of the recessive character is itself the evidence that segregation occurs.
Because the two alleles separate, the gametes which are formed are always pure for a particular character (trait). A gamete may carry either the dominant factor or the recessive factor, but never both. That is why this law is also called the law of purity of gametes.
The physical basis of this behaviour is meiosis, which you have already studied. The two alleles of a pair sit at the same locus on two homologous chromosomes; when the homologues separate in meiosis, the alleles are carried apart with them into different gametes. Mendel deduced this separation from counted peas, decades before anyone had watched it happen down a microscope.
3. Law of Independent Assortment
The third law is based on the dihybrid cross. It is a basic principle of genetics, developed by Mendel, and it describes how different genes or alleles present on separate chromosomes separate independently of each other during the formation of gametes. These alleles are then randomly united at fertilization.
In the dihybrid cross, the F2 phenotypic ratio \(9 : 3 : 3 : 1\) is the evidence. This ratio indicates that the two pairs of characters behave independently of each other. Had the two characters been forced to travel together, only the two parental combinations would have reappeared in F2, and no new combinations would have been seen. It can therefore be concluded that the two characters under consideration are assorted independently, giving rise to different combinations.
The grouping of the sixteen F2 boxes into four phenotypic classes is shown in Fig. 3.3a, and the logic of independent assortment is set out in FC 3.3.
Diagram
Fig. 3.3a – The 4 × 4 dihybrid F2 checker board with the sixteen boxes shaded into four phenotype groups — 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green — and the ratio 9 : 3 : 3 : 1 written below.
Flowchart
FC 3.3 – Independent assortment in the F1 dihybrid RrYy: R separates from r (segregation) AND Y separates from y (segregation) → the separation of R/r does not influence the separation of Y/y → each gamete gets one seed-shape allele and one seed-colour allele in any combination → four gamete types RY, Ry, rY, ry, formed in equal numbers → random fusion at fertilization → 16 F2 combinations → 9 : 3 : 3 : 1
Two of the sixteen F2 phenotypic classes — round green and wrinkled yellow — are recombinant types: combinations that neither parent possessed. They exist only because the two pairs of factors assorted independently. This shuffling of parental characters into new combinations is one of the main sources of variation in sexually reproducing organisms.
For NEET and board questions, all three laws are most often tested as assertion–reason items or as “which law is illustrated by this ratio” items. A safe rule of thumb: a \(3 : 1\) phenotypic ratio in F2 points to dominance and segregation together, a \(1 : 2 : 1\) genotypic ratio points to segregation alone, and a \(9 : 3 : 3 : 1\) ratio points to independent assortment.
3.4 Back Cross and Test Cross
a. Back Cross
The F1 individuals obtained in a cross are usually selfed to get the F2 progeny. But that is not the only thing a geneticist can do with them. The F1 plants can also be crossed with one of the two parents from which they were derived — either the recessive parent or the dominant parent. Such a cross is known as a back cross.
So a back cross of a hybrid tall pea plant may be \(Tt \times TT\) (with the dominant parent) or \(Tt \times tt\) (with the recessive parent). Both are back crosses, because in each case the hybrid is being crossed “back” to a parental type.
b. Test Cross
The cross of an F1 hybrid with the homozygous recessive parent is known as a test cross. It is used to test whether an individual is homozygous (pure) or heterozygous (hybrid). A test cross is easy, simple, repeatable and predictable — which is why it remains the standard genetic tool for this job.
Why a Test Cross Works
A test cross can be used to find out the genotype of any plant with a dominant expression — a plant whose appearance shows the dominant character, but where it is not known whether it is homozygous (pure) or heterozygous for that trait. The reason such a plant is a puzzle is that dominance hides one of the two possible genotypes behind a single appearance.
The recessive parent is chosen deliberately, and this is the heart of the method. A homozygous recessive individual can contribute only recessive alleles to the offspring. It therefore adds nothing that could mask anything. Whatever allele the unknown parent supplies will show itself directly in the phenotype of the offspring. In effect, the recessive parent turns the offspring into a readout of the unknown parent’s gametes.
Reading the Result
Consider a pea plant having violet (purple) flowers, whose genotype is unknown. It is crossed with a pea plant having white flowers, which must be homozygous recessive.
If all the flowers produced in the offspring are violet, we can conclude that the unknown plant is pure, or homozygous. If we get violet and white flowers in a \(1 : 1\) ratio, we can conclude that the unknown plant is heterozygous. The reasoning is set out as a decision chart in FC 3.4, and the whole cross is represented graphically in Fig. 3.1.
Flowchart
FC 3.4 – Test cross logic: plant with dominant phenotype, genotype unknown → cross with homozygous recessive plant → observe offspring; if all offspring show the dominant phenotype → the unknown plant is homozygous dominant; if offspring show dominant and recessive phenotypes in 1 : 1 → the unknown plant is heterozygous
Diagram
Fig. 3.1 – Graphical representation of a test cross: on the left, a plant of unknown dominant genotype crossed with a homozygous recessive plant giving all violet offspring (result: unknown plant is homozygous dominant); on the right, the same cross giving half violet and half white offspring (result: unknown plant is heterozygous); label parents, gametes, offspring, Result and Interpretation.
Another Use of the Test Cross
The test cross is not only a diagnostic tool. It is also used to introduce useful recessive traits into the hybrids of self-pollinated plants during rapid crop improvement programmes. Because the recessive parent contributes only recessive alleles, crossing to it is a reliable way of moving a desirable recessive character — say a gene for a particular grain quality — into a chosen line.
3.5 Deviations from Mendel’s Findings
On the basis of his experiments with the garden pea, Mendel arrived at a few generalizations — statements that were true of every character he had studied. They can be set out as follows:
- A single trait is controlled by a single gene, and that gene has two alleles.
- The two alleles show an interaction in which one of them is completely dominant over the other.
- Factors (genes) for different traits are present on different chromosomes and therefore assort independently.
These generalizations were honestly drawn from good data. But Mendel’s data came from seven characters of one plant. With the passage of time, a number of deviations were observed and identified in the post-Mendelian era, and these gave additional information about the patterns of inheritance. Such deviations are described together as Neo-Mendelism.
It is important to see clearly what these deviations do and do not do. They do not overturn segregation — alleles still separate cleanly into gametes. What they modify is the relationship between alleles, the number of alleles, the number of genes controlling a trait, and the number of traits controlled by a gene.
Two Kinds of Gene Interaction
It was observed that the phenotypic expression of a gene can be modified or influenced by another gene. These gene interactions are of two types.
Intragenic interactions occur between the alleles of the same gene, for example incomplete dominance and co-dominance. Such interactions also occur between the members of a multiple allele series of a gene.
Intergenic (non-allelic) interactions occur between the alleles of different genes present on the same or on different chromosomes, for example pleiotropy, polygenes, epistasis, supplementary and complementary genes, and so on.
Some of these interactions are discussed below.
a. Incomplete Dominance
In incomplete dominance, both the alleles (genes) of an allelomorphic pair express themselves partially. One allele cannot suppress the expression of the other allele completely. In such a case, there is an intermediate expression in the F1 hybrid — the hybrid looks like neither parent, but like something in between.
A well-known example is the flower colour of Mirabilis jalapa, the four o’clock plant. If a red-flowered (\(RR\)) plant is crossed with a white-flowered (\(rr\)) plant, then the F1 offspring have pink (\(Rr\)) flowers. The cross is worked out in Fig. 3.5a.
Diagram
Fig. 3.5a – Incomplete dominance in Mirabilis jalapa: parents red (RR) × white (rr) → gametes R and r → F1 all pink (Rr) → selfing of F1 → 2 × 2 Punnett square giving RR (red), Rr (pink), Rr (pink), rr (white); label the F2 boxes with their colours.
When the pink F1 plants are selfed, the results are:
Genotypic ratio — \(1\ RR : 2\ Rr : 1\ rr\) Phenotypic ratio — \(1\ \text{Red} : 2\ \text{Pink} : 1\ \text{White}\)
Notice a striking feature of incomplete dominance: because each genotype has its own appearance, the F2 phenotypic and genotypic ratios are identical (\(1 : 2 : 1\)). Whenever a question tells you that the phenotypic and genotypic ratios are the same, incomplete dominance (or co-dominance) is being tested.
b. Co-dominance
In co-dominance, both the alleles (genes) of an allelomorphic pair express themselves equally in the F1 hybrid. Such alleles, which are able to express themselves equally and independently in hybrids, are called co-dominant alleles. Thus, in co-dominance, both alleles are expressed — neither is masked, and neither is diluted.
The classic example of co-dominance is coat colour in cattle. There are two types: one with a red coat (made of red-coloured hair) and the other with a white coat (made of white hair). When red cattle (\(RR\)) are crossed with white cattle (\(WW\)), the F1 hybrids (\(RW\)) are roan, as shown in Fig. 3.2.
