Chapter 3: Genetics — Some Basic Fundamentals
In 1905, in a laboratory at Bryn Mawr College in Pennsylvania, an American biologist named Nettie Stevens sat at a microscope looking at the cells of the mealworm beetle, Tenebrio molitor — the beetle whose grubs are sold in pet shops as feed.
She was chasing an old question. Everyone could see that young ones resemble their parents. Nobody could say what decided whether a newborn was male or female. Warmth had been suggested. So had food, and the mother’s health.
Stevens simply counted. Inside the nucleus of each body cell lay a set of tiny threads called chromosomes — the structures that carry the instructions passed from parents to offspring. In the female beetle’s cells she counted twenty threads, all of much the same size. In the male’s cells she counted twenty as well, but one of them was clearly smaller than its partner.
Then she looked at the sperm, and found two kinds. Half carried the large chromosome, half carried the small one. Every egg carried only the large one. The conclusion was hard to escape: sex was not settled by warmth or by food, but by which kind of sperm reached the egg. Fig. 3.0-1 shows what she saw.
At about the same time, Edmund Beecher Wilson, working on other insects, reached a similar conclusion.
Diagram
Fig. 3.0-1 — What Nettie Stevens Saw In The Mealworm Beetle Simple line drawing in two columns. Left column: the chromosome set from a body cell of a female mealworm beetle, drawn as ten matched pairs of similar size; below it, one egg containing a large chromosome. Right column: the chromosome set from a body cell of a male, drawn as nine matched pairs of similar size plus one pair in which one member is much smaller; below it, two sperm side by side, one containing the large chromosome and one containing the small one. Label female body cell, male body cell, the smaller chromosome, egg, and the two kinds of sperm.
3.1 What Is Genetics
When a baby is born, the family almost always begins to compare. The eyes are the mother’s, someone says; the chin is the father’s. Sometimes a feature seems to have skipped a generation and come from a grandparent.
These comparisons are not idle. They point to something real: body features are passed from parents to their offspring, and they are passed on according to rules. The branch of biology that studies those rules is genetics.
Genetics deals with both halves of the family resemblance. It explains why children resemble their parents, and it explains, just as carefully, why they are never exact copies of them.
The Father Of Genetics
The rules were first worked out by Gregor Johann Mendel (1822–1884), an Austrian monk. He was born into a farming family, studied science and mathematics at the University of Vienna, and failed the examination for a teaching certificate — twice.
He returned to his monastery at Brno, where he was later made abbot, and where the garden gave him room to test his ideas on the garden pea. His results, published in 1866, are the reason he is called the Father of Genetics. You will meet his experiments in detail later in this chapter.
3.1.1 Heredity And The Rule Of Like Begets Like
The passing on of features from one generation to the next is called heredity. It is what makes a family a family, biologically speaking.
Heredity shows itself first as a very blunt rule, easy to state and never broken: like begets like. Every organism produces its own kind.
- Cats produce kittens, never puppies.
- A mango seed germinates into a mango tree, never into a guava.
- Human beings give birth to human beings, and to nothing else.
- The bacteria that turn milk into curd pass through hundreds of generations in a day, and every generation is the same kind of bacterium.
The rule holds at every level of life, from a bacterium too small to see to a tree that outlives the person who planted it. Whatever is being handed down must therefore be present in the cells of all of them.
And yet the rule is not a rule of copying. Kittens from one litter differ in coat and size; two mango trees grown from seeds of the same fruit will not bear identical fruit. Offspring resemble their parents closely, but never exactly — and that gap is the subject of the next section.
3.1.2 Characters And Their Traits
To study inheritance you must first be able to name what is being inherited. Biology does this in two steps, and the two steps are easy to confuse.
Any inheritable feature of an organism is called a character — eye colour, for example, or the shape of the ear lobe. The alternative forms in which that character appears are called its traits. Eye colour is the character; brown and blue are two of its traits.
We inherit thousands of characters from our two parents, who in turn received them from theirs. This is why brothers, sisters and cousins tend to resemble one another. Two such characters are drawn in Fig. 3.1-1, and a longer list is set out in Table 3.1.
Diagram
Fig. 3.1-1 — Two Human Characters And Their Contrasting Traits Four small line drawings in two labelled rows. Row 1, the character ‘ear lobe’: a side view of a human ear with the lobe hanging free, and beside it a side view with the lobe joined directly to the side of the head; label free ear lobe and attached ear lobe. Row 2, the character ‘hair on the middle joint of the fingers’: the back of a hand with fine hair drawn on the middle joint of the fingers, and beside it the same view with no hair there; label hair present and hair absent.
A word of caution about a table like this one. It splits each character neatly into two, because that makes the character easy to study, but a few of them are not really so tidy. Eye colour, skin colour and hand use are each influenced by several genes together, so they come in a range rather than in two clean alternatives.
3.2 Variations In A Population
Stand at the door of your classroom and look down the rows. Everyone in front of you belongs to the same species, Homo sapiens. A species is a group of organisms so alike that they can breed among themselves and produce fertile young.
Because they belong to one species, all of them share the features that make a human being human: the same body plan, the same number of bones, upright walking, the same set of organs doing the same work. Not one of these is in doubt from one student to the next.
And yet no two of them are alike. They differ in height, in the shape of the nose and the chin, in the colour and texture of hair, in the colour of the eyes, in blood group, in the sound of the voice. These small differences between individuals of the same species are called variations.