Diagram
Fig. 3.2 – Representation of co-dominance in cattle: parents red (RR) × white (WW) → gametes R and W → F1 all roan (RW) → F2 giving red (RR) : roan (RW) : white (WW) in 1 : 2 : 1; show the roan coat as a mixture of red and white hairs.
Roans are not pink cattle. Their coat is a mixture of red and white coloured hairs, sitting side by side — both traits are expressed equally, each in its own hairs. In the F2 generation, red (\(RR\)), roan (\(RW\)) and white (\(WW\)) animals are produced in the ratio \(1 : 2 : 1\). Thus in co-dominance also, the genotypic and phenotypic ratios are identical.
c. Multiple Alleles
More than two alternative forms (alleles) of a gene in a population, occupying the same locus on a chromosome or its homologue, are known as multiple alleles.
Multiple alleles arise by mutations of the wild type of gene. A gene can mutate several times, producing a whole series of alternative expressions. The different alleles in such a series may show a dominant–recessive relation with one another, or they may show co-dominance or incomplete dominance among themselves. The wild type is dominant over all the other mutant alleles.
In Drosophila, a large number of multiple alleles are known. For example, the size of the wings ranges from normal wings all the way down to vestigial (no) wings — that is, just stumps — the vestigial condition being due to one allele (\(vg\)) in the homozygous condition. The normal wing is the wild type, while the vestigial wing is the recessive type. Some members of this allele series are listed in Table 3.3.
Table 3.3: A few phenotypes and genotypes in Drosophila
| Phenotype | Genotype |
|---|---|
| Normal wings | \(vg^{+}\) |
| Nicked wings | \(vg^{ni}\) |
| Notched wings | \(vg^{no}\) |
| Strap wings | \(vg^{st}\) |
| Vestigial wings | \(vg\) |
Another good example of multiple alleles is the A, B, O blood grouping in human beings.
d. Pleiotropy
When a single gene controls two (or more) different, non-related traits, it is called a pleiotropic gene, and the phenomenon is called pleiotropy or pleiotropism. Here the deviation is not in how alleles interact, but in how far a single gene’s influence reaches.
The standard example is sickle-cell anaemia, a disease caused by the gene \(Hb^{s}\). The normal or healthy gene \(Hb^{A}\) is dominant.
The carriers — the heterozygotes \(Hb^{A}/Hb^{s}\) — show signs of mild anaemia, because their RBCs become sickle-shaped, that is, half-moon-shaped, but only under abnormally low \(O_2\) concentration. The homozygotes with the recessive gene \(Hb^{s}\), however, die of total anaemia.
Thus the gene for sickle-cell anaemia is lethal in the homozygous condition and produces the sickle-cell trait in the heterozygous carrier. Two different expressions are produced by a single gene — which is exactly what makes it pleiotropic.
Because of the death of the recessive homozygote, the ratio observed is \(1 : 2\) instead of \(3 : 1\). A marriage between two carriers will produce normal, carrier and sickle-cell anaemic children in the ratio \(1 : 2 : 1\), as shown in Fig. 3.4. The sickle-cell anaemics die, leaving carriers and normals in the ratio \(2 : 1\). The heterozygotes, or carriers, can be identified by microscopic examination of the blood.
Diagram
Fig. 3.4 – Representation of pleiotropy: parents, both sickle-cell carriers (Hb^A Hb^S × Hb^A Hb^S) → gametes Hb^A and Hb^S from each → offspring Hb^A Hb^A (normal, 1), Hb^A Hb^S (sickle-cell carrier, 2), Hb^S Hb^S (sickle-cell anaemic, 1 — dies); label each offspring class.
3.6 Chromosomal Theory of Inheritance
Why Mendel’s Work Was Ignored
Gregor Johann Mendel published his work on the inheritance of traits in 1866, but for several reasons it remained unnoticed and unrecognised till 1900. Communication was not easy in those days, and his paper simply did not travel far. His work was therefore not widely recognized.
There were deeper reasons too. His approach of using mathematics and statistics to explain a biological phenomenon was totally new, and unacceptable to the biologists of his time. Biology then was a science of description, not of counting.
Nor did his central idea fit what naturalists thought they saw. Continuous variations — smooth gradations of size, shape and colour — were being observed everywhere in nature. Against this background, Mendel’s concept of factors (genes) as stable and discrete units which controlled the expression of characters, and his insistence that a pair of alleles did not “blend” with each other, was not accepted by his contemporaries.
Finally, there was an honest gap in his own explanation. Mendel did not know the physical location of the ‘factors’ (genes) in the gametic cell. He had described how they behave without being able to say where they are.
The Rediscovery of 1900
In 1900, three scientists — Hugo de Vries, Correns and von Tschermak — independently rediscovered Mendel’s work on the inheritance of traits. Three separate researchers, working on different plants, arrived at the same rules and then found that a monk had published them thirty-four years earlier.
By then, the scientific setting had changed. Due to advancements in microscopy, scientists were able to observe cell division and the structure of chromosomes under the microscope. The invisible had become visible — and what they were watching looked oddly familiar.
Sutton and Boveri: A Parallel Behaviour
Walter Sutton, along with Theodor Boveri (1903), studied the parallel behaviour of Mendel’s factors (genes) and the behaviour of chromosomes at the time of meiosis. They noticed that the two behave in the same way at every step.
Based on these observations, the chromosomal theory of inheritance was put forth by Sutton and Boveri. This theory identifies chromosomes as the carriers of the genetic material.
This point-for-point matching is the whole argument of the theory, and it is a favourite for assertion–reason questions: genes behave as they do because they are located on chromosomes.
Diagram
Fig. 3.6a – A cell with one pair of homologous chromosomes carrying alleles A and a, going through Meiosis I (the two homologues separate into two cells) and Meiosis II (each chromatid separates), producing four gametes — two carrying A and two carrying a.
The sequence just described is summarised in FC 3.6.
Flowchart
FC 3.6 – Chromosomal basis of inheritance: diploid somatic cell (chromosomes in homologous pairs, alleles in pairs) → meiosis begins → homologous chromosomes pair (synapse) → homologues segregate, and different pairs assort independently → each gamete receives only one chromosome from each pair (haploid) → fusion of haploid male gamete and haploid female gamete → diploid zygote with the species’ chromosome number restored
Gametes as the Link Between Generations
The nucleus of a gamete contains chromosomes, which carry all the hereditary traits. Male and female gametes — sperms and eggs — carry all the hereditary traits, and they are the physical link between parents and offspring. Nothing else passes from parent to child.
The fusion of the haploid male gamete and the haploid female gamete restores the diploid number of chromosomes of the species. This is why the chromosome number of a species stays constant generation after generation, instead of doubling each time: meiosis halves it, fertilization doubles it back.
The theory tells us that chromosomes carry the genes. The obvious next question is: what exactly is a chromosome, and how is it built? That is the subject of the next section.
3.7 Chromosomes
Chromosomes are filamentous (thread-like) bodies present in the eukaryotic nucleus. The term chromosome (Gr. chromo = colour, soma = body) was coined by W. Waldeyer (1888) — a name given simply because these bodies took up dye strongly and stood out as coloured threads under the microscope.
The size of a chromosome varies from species to species. Each metaphase chromosome varies from \(0.1\) to \(33\ \mu m\) in length and from \(0.2\) to \(2\ \mu m\) in thickness. Chromosomes are visible during cell division, when they condense; at other times they are far too fine and extended to be seen as separate bodies.
Chromosomes are capable of self-replication, and they play a vital role in heredity, mutation, variation and the evolutionary development of eukaryotic species. Chemically, eukaryotic chromosomes are made up of DNA, histone proteins and non-histone proteins.
Function
Chromosomes mainly act as carriers of heredity. Everything else about them — their number, their shape, their behaviour in cell division — matters because of this single job.
Number of Chromosomes
The number of chromosomes is specific and constant for a particular species. Because of this constancy, chromosome number is of great importance in the study of the phylogeny and taxonomy of a species — two organisms that look alike may be told apart by counting their chromosomes.
The term ploidy speaks for the degree of repetition of the primary basic number of chromosomes (i.e. ‘x’) in a cell.
When the chromosome number in a cell is an exact multiple of the primary basic number, it is called euploidy. Euploids include monoploid or haploid cells (with one set of chromosomes, where \(x = n\)), diploids (\(2n\) — two sets of chromosomes), triploids (\(3n\) — three sets), tetraploids (\(4n\) — four sets), and so on.
When the chromosome number is not the exact multiple of the haploid set, the condition is described as aneuploidy. Aneuploidy is either the addition or the deletion of one or more chromosome(s) to the total number of chromosomes in a cell.
The whole classification of ploidy is set out in FC 3.7.
Flowchart
FC 3.7 – Variation in chromosome number (ploidy): Ploidy → branches into Euploidy and Aneuploidy. Euploidy → Monoploidy (n), Diploidy (2n), Polyploidy (3n, 4n, 5n, etc.). Aneuploidy → Hypoploidy and Hyperploidy; Hypoploidy → Monosomy (2n−1), Nullisomy (2n−2); Hyperploidy → Trisomy (2n+1), Tetrasomy (2n+2)
Structure of Chromosome
Chromosomes are best visible under the microscope when the cell is at the metaphase stage, because at this stage chromosomes are highly condensed. This is why karyotypes are always prepared from metaphase cells. The parts of a typical chromosome are shown in Fig. 3.6.
Diagram
Fig. 3.6 – Structure of chromosome: (A) parts of a chromosome — two chromatids joined at the centromere, with chromonemata, chromomeres, matrix and telomeres labelled; (B) a chromosome showing primary constriction (centromere), secondary constriction (i) or nucleolar organizer, secondary constriction (ii), satellite body and nucleolus.
A typical chromosome consists of two chromatids joined together at the centromere, also called the primary constriction. The primary constriction consists of a disc-shaped plate called the kinetochore. It is at the kinetochore that spindle fibres get attached during cell division.
Besides the primary constriction, a few chromosomes possess an additional one or two constrictions, called secondary constrictions. At secondary constriction I, the nucleolus becomes organized during interphase — which is why this region is also called the nucleolar organizer. A satellite body (SAT body) is attached at secondary constriction II, in very few chromosomes.
Each chromatid in turn contains a long, unbranched, slender, highly coiled DNA thread called the chromonema, extending through the length of the chromatid. A chromatid consists of a double-stranded DNA molecule which extends from one end of the chromosome to the other.
Types of Chromosomes
Depending upon the position of the centromere, there are four types (shapes) of chromosomes: acrocentric (j-shaped), telocentric (i-shaped), submetacentric (L-shaped) and metacentric (V-shaped). The ends of a chromosome — that is, of the chromatids — are known as telomeres.
Sex Chromosomes
The chromosomes which are responsible for the determination of sex are known as sex chromosomes (allosomes). Human beings and other mammals have X and Y chromosomes as their sex chromosomes.
The X chromosome is straight, rod-like and longer than the Y chromosome. The X chromosome is metacentric, while the Y chromosome is acrocentric.
The X chromosome has a large amount of euchromatin (the extended region) and a small amount of heterochromatin (the highly condensed region). Euchromatin has DNA in the extended state, and hence it is metabolically more active than heterochromatin. The Y chromosome, in contrast, has a small amount of euchromatin and a large amount of heterochromatin, and hence it is genetically less active, or inert.
Both the X and the Y chromosome show homologous and non-homologous regions, as seen in Fig. 3.7. The homologous regions show similar genes, while the non-homologous regions show dissimilar genes.
Diagram
Fig. 3.7 – Structure of X and Y chromosomes in humans, placed side by side, with the centromere marked on each, the shorter homologous region shaded at one end, and the longer non-homologous regions of both chromosomes labelled.
Crossing over occurs only between the homologous regions of the X and Y chromosomes. The non-homologous region of the X chromosome is longer and contains more genes than that of the Y chromosome. X-linked genes are present on the non-homologous region of the X chromosome, while Y-linked genes are present on the non-homologous region of the Y chromosome.
This asymmetry between X and Y is not a curiosity of anatomy. It is the reason why certain diseases strike men far more often than women — a consequence we will follow through in Section 3.10.
3.8 Linkage and Crossing Over
Linkage
Mendel’s law of independent assortment rests on a quiet assumption: that the two genes being studied sit on different chromosomes. But a chromosome is long and a gene is short, so a single chromosome must carry many genes. What happens to those genes?
It is a known fact that several genes are closely located on the same chromosome. As chromosomes are the carriers of heredity, and as a whole chromosome travels into a gamete as one piece, these genes have a tendency to be inherited together. Such genes are called linked genes.
Linkage was discovered in plants by Bateson and Punnett, and in animals by T. H. Morgan.
Linkage is of two kinds — complete and incomplete.
I. Complete Linkage
The linked genes which are closely located on the chromosome do not separate — no crossing over occurs between them — and they are inherited together. They are called completely linked (strongly linked) genes, and the phenomenon of their inheritance is called complete linkage. Because the genes travel together exactly as they came from the parent, the parental traits are inherited in the offspring, and no new combinations appear.
For example, the X chromosome of Drosophila males shows complete linkage.
II. Incomplete Linkage
The linked genes which are distantly located on the same chromosome, and therefore have chances of separation by crossing over, are known as incompletely linked (weakly linked) genes. The phenomenon of their inheritance is called incomplete linkage. Here, because the genes can be separated, new traits (new combinations) occur in the offspring.
For example, in Zea mays, the colour and shape of the grain show incomplete linkage.
Linkage Groups
All the linked genes present in a particular chromosome together constitute a linkage group. Since every gene on a chromosome belongs to that chromosome’s linkage group, the number of linkage groups of a particular species corresponds to its haploid number of chromosomes.
For example, Drosophila melanogaster has 4 linkage groups, corresponding to its 4 pairs of chromosomes. The garden pea has 7 linkage groups and 7 pairs of chromosomes.
Sex-Linkage
The transmission (inheritance) of X-linked and Y-linked genes from parents to offspring is called sex-linked inheritance. Sex-linked inheritance is of three types, viz. X-linked, Y-linked and XY-linked. Sex linkage itself is of two kinds.
a. Complete Sex Linkage
Complete sex linkage is exhibited by genes located on the non-homologous regions of the X and Y chromosomes. These genes inherit together, because crossing over does not occur in this region — there is no partner segment to exchange with.
Examples of X-linked traits are haemophilia, red-green colour blindness, myopia (near-sightedness) and Ichthyosis. Examples of Y-linked traits are hypertrichosis and the H-Y antigen gene.
b. Incomplete Sex Linkage
Incomplete sex linkage is exhibited by genes located on the homologous regions of the X and Y chromosomes. These genes do not inherit together, because crossing over does occur in this region.
Examples of X-Y linked traits are total colour blindness, nephritis and retinitis pigmentosa.
Crossing Over
Crossing over is a process that produces new combinations (recombinations) of genes by the interchange and exchange of corresponding segments between the non-sister chromatids of homologous chromosomes.
It occurs during pachytene of prophase I of meiosis. The term crossing over was coined by Morgan. The mechanism of crossing over consists of four sequential steps: synapsis, tetrad formation, crossing over and terminalization — which you have already studied in the chapter on cell division in Class XI. The sequence is summarised in FC 3.8, and the exchange itself is shown in Fig. 3.8a.
Flowchart
FC 3.8 – Mechanism of crossing over: homologous chromosomes come together and pair → synapsis → each chromosome has two chromatids, so the paired structure has four → tetrad formation → non-sister chromatids break at corresponding points and exchange segments at a chiasma → crossing over → the chiasmata move towards the ends of the chromatids → terminalization → homologues separate, now carrying recombined gene combinations
Diagram
Fig. 3.8a – Two homologous chromosomes paired as a tetrad, with a chiasma between two non-sister chromatids; show the chromatids before and after the exchange, so that two are parental and two are recombinant; label non-sister chromatids, chiasma, and the recombinant chromatids.
The phenomenon of crossing over is by and large universal, except in male Drosophila. It is necessary for natural selection, because it increases the chances of variation — it keeps producing gene combinations that neither parent possessed, and natural selection can only act on variation that exists.
Morgan’s Experiments Showing Linkage and Crossing Over
Morgan used Drosophila melanogaster (the fruit fly) for his experiments, and the choice was as shrewd as Mendel’s choice of the pea. Drosophila can easily be cultured in the laboratory. Its life span is short, about two weeks. Moreover, it has a high rate of reproduction — so a great many offspring can be counted in a short time, which is exactly what a study of rare recombinants requires.
Morgan carried out several dihybrid cross experiments in the fruit fly to study genes that are sex-linked. The crosses were similar to the dihybrid crosses carried out by Mendel in pea. For example, Morgan and his group crossed yellow-bodied, white-eyed females with wild-type brown-bodied, red-eyed males, and then intercrossed their F1 progeny. Here the dominant wild-type alleles are represented with a plus \((+)\) sign. The two crosses are shown in Fig. 3.8.
Diagram
Fig. 3.8 – Linkage and crossing over in Drosophila: Cross I (yellow body, white eye × wild type) and Cross II (white eye, miniature wing × wild type), each showing the parental cross, the F1 generation, and the F2 generation split into parental types (98.7 % in Cross I; 62.8 % in Cross II) and recombinant types (1.3 % in Cross I; 37.2 % in Cross II); label the alleles y, w, m and their wild-type forms y⁺, w⁺, m⁺.
He observed that the two genes did not segregate independently of each other, and that the F2 ratio deviated very significantly from the \(9 : 3 : 3 : 1\) ratio expected for a dihybrid cross.
Morgan and his group knew that the genes concerned were located on the X chromosome. They stated that when two genes in a dihybrid cross are situated on the same chromosome, then the proportion of the parental combination is much higher than that of the non-parental type. This occurs due to the physical association, or linkage, of the two genes.
He also found something more precise. When genes are grouped on the same chromosome, some genes are strongly linked: they show very few recombinations (\(1.3\ \%\)). When genes are loosely linked — that is, present far away from each other on the chromosome — they show more (higher) recombinations (\(37.2\ \%\)).
For example, the genes for yellow body and white eye were strongly linked and showed only \(1.3\) per cent recombination (in Cross I). The genes for white eye and miniature wing showed \(37.2\) per cent recombination (in Cross II). Cross I shows crossing over between the genes y and w; Cross II shows crossing over between the genes white (w) and miniature wing (m).
3.9 Autosomal Inheritance
A human somatic (\(2n\)) cell contains 23 pairs of chromosomes. These can be divided functionally into two groups: autosomes and sex chromosomes. A single pair of chromosomes is involved in sex determination, and the remaining 22 pairs are called autosomes.
Autosomes control a wide variety of traits other than sex, and such traits are called autosome-linked traits. The transmission of body characters — that is, of characters other than the sex-linked traits — from parents to their offspring through the autosomes is called autosomal inheritance.
Dominant and Recessive Autosomal Traits
Some characters are influenced by dominant genes, while some others are influenced by recessive genes, present on the autosomes. For example:
- Autosomal dominant traits such as Widow’s peak and Huntington’s disease.
- Autosomal recessive traits such as Phenylketonuria (PKU), Cystic fibrosis and Sickle-cell anaemia.
The distinction is not a matter of how serious the disease is. It is a matter of how many faulty copies of the gene are needed before the character shows itself — one copy for a dominant trait, two copies for a recessive one.
a. Widow’s Peak
A prominent “V”-shaped hairline on the forehead is described as a widow’s peak. It is determined by an autosomal dominant gene. Widow’s peak therefore occurs in homozygous dominant (\(WW\)) individuals and also in heterozygous (\(Ww\)) individuals — one copy of the dominant allele is enough.
Individuals with the homozygous recessive (\(ww\)) genotype have a straight hairline, that is, no widow’s peak. The two hairlines are compared in Fig. 3.9.
Diagram
Fig. 3.9 – Two front views of a human forehead side by side: on the left, a hairline coming to a distinct “V” in the middle, labelled Widow’s peak; on the right, a straight hairline, labelled No widow’s peak.
Since the gene lies on an autosome and not on a sex chromosome, both males and females have an equal chance of inheriting it. This is the signature of autosomal inheritance, and it is the first thing to look for while reading a pedigree chart: if a trait appears just as often in males as in females, it is very likely autosomal rather than sex-linked.
b. Phenylketonuria (PKU)
Phenylketonuria is an inborn metabolic disorder, caused due to recessive autosomal genes. It shows itself only when the recessive genes are present in the homozygous condition.
In such an individual, the enzyme phenylalanine hydroxylase is not produced. This enzyme is essential for the conversion of the amino acid phenylalanine into tyrosine. Due to the absence of the enzyme, phenylalanine is not converted into tyrosine.
Hence phenylalanine and its derivatives accumulate in the blood and in the cerebrospinal fluid (CSF). This accumulation affects the development of the brain and causes mental retardation. Excess phenylalanine is excreted in the urine, and it is from this that the disease gets its name — phenylketonuria. The block in the pathway is shown in FC 3.9.
Flowchart
FC 3.9 – Phenylketonuria: Normal pathway — phenylalanine (from the diet) → enzyme phenylalanine hydroxylase acts → tyrosine. In PKU — homozygous recessive genotype → phenylalanine hydroxylase not produced → phenylalanine is not converted to tyrosine → phenylalanine and its derivatives accumulate in blood and cerebrospinal fluid → brain development affected → mental retardation; and excess phenylalanine → excreted in urine
PKU is the clearest illustration in this chapter of what a gene actually does. A gene does not directly build a hairline or an eye colour. It carries the instructions for a protein — very often an enzyme — and the character we see is the downstream consequence of that protein working, or failing to work.
The Pattern of Autosomal Recessive Traits
Autosomal recessive traits appear in both sexes with equal frequency, and they tend to skip generations.
3.10 Sex Linked Inheritance
Genes located on the non-homologous region of the sex chromosomes are called sex-linked genes. The traits that are determined by sex-linked genes are called sex-linked traits. The inheritance of sex-linked genes from parents to their offspring is called sex-linked inheritance.
There are two types of sex-linked genes: X-linked genes and Y-linked genes.
a. X-linked (Sex-linked) Genes
The X-linked genes are located on the non-homologous region of the X chromosome, and these genes do not have corresponding alleles on the Y chromosome. This single anatomical fact drives everything that follows.
A female has two X chromosomes. In a female, therefore, two recessive sex-linked genes are required for the expression of a sex-linked trait. If one X chromosome carries a recessive gene for a sex-linked trait (a defect), its effect is suppressed by the dominant gene present on the other X chromosome. Females with one such recessive gene are called carriers. A carrier female is physically normal, as she does not suffer from the disease (disorder), but she can pass the recessive gene on.
A male has only one X chromosome. If that X chromosome carries an X-linked recessive gene for a sex-linked trait, it is expressed phenotypically — because there is no dominant gene on the Y chromosome to suppress its effect. A male in this situation is described as hemizygous: he has only a single copy of the gene, not a pair, so there is no second allele to mask it.
Therefore, sex-linked (X-linked) traits appear more frequently in males than in females. Examples of X-linked traits include haemophilia, colour blindness, night blindness, myopia and muscular dystrophy.
b. Y-linked (Holandric) Genes
Genes located on the non-homologous region of the Y chromosome are called Y-linked genes, or holandric genes. Since only males possess a Y chromosome, and since a father passes his Y chromosome to every son, the Y-linked genes are inherited directly from male to male.
In man, a Y-linked gene such as that for hypertrichosis is responsible for the excessive development of hair on the pinna of the ear. This character is transmitted directly from father to son, and it never appears in a daughter.
Colour Blindness
Colour blindness is an X-linked recessive disorder in which the person is unable to distinguish between red and green colours, as both the colours appear grey. It is caused due to recessive X-linked genes (\(X^c\)), which prevent the formation of the colour-sensitive cells — the cones — in the retina of the eye. The dominant X-linked gene (\(X^C\)) is necessary for the formation of these colour-sensitive cells.
The homozygous recessive females (\(X^c X^c\)) and the hemizygous recessive males (\(X^c Y\)) are unable to distinguish between red and green colours. The frequency of colour-blind women is much less than that of colour-blind men.
Thus the genotypes of male and female individuals can be represented as follows:
| Sex | Normal | Colourblind | Carrier |
|---|---|---|---|
| Male | \(X^C Y\) | \(X^c Y\) | — |
| Female | \(X^C X^C\) | \(X^c X^c\) | \(X^C X^c\) |
The inheritance of colour blindness can be studied in the following two types of marriages.
1. Colour-blind male × normal female
A marriage between a colour-blind male and a normal female will produce normal-visioned male and female offspring in F1. The sons have normal vision, but the daughters will be carriers for the disease. The cross is worked out in Fig. 3.10.
Diagram
Fig. 3.10 – Sex-linked inheritance (colour blindness): parents colour-blind male (X^c Y) × normal female (X^C X^C) → gametes X^c and Y from the father, X^C from the mother → F1 shows carrier daughters (X^C X^c, 50 %) and normal sons (X^C Y, 50 %); label each offspring class.
Look carefully at where the alleles have gone. The father’s single X, carrying the defective gene, has passed to every daughter — a father gives his X chromosome only to his daughters. His sons received his Y chromosome, and therefore none of his colour-blind allele at all.
2. Carrier female × normal male
A marriage between a carrier female (the daughter above) and a normal male will produce female offspring with normal vision, but half of them will be carriers for the disease. Half of the male offspring will be normal, while the remaining half will be colour blind. The cross is shown in Fig. 3.11.
Diagram
Fig. 3.11 – Sex-linked inheritance (colour blindness): parents carrier daughter (X^C X^c) × normal male (X^C Y) → gametes X^C and X^c from the mother, X^C and Y from the father → F1 shows normal female (X^C X^C, 25 %), normal male (X^C Y, 25 %), carrier female (X^C X^c, 25 %) and colour-blind male (X^c Y, 25 %).
Criss-cross Inheritance
Put the two marriages side by side and a pattern emerges. From the above examples it is clear that the X-linked recessive gene for colour blindness is inherited from the colour-blind father to his grandson, through his daughter. This type of inheritance is called criss-cross inheritance.
Flowchart
FC 3.10 – Criss-cross inheritance: colour-blind grandfather (X^c Y) → passes X^c to all his daughters → each daughter is a carrier (X^C X^c), phenotypically normal → she passes X^c to half her children → a son receiving X^c is colour blind (X^c Y); a daughter receiving X^c is again a carrier → trait reappears in the grandson
Haemophilia (Bleeder’s Disease)
Haemophilia is an X-linked recessive disorder in which the blood fails to clot, or coagulates very slowly. The genes for normal clotting are dominant over the recessive genes for haemophilia.
A person having the recessive gene for haemophilia is deficient in the clotting factors (VIII or IX) in the blood. Even minor injuries cause continuous bleeding, and hence haemophilia is also called bleeder’s disease.
The recessive gene for haemophilia is located on the non-homologous region of the X chromosome. As there is no corresponding allele on the Y chromosome to suppress its expression, men suffer from this disease. Women suffer only when both of their X chromosomes carry the recessive genes (alleles).
The genotypes of male and female individuals can be represented as follows:
| Sex | Normal | Haemophilic | Carrier |
|---|---|---|---|
| Male | \(X^H Y\) | \(X^h Y\) | — |
| Female | \(X^H X^H\) | \(X^h X^h\) | \(X^H X^h\) |
Like colour blindness, haemophilia also shows criss-cross inheritance. Its inheritance can be studied with the help of the following examples.
1. Haemophilic male × normal female
The cross gives carrier daughters (50 %) and normal sons (50 %), as shown in Fig. 3.12. Once again, every daughter of an affected father is a carrier, and no son of his is affected by the gene he carries.
Diagram
Fig. 3.12 – Sex-linked inheritance (haemophilia): parents haemophilic male (X^h Y) × normal female (X^H X^H) → gametes X^h and Y from the father, X^H from the mother → F1 shows carrier daughters (X^H X^h, 50 %) and normal sons (X^H Y, 50 %).
2. Carrier female × normal male
The cross gives normal female (25 %), normal male (25 %), carrier female (25 %) and haemophilic male (25 %), as shown in Fig. 3.13.
Diagram
Fig. 3.13 – Sex-linked inheritance (haemophilia): parents carrier daughter (X^H X^h) × normal male (X^H Y) → gametes X^H and X^h from the mother, X^H and Y from the father → F1 shows normal female (X^H X^H, 25 %), normal male (X^H Y, 25 %), carrier female (X^H X^h, 25 %) and haemophilic male (X^h Y, 25 %).
3.11 Sex Determination
The mechanism by which sex is established is termed sex determination. The term sex here refers to the sexual phenotype — whether an individual develops as a male or as a female.
In some species, both male and female reproductive organs are present in the same organism. Such an organism is described as bisexual, or hermaphrodite, or monoecious. On the other hand, some species have organisms possessing either male or female reproductive organs, and these are said to be dioecious or unisexual. Human beings are dioecious.
The Discovery of the X Chromosome
a. Sex Determination in Human Beings
The chromosomal mechanism of sex determination in human beings is of the XX-XY type.
In human beings, the nucleus of each somatic cell contains 46 chromosomes, or 23 pairs of chromosomes. Out of these, 22 pairs are autosomes and one pair consists of the sex chromosomes.
The human female has a pair of XX — homomorphic sex chromosomes, since the two members look alike. The human male has XY — heteromorphic sex chromosomes, since the two members are unlike each other.
Thus the genotype of:
Female = 44 Autosomes + XX Male = 44 Autosomes + XY
During gamete formation in the male, the diploid germ cells in the testis undergo spermatogenesis to produce two types of haploid sperms: 50 % of the sperms contain 22 autosomes and an X chromosome, while 50 % of the sperms contain 22 autosomes and a Y chromosome.
In the female, the diploid germ cells in the ovaries undergo oogenesis to produce only one type of egg. All the eggs contain 22 autosomes and an X chromosome.
Thus the human male is heterogametic (producing two kinds of gametes) and the human female is homogametic (producing only one kind of gamete).
If a sperm containing the X chromosome fertilizes the egg (ovum), then a diploid zygote is formed which grows into a female child. If a sperm containing the Y chromosome fertilizes the egg, then the diploid zygote formed grows into a male child. The whole mechanism is shown in Fig. 3.14, where it is also clear why a \(1 : 1\) sex ratio is expected.
Diagram
Fig. 3.14 – Sex determination in human beings: P generation male (XY) × female (XX) → meiosis → sperms carrying X and Y in equal numbers, eggs all carrying X → fertilization → F1 Punnett square giving XX (female), XY (male), XX (female), XY (male); conclusion — a 1 : 1 sex ratio is produced.
This indicates that the sex of a child depends on the type of sperm that fertilizes the egg, and hence the father is responsible for the determination of the sex of the child, and not the mother. Due to lack of knowledge, women are often blamed for giving birth to a female child — a belief that this simple piece of genetics shows to be baseless. The mother’s egg always carries an X; she has no other kind of egg to give.
b. Sex Determination in Birds
In birds, the chromosomal mechanism of sex determination is of the ZW-ZZ type. Here the situation is the reverse of the human one.
In this type, the females are heterogametic and produce two types of eggs: 50 % of the eggs carry the Z chromosome, while 50 % of the eggs carry the W chromosome.
The males are homogametic and produce only one type of sperm. Each sperm carries a Z chromosome. Thus the sex of the individual depends on the kind of egg (ovum) that is fertilized by the sperm, as shown in Fig. 3.15.
Diagram
Fig. 3.15 – Sex determination in birds: parents male (2n + ZZ) × female (2n + ZW) → gametes n + Z from the male, and n + Z and n + W from the female → F1 gives 2n + ZZ (male bird) and 2n + ZW (female bird).
c. Sex Determination in Honey Bees
In honey bees, the chromosomal mechanism of sex determination is of the haplo-diploid type. In this type, the sex of an individual is determined by the number of sets of chromosomes it receives — not by any special sex chromosome at all.
Females are diploid (\(2n = 32\)) and males are haploid (\(n = 16\)).
The female produces haploid eggs (\(n = 16\)) by meiosis, while the male produces haploid sperms (\(n = 16\)) by mitosis. A haploid male has no second set of chromosomes to halve, so meiosis is neither possible nor necessary; mitosis alone gives him gametes.
If the egg is fertilized by a sperm, the zygote develops into a diploid female (\(2n = 32\)) — a queen or a worker. An unfertilised egg develops into a haploid male (\(n = 16\)), the drone, by way of parthenogenesis — development from an unfertilised egg. The mechanism is shown in Fig. 3.16 and summarised in FC 3.11.
Diagram
Fig. 3.16 – Sex determination in honey bee: female (2n = 32) undergoes meiosis to give eggs (n = 16); male (n = 16) undergoes mitosis to give sperms (n = 16); a fertilized egg gives 2n = 32 (diploid female); an unfertilised egg develops without fertilization (parthenogenesis) into n = 16 (haploid male, the drone).
Flowchart
FC 3.11 – Haplo-diploid sex determination in the honey bee: queen produces haploid eggs (n = 16) by meiosis → if an egg is fertilized by a haploid sperm (n = 16) → diploid zygote (2n = 32) → develops into a female → if that larva is fed royal jelly → queen (fertile female); if fed ordinary food → worker (sterile female). If the egg is NOT fertilized → parthenogenesis → haploid individual (n = 16) → drone (male)
The diploid female gets differentiated into either a worker or a queen, depending on the food consumed during development. Diploid larvae which get royal jelly as food develop into the queen (a fertile female), while the others develop into workers (sterile females). Here, then, the genotype decides sex, but the diet decides caste — two different levels of control operating in the same insect.
3.12 Genetic Disorders
Genetic disorders are broadly grouped into two categories: Mendelian disorders and chromosomal disorders. The difference between them is a difference of scale — whether the fault lies in a single gene, or in a whole chromosome.
Mendelian disorders are mainly caused due to an alteration or mutation in the gene. Examples include thalassemia, sickle-cell anaemia, colour blindness, haemophilia and phenylketonuria. They are called Mendelian because they are inherited in the patterns Mendel described — dominant or recessive, autosomal or sex-linked — and because their transmission through a family can be traced with a Punnett square.
Chromosomal disorders, on the other hand, are caused due to the absence or excess of one or more chromosomes, or due to their abnormal arrangement. For example, Down’s syndrome, Turner’s syndrome and Klinefelter’s syndrome.
Thalassemia
Thalassemia is an autosomal, inherited recessive disease. To understand it, recall the structure of the molecule that is at fault.
The haemoglobin molecule is made of four polypeptide chains — 2 alpha (\(\alpha\)) chains and 2 beta (\(\beta\)) chains. The synthesis of the alpha chains is controlled by two closely linked genes, HBA1 and HBA2, on chromosome 16, while the synthesis of the beta chain is controlled by a single gene, HBB, on chromosome 11.
Depending upon which chain of haemoglobin is affected, thalassemia is classified as alpha-thalassemia and beta-thalassemia. It is caused due to the deletion or mutation of the gene which codes for the alpha (\(\alpha\)) and beta (\(\beta\)) globin chains, and this results in the abnormal synthesis of haemoglobin.
In thalassemia, the person shows symptoms such as anaemia, pale yellow skin, change in the size and shape of the RBCs, slow growth and development, dark urine, and so on. Massive blood transfusion is needed for these patients.
Thalassemia differs from sickle-cell anaemia. Thalassemia is a quantitative problem — too few globin molecules are synthesised. Sickle-cell anaemia is a qualitative problem — an incorrectly functional globin is synthesised.
Non-disjunction: The Common Cause of Chromosomal Disorders
Before looking at the three chromosomal disorders individually, it is worth being clear about the accident that produces all of them.
Non-disjunction is the failure of chromosomes to separate during gamete formation. Instead of one member of a pair going to each gamete, both go to the same gamete — leaving that gamete with one chromosome too many, and the other gamete with one too few.
Flowchart
FC 3.12 – Non-disjunction and its outcome: normal gamete formation → homologous chromosomes separate → each gamete gets one chromosome of the pair → normal 2n zygote after fertilization. In non-disjunction → the pair fails to separate → one gamete gets both chromosomes (n + 1) and the other gets none (n − 1) → after fertilization with a normal gamete → zygote is either 2n + 1 (trisomy, e.g. Down’s syndrome, Klinefelter’s syndrome) or 2n − 1 (monosomy, e.g. Turner’s syndrome)
Notice that this connects directly to the ploidy chart of Section 3.7: a trisomy (\(2n + 1\)) is a hyperploid condition and a monosomy (\(2n - 1\)) is a hypoploid condition. Both are forms of aneuploidy.
Down’s Syndrome (21st Trisomy)
Down’s syndrome is named after the physician John Langdon Down, who first described this autosomal chromosomal disorder in 1866.
This syndrome is caused due to an extra copy of chromosome number 21. It shows the presence of three copies of the 21st chromosome instead of a homologous pair. These individuals will therefore have 47 chromosomes instead of the normal number of 46, as seen in the karyotype in Fig. 3.18.
The 21st trisomy occurs due to non-disjunction, or failure of separation of chromosomes (autosomes) during gamete formation. The incidence of non-disjunction is distinctly higher in mothers who are over 45 years old.
These patients have mild or moderate mental retardation, and skeletal development is poor. There are distinct facial features: a small head, small ears and mouth; the face is typically flat and rounded, with a flat nose, an open mouth and a protruding tongue; the eyes slant up and out with internal epicanthal folds; the hands are flat with stubby fingers; and the palm is broad with a single palmar crease. Fig. 3.17 shows a child with Down’s syndrome.
Diagram
Fig. 3.17 – A photograph or drawing of a child with Down’s syndrome, showing the flat rounded face, flat nose, slanting eyes with epicanthal folds, and small ears.
Diagram
Fig. 3.18 – Karyotype of Down’s syndrome: the 23 pairs of human chromosomes arranged in order 1 to 22 plus the sex chromosomes, with three copies (instead of two) circled at position 21 and labelled Trisomy; total count 47.
Turner’s Syndrome (X Monosomy / XO Females)
Turner’s syndrome is a sex chromosomal disorder, caused due to the non-disjunction of a chromosome during the formation of the egg. An individual born with Turner’s syndrome has 44 autosomes with XO — that is, a single X chromosome, with no second sex chromosome at all.
Such individuals are phenotypically female. They have a short stature (height) and a webbed neck, a lower posterior hair line, a broad shield-shaped chest, poorly developed ovaries and breasts, and low intelligence.
Klinefelter’s Syndrome (XXY Males)
Klinefelter’s syndrome is a chromosomal disorder caused due to an extra X chromosome in males. The genotype of such an individual is 44 + XXY. They are described as feminized males.
The extra chromosome is the result of the non-disjunction of the X chromosome during meiosis in the formation of the ovum. Such an individual is male, and has overall masculine development. The voice pitch is harsh, and the testes are underdeveloped. These individuals are tall with long arms, and they show some feminine development — the development of the breast, that is, gynaecomastia. There is no spermatogenesis, and therefore such individuals are sterile.
3.13 Summary
Heredity (inheritance) is the transmission of genetic information from one generation to the next; variation is the set of differences that still appear among individuals. Mendel, working on the garden pea (Pisum sativum) with the hybridization technique, was the first to explain the mechanism of inheritance accurately — before chromosomes or genes were known.
Mendel studied seven pairs of contrasting characters in pea, one at a time and in combinations, and processed his data mathematically. He proposed that characters are carried by particulate factors (now called genes), which occur in pairs, segregate during gamete formation, and do not blend. His success rested on carefully planned experiments with large samples and on expressing results as ratios.
Key terms: a character is a feature, a trait is one detectable form of it; alleles are alternative forms of a gene at identical loci on homologous chromosomes; dominant alleles express in F1, recessive alleles do not; phenotype is the appearance, genotype the genetic make-up; homozygous individuals breed true and form one type of gamete, heterozygous (hybrid) individuals do not and form two types. The Punnett square predicts the progeny of a cross.
A monohybrid cross (pure tall × pure dwarf) gives all tall in F1, and in F2 a phenotypic ratio of \(3\ \text{Tall} : 1\ \text{dwarf}\) with a genotypic ratio of \(1\ TT : 2\ Tt : 1\ tt\). A dihybrid cross (\(RRYY \times rryy\)) gives all round yellow in F1, and an F2 phenotypic ratio of \(9 : 3 : 3 : 1\).
Mendel’s three laws: the Law of Dominance (of a contrasting pair, the allele appearing in F1 is dominant and the one that does not appear is recessive); the Law of Segregation, or law of purity of gametes (when the hybrid forms gametes, the two alleles separate and enter different gametes, each gamete carrying only one allele); and the Law of Independent Assortment (when a hybrid with two or more pairs of contrasting factors forms gametes, the factors of each pair segregate independently of the other pair).
A back cross is a cross of F1 with either parent. A test cross is a cross of an individual showing the dominant phenotype with the homozygous recessive parent; all-dominant offspring indicate a homozygous individual, while a \(1 : 1\) ratio indicates a heterozygote. Every test cross is a back cross, but not every back cross is a test cross.
Deviations from Mendelism (Neo-Mendelism) arise from gene interactions — intragenic (between alleles of the same gene) and intergenic (between alleles of different genes). In incomplete dominance (Mirabilis jalapa), both alleles express partially and F1 is intermediate (pink); F2 is \(1\ \text{Red} : 2\ \text{Pink} : 1\ \text{White}\). In co-dominance (roan coat in cattle), both alleles express fully and equally; F2 is \(1 : 2 : 1\). In both, the phenotypic and genotypic ratios are identical.
Multiple alleles are more than two alternative forms of a gene in a population at the same locus, arising by mutation of the wild type, which is dominant over all mutant alleles (wing alleles in Drosophila; ABO blood groups). Pleiotropy is one gene controlling two or more unrelated traits — sickle-cell anaemia, where \(Hb^{s}\) is lethal in the homozygote and gives the sickle-cell trait in the heterozygous carrier, so that the ratio becomes \(1 : 2\) instead of \(3 : 1\).
The chromosomal theory of inheritance (Sutton and Boveri, 1903) identifies chromosomes as the carriers of genetic material: chromosomes occur in pairs in somatic cells, homologous chromosomes pair, segregate and assort independently during meiosis, and each gamete receives only one chromosome of a pair. Fusion of the haploid gametes restores the diploid number.
Chromosomes are filamentous bodies of the eukaryotic nucleus, made of DNA, histone and non-histone proteins; the term was coined by W. Waldeyer (1888). A metaphase chromosome is \(0.1\)–\(33\ \mu m\) long and \(0.2\)–\(2\ \mu m\) thick. It has two chromatids joined at the centromere (primary constriction), which bears the kinetochore for spindle attachment; some chromosomes have secondary constrictions, a nucleolar organizer and a satellite (SAT) body; the ends are telomeres. By centromere position, chromosomes are metacentric (V), submetacentric (L), acrocentric (j) and telocentric (i).
Ploidy describes the repetition of the basic number x. Euploidy (exact multiples: monoploid n, diploid 2n, polyploid 3n, 4n …) is distinguished from aneuploidy (not an exact multiple), which includes hypoploidy — monosomy (\(2n-1\)), nullisomy (\(2n-2\)) — and hyperploidy — trisomy (\(2n+1\)), tetrasomy (\(2n+2\)).
Sex chromosomes (allosomes) determine sex. The X is longer and metacentric, rich in euchromatin and metabolically active; the Y is shorter and acrocentric, rich in heterochromatin and genetically less active. Crossing over occurs only between their homologous regions.
Linkage is the tendency of genes on the same chromosome to be inherited together (discovered by Bateson and Punnett in plants, and by Morgan in animals). Complete linkage (closely placed genes, no crossing over) gives only parental types; incomplete linkage (distantly placed genes, crossing over possible) gives new combinations. The number of linkage groups equals the haploid chromosome number — 4 in Drosophila melanogaster, 7 in the garden pea.
Crossing over is the exchange of corresponding segments between non-sister chromatids of homologous chromosomes during pachytene of prophase I, through synapsis, tetrad formation, crossing over and terminalization. It is a major source of variation. Morgan’s Drosophila crosses showed that linked genes give more parental than recombinant types: yellow body and white eye recombined only \(1.3\ \%\) (strongly linked), while white eye and miniature wing recombined \(37.2\ \%\) (loosely linked).
Sex linkage: complete sex linkage involves genes on the non-homologous regions of X and Y (haemophilia, red-green colour blindness, myopia, Ichthyosis; hypertrichosis and the H-Y antigen gene on Y), while incomplete sex linkage involves genes on the homologous regions (total colour blindness, nephritis, retinitis pigmentosa).
Autosomal inheritance works through the 22 pairs of autosomes and affects both sexes equally. Widow’s peak and Huntington’s disease are autosomal dominant; phenylketonuria (absence of phenylalanine hydroxylase, so phenylalanine is not converted to tyrosine and accumulates in blood and CSF, causing mental retardation), cystic fibrosis and sickle-cell anaemia are autosomal recessive. Autosomal recessive traits appear equally in both sexes and tend to skip generations.
X-linked recessive disorders appear far more often in males, who are hemizygous and have no masking allele on Y. In colour blindness (\(X^c\)) the cones of the retina are not formed; in haemophilia (\(X^h\)) clotting factor VIII or IX is deficient and blood fails to clot. Both show criss-cross inheritance — from the affected father, through his carrier daughter, to his grandson.
Sex determination: in humans it is of the XX-XY type, with the male heterogametic; the sex of the child is decided by the type of sperm, so the father — not the mother — determines the sex of the child, and a \(1 : 1\) sex ratio results. In birds it is ZW-ZZ, with the female heterogametic. In honey bees it is haplo-diploid: fertilized eggs give diploid females (\(2n=32\), queen or worker, depending on royal jelly), and unfertilised eggs give haploid males (\(n=16\), drones) by parthenogenesis. Henking’s “x-body” in Anasa tristis (1891) was the first sighting of the X chromosome.
Genetic disorders are of two kinds. Mendelian disorders arise from gene mutation — thalassemia (a quantitative defect: too few \(\alpha\) or \(\beta\) globin chains, from genes HBA1 and HBA2 on chromosome 16 and HBB on chromosome 11), sickle-cell anaemia (a qualitative defect), colour blindness, haemophilia, phenylketonuria. Chromosomal disorders arise from non-disjunction — Down’s syndrome (21st trisomy, 47 chromosomes), Turner’s syndrome (44 + XO, phenotypically female, sterile) and Klinefelter’s syndrome (44 + XXY, feminized sterile male).
The chapter’s overall logic — from a factor to a person — is condensed in FC 3.13.
Flowchart
FC 3.13 – The chapter in one chain: gene (a segment of DNA) → located at a locus on a chromosome → chromosomes occur in homologous pairs in the diploid cell → meiosis: segregation, independent assortment and crossing over → gametes, each with one allele of every pair → fertilization → diploid zygote with a new combination of alleles → expression of alleles (dominance, incomplete dominance, co-dominance, multiple alleles, pleiotropy) → phenotype of the offspring → if the gene is altered → Mendelian disorder; if the chromosome number is altered by non-disjunction → chromosomal disorder
3.14 Points to Ponder
Mendel could not see a gene, a chromosome, or a gamete’s contents. He arrived at the laws of heredity purely by counting plants and comparing ratios. What does this tell you about how a scientific explanation can run ahead of the technology available to test it — and what does it cost a scientist when it does?
Mendel’s seven characters in the pea happened to behave as though they were on seven different chromosomes, and the pea does have seven pairs of chromosomes. Suppose two of his chosen characters had been closely linked on the same chromosome. Would he still have arrived at the law of independent assortment? What might the history of genetics have looked like then?
The law of dominance is often described as “not universal”. After studying incomplete dominance, co-dominance and pleiotropy, would you say Mendel’s laws were wrong, or that they were correct within limits he could not have known? Where exactly is the boundary between a law being wrong and a law being incomplete?
Crossing over breaks up combinations of genes that a parent had assembled successfully enough to survive and reproduce. Why, then, should an organism risk shuffling them? Think about what a population would lose if crossing over stopped altogether.
In Bonellia viridis the environment settles the sex of the individual; in the honey bee, the diet of a diploid larva settles whether she becomes a queen or a worker. If genes carry the instructions, how much of an organism is actually decided by the instructions, and how much by the conditions in which they are read?
A phenylketonuric child on a controlled diet may develop entirely normally. The genotype has not changed at all; only the surroundings have. What does this example do to the common idea that “genetic” means “unchangeable”?
A carrier of sickle-cell anaemia or of thalassemia is usually healthy, and the allele passes silently through generations of such carriers. Why does a harmful recessive allele not simply disappear from a population over time?
The father, not the mother, determines the sex of a child — yet in many societies women are still blamed for the birth of a daughter. What responsibility does a student who understands the biology have when confronted with such a belief, in her or his own family or neighbourhood?
Down’s syndrome, Turner’s syndrome and Klinefelter’s syndrome all arise from the same accident — non-disjunction — yet they produce very different individuals. What does this suggest about how important the balance of chromosomes is, as against their mere number?
Karyotyping and carrier screening can now tell a couple the risk their future child faces. Knowing a risk, however, is not the same as knowing what to do about it. What questions, beyond the strictly biological ones, does such knowledge raise for the people who receive it?
3.15 Objective Questions
- The F2 phenotypic ratio of incomplete dominance in Mirabilis jalapa is —
- 2 : 1 : 1
- 1 : 2 : 1
- 3 : 1
- 2 : 2
- In a dihybrid cross, the F2 generation offspring show four different phenotypes, while the genotypes are …………….
- six
- nine
- eight
- sixteen
- A cross between an individual with an unknown genotype for a trait and a recessive plant for that trait is a …………….
- back cross
- reciprocal cross
- monohybrid cross
- test cross
- When the F2 phenotypic and genotypic ratios are the same, it is an example of …………….
- incomplete dominance
- complete dominance
- multiple alleles
- cytoplasmic inheritance
- If the centromere is situated near the end of the chromosome, the chromosome is called …………….
- Metacentric
- Acrocentric
- Sub-metacentric
- Telocentric
- The chromosomal theory of inheritance was proposed by …………….
- Sutton and Boveri
- Watson and Crick
- Miller and Urey
- Oparin and Haldane
- If the genes are located on a chromosome in the order p-q-r-s-t, which of the following gene pairs will have the least probability of being inherited together?
- p and q
- r and s
- s and t
- p and s
- Find the mismatched pair.
- Down’s syndrome = 44 + XY
- Turner’s syndrome = 44 + XO
- Klinefelter’s syndrome = 44 + XXY
- Normal female = 44 + XXX
- A colour-blind man marries a woman who is homozygous for normal colour vision. The probability of their son being colour blind is —
- 0 %
- 25 %
- 50 %
- 100 %
- Match Column-I with Column-II and rewrite the matching pairs.
| Column-I | Column-II |
|---|---|
| a. 21 trisomy | i. Turner’s syndrome |
| b. X-monosomy | ii. Klinefelter’s syndrome |
| c. Holandric traits | iii. Down’s syndrome |
| d. Feminized male | iv. Hypertrichosis |
3.16 Check Your Concepts
Assertion: A gamete can never carry both alleles of a gene pair. Reason: Gametes are haploid and receive only one chromosome from each homologous pair. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Assertion: X-linked recessive disorders are far more common in men than in women. Reason: The Y chromosome carries no allele corresponding to the genes on the non-homologous region of the X chromosome. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Assertion: In co-dominance, the F2 phenotypic and genotypic ratios are identical. Reason: In co-dominance, the heterozygote has a phenotype of its own, distinct from both homozygotes. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Why did Mendel deliberately begin his experiments with pure lines? What would have gone wrong with his results if the parent plants had not been true-breeding?
Both the pink flower of Mirabilis jalapa and the roan coat of cattle appear in the F1 hybrid, and both give a 1 : 2 : 1 ratio in F2. Explain how you would still tell incomplete dominance apart from co-dominance simply by looking at the F1 individual.
What happens to Mendel’s law of independent assortment when the two genes under study happen to lie close together on the same chromosome? Support your answer with reference to Morgan’s Drosophila crosses.
Explain why crossing over between sister chromatids would produce no new gene combinations, whereas crossing over between non-sister chromatids does.
Arrange the following steps of crossing over in the correct sequence: terminalization, synapsis, crossing over, tetrad formation.
Arrange the following in the correct sequence of events for the appearance of a dwarf plant in F2, starting from a cross between pure tall and pure dwarf parents: formation of the tt genotype; segregation of T and t during gamete formation in F1; fusion of a t-bearing pollen grain with a t-bearing egg; formation of the Tt hybrid.
Why is a gene said to be “expressed” through a protein? Use phenylketonuria to explain how a defect in a single gene produces a whole-body disorder.
What would happen to the chromosome number of a species, generation after generation, if gametes were formed by mitosis instead of meiosis?
In sickle-cell anaemia, the expected F2 ratio of 3 : 1 becomes 2 : 1 among the surviving individuals. Explain why.
A boy has hair on the pinna of his ear, exactly like his father and his grandfather, but no woman in the family has ever shown this trait. Identify the type of inheritance and justify your answer.
A woman with normal colour vision, whose father was colour blind, marries a man with normal colour vision. What is the chance that their son will be colour blind, and why? Explain your reasoning using the idea of criss-cross inheritance.
Draw a labelled diagram of a metaphase chromosome and label the following: centromere, kinetochore, chromatids, secondary constriction, satellite body and telomere.
Draw and label a diagram of the X and Y chromosomes of a human being, marking the homologous and the non-homologous regions. Explain why crossing over is restricted to only one of these regions.
Match the following:
| Column-I | Column-II |
|---|---|
| a. Metacentric | i. j-shaped |
| b. Submetacentric | ii. i-shaped |
| c. Acrocentric | iii. V-shaped |
| d. Telocentric | iv. L-shaped |
- Match the following:
| Column-I | Column-II |
|---|---|
| a. Monosomy | i. 2n + 1 |
| b. Nullisomy | ii. 2n − 2 |
| c. Trisomy | iii. 2n + 2 |
| d. Tetrasomy | iv. 2n − 1 |
- Match the following:
| Column-I | Column-II |
|---|---|
| a. Bateson and Punnett | i. Coined the term chromosome |
| b. T. H. Morgan | ii. Discovered linkage in animals |
| c. W. Waldeyer | iii. Described the x-body in Anasa tristis |
| d. Henking | iv. Discovered linkage in plants |
Both Down’s syndrome and Klinefelter’s syndrome give a total of 47 chromosomes, yet the two conditions are entirely different. Explain why, referring to the chromosome involved in each case.
Distinguish between euploidy and aneuploidy, and state which of the two is deliberately used in crop improvement, giving your reason.
A karyotype shows three copies of chromosome 21 and 46 other chromosomes arranged normally. Identify the disorder, name the accident during gamete formation that causes it, and state which parent’s age raises its likelihood.
Why can a male never be a “carrier” of an X-linked recessive trait, while a female can?
Compare thalassemia and sickle-cell anaemia in terms of the defect in globin synthesis. Which of the two is a quantitative problem and which is a qualitative one?
In the honey bee, the male produces sperms by mitosis rather than meiosis. Explain why meiosis is neither possible nor necessary in a drone.
A queen bee and a worker bee have exactly the same chromosome number and are produced from the same kind of fertilized egg, yet one is fertile and the other is not. What decides the difference, and what does this tell you about the relationship between genotype and phenotype?
“The chromosomal theory of inheritance did not discover anything new about factors; it simply told us where they live.” Do you agree? Support your answer by comparing the behaviour of genes with the behaviour of chromosomes during meiosis.
Why is the number of linkage groups in a species equal to its haploid chromosome number and not its diploid number?
Explain the statement: “A test cross is a back cross, but a back cross is not necessarily a test cross.” (30–40 words)
Explain the statement: “The law of dominance is not universal.” (30–40 words)
Define the following terms: (30–40 words)
- Dihybrid cross
- Homozygous
- Heterozygous
- Test cross
What is an allosome? (30–40 words)
What is crossing over? (30–40 words)
Give one example of an autosomal recessive disorder. (30–40 words)
What are X-linked genes? (30–40 words)
What are holandric traits? (30–40 words)
Give an example of a chromosomal disorder caused due to non-disjunction of autosomes. (30–40 words)
Give one example of complete sex linkage. (30–40 words)
Enlist the seven traits of the pea plant selected and studied by Mendel. (80–100 words)
Why is the law of segregation also called the law of purity of gametes? (80–100 words)
Write a note on pleiotropy. (80–100 words)
What are the reasons for Mendel’s success? (80–100 words)
“In humans, the father is responsible for the determination of the sex of the child, and not the mother.” Justify this statement. (80–100 words)
What is linkage? How many linkage groups occur in a human being? (80–100 words)
Write a note on PKU. (80–100 words)
Compare the X-chromosome and the Y-chromosome. (80–100 words)
Explain the chromosomal theory of inheritance. (80–100 words)
What is a dihybrid cross? Explain it with a suitable example and the checker board method. (150–200 words)
Explain independent assortment with a suitable example. (150–200 words)
Define test cross and explain its significance. (150–200 words)
What is parthenogenesis? Explain the haplo-diploid method of sex determination in the honey bee. (150–200 words)
In his answer on the inheritance of X-linked genes, Madhav had shown a carrier male. His answer was marked incorrect, and he wondered why his marks were cut. Explain the reason. (150–200 words)
With the help of a neat, labelled diagram, describe the structure of a chromosome. (150–200 words)
What is criss-cross inheritance? Explain it with a suitable example. (150–200 words)
Describe the different types of chromosomes. (150–200 words)
3.17 Practice With Numericals
A pure tall pea plant (TT) is crossed with a pure dwarf plant (tt). The F1 plants are selfed. Out of 480 F2 plants obtained, how many would you expect to be tall and how many dwarf? How many of the tall plants would be expected to be homozygous?
A dihybrid pea plant (RrYy) is selfed. Out of 320 offspring, calculate the expected number of plants in each of the four phenotypic classes.
A dihybrid plant (RrYy) is test-crossed with a double homozygous recessive plant (rryy). Using a Punnett square, work out the expected phenotypic ratio of the offspring.
A pink-flowered Mirabilis jalapa plant is crossed with a white-flowered plant. Work out the expected genotypic and phenotypic ratios of the offspring.
A roan bull (RW) is crossed with a roan cow (RW). Out of 100 calves, how many would be expected to be red, how many roan and how many white?
Two carriers of sickle-cell anaemia (Hb^A Hb^S) marry. Work out the expected ratio of normal, carrier and sickle-cell anaemic children among their offspring, and state the percentage of their children who would be at risk of dying of total anaemia.
A carrier woman for haemophilia (X^H X^h) marries a normal man (X^H Y). Using a Punnett square, calculate the percentage of their sons who would be haemophilic and the percentage of their daughters who would be carriers.
A colour-blind woman (X^c X^c) marries a man with normal colour vision (X^C Y). Work out the genotypes and phenotypes of their sons and daughters, and state what percentage of the children would be colour blind.
In a cross involving two linked genes, 2000 offspring are obtained. Of these, 1740 are of the two parental types and 260 are of the two recombinant types. Calculate the percentage of recombination and state whether the two genes are strongly or loosely linked.
In one of Morgan’s crosses, the recombinants formed 37.2 % of the total progeny. If 500 flies were scored in this cross, how many would be expected to be of the parental type?
A couple has four children, all of them daughters. Calculate the probability that their fifth child will be a son, and explain your answer in terms of the two kinds of sperm produced by the father.
3.18 Case-Based Questions
- Observe the given pedigree chart and answer the following questions. (80–100 words)
Diagram
Fig. E.1 – Three-generation pedigree chart. Generation I: a male (square) married to a female (circle), joined by a horizontal marriage line, with a vertical line dropping to a horizontal sibship line. Generation II: five children of this couple, from left to right — an unaffected male (open square), an affected male (filled square), an unaffected female (open circle), an unaffected female (open circle) and an unaffected male (open square); the third child, a female, is married to an unaffected male (open square) drawn to her left, outside the sibship. Generation III: a vertical line from this couple drops to a sibship line bearing three children, from left to right — an affected male (filled square), an unaffected female (open circle) and an unaffected female (open circle). Key: square = male, circle = female, filled symbol = affected, open symbol = unaffected.
- Identify whether the trait is sex-linked or autosomal.
- Give an example of a trait in human beings which shows such a pattern of inheritance.