Variations show up most clearly when one character is measured across a whole group, as in Fig. 3.2-1. Take height in a class of students of the same age. There is no single ‘correct’ height; instead there is a shortest, a tallest, and a spread of individuals between them.
Diagram
Fig. 3.2-1 — Variation In A Single Character Line drawing of eight students of the same age standing in a row against a vertical scale marked in centimetres, arranged from shortest at the left to tallest at the right, with a dotted line carried across from the top of each head to the scale. Label the scale ‘height in centimetres’, and mark the shortest and the tallest of the eight.
The differences do not disappear as the group gets smaller. People living in different parts of the world differ in features such as skin colour, hair form and average height. Narrow it down to a single town and the differences are still there. Narrow it down to one family, where brothers and sisters have exactly the same two parents, and they are still there.
The same is true of every other species. There are dozens of recognised breeds of cattle in India — Sahiwal, Gir and Tharparkar among them — and each breed has its own build, horn shape and coat. But put twenty animals of a single breed together and you will still find differences in size, in markings and in milk yield among them.
So the rule of like begets like turns out to have a second half. Every organism produces its own kind, and every organism produces individuals that are slightly its own.
3.3 Chromosomes — The Carriers Of Heredity
If features travel from parents to children, something physical must carry them. And there is only one physical bridge between a mother, a father and their child: the sex cells that fuse at fertilisation. Whatever is inherited must be packed inside those two cells.
Inside each of them, in the nucleus, lie the chromosomes. You met them in the chapter on cell division, where each chromosome had to be copied and shared out before a cell could divide.
Chromosomes are not visible in a resting cell, even under a good microscope. For most of the cell’s life they are extremely long, thin and tangled together. Only when the nucleus is about to divide do they coil up tightly into short, thick rods that take up stain and can be seen and counted.
At that stage each chromosome has already been copied. It appears as two identical threads, called chromatids, lying side by side and held together at one point by a small constriction called the centromere. Fig. 3.3-1 shows this.
Diagram
Fig. 3.3-1 — A Chromosome As It Appears Before Cell Division Line drawing of a single condensed chromosome shown as two identical rod-like threads lying side by side and joined at a narrow constriction part-way along their length. Label the two chromatids and the centromere, with a bracket showing that both chromatids together make one chromosome.
3.3.1 The Karyotype
Because chromosomes show up only during division, a cell that is dividing is photographed through a high-power light microscope. The photograph is then cut up, and the chromosomes are sorted and arranged in order of their size and shape on a chart.
Such an arranged set is called a karyotype. It is not something that happens inside the body; it is a way of laying out the chromosomes of a cell so that they can be counted, compared and matched. The human karyotype is shown in Fig. 3.3-2.
Diagram
Fig. 3.3-2 — The Human Karyotype Line drawing of the chromosomes of a human body cell arranged as a karyotype: twenty-two numbered pairs laid out in rows in order of decreasing size, each pair drawn as two chromosomes of matching size and shape with the centromere at the same position in both, followed by the twenty-third pair shown twice — as two matching chromosomes labelled X and X, and as one large chromosome labelled X with a much smaller one labelled Y. Label the two versions of the twenty-third pair ‘female’ and ‘male’, and number the pairs 1 to 23.
3.3.2 Chromosome Number
Once chromosomes can be counted, a striking fact appears. The number is fixed for a species, and every body cell of every individual of that species carries the same number. A human being has 46 chromosomes in each body cell — in the skin, in the liver, in the bone marrow, everywhere.
Other species have their own fixed numbers, as Table 3.2 shows.
3.3.3 Homologous Chromosomes
The numbers in Table 3.2 are all even because chromosomes come in pairs. The 46 human chromosomes are really 23 pairs.
The two members of a pair are alike in length and shape, and their centromeres lie at the same position, so they can be picked out and set side by side on the karyotype. Such a matching pair is called a pair of homologous chromosomes.
Why one from each parent? Because a sex cell does not carry the full set. A sperm carries 23 chromosomes and an egg carries 23, and fertilisation brings the two half-sets together to make up the 46 of the new individual.
That single sentence explains the pairing. Every chromosome in your body cells has a partner of matching size and shape, and for each of those 23 pairs, one member came from your mother and the other from your father.
3.4 Autosomes And Sex Chromosomes
Go back to the human karyotype in Fig. 3.3-2 and look at it pair by pair. For the first twenty-two pairs there is nothing to report. Each pair is a matched pair, and the karyotype of a man and the karyotype of a woman look the same all the way down.
The twenty-third pair is where the two karyotypes part company. In a woman it is another matched pair, two chromosomes of the same size and shape, each called X. In a man it is a mismatched pair: one X, and beside it a chromosome that is much shorter, called Y.
This one difference splits the chromosome set into two categories.
The twenty-two matched pairs are the autosomes. They carry the genes for general body features — height, complexion, the shape of the ear lobe, and thousands more — and they are the same in both sexes.
The twenty-third pair are the sex chromosomes, also called allosomes. They are the pair that decides whether the individual develops as male or female. Every human being has exactly one such pair. Fig. 3.4-1 compares the two sex chromosomes.
Diagram
Fig. 3.4-1 — The Human X And Y Chromosomes Line drawing of the human X and Y chromosomes placed side by side and drawn to relative scale, the Y clearly much shorter than the X. Show each as two chromatids joined at a centromere, and label X chromosome, Y chromosome, chromatid and centromere.
So the full chromosome constitution of a human being can be written in two lines: a female is \(44 + XX\) and a male is \(44 + XY\). In both cases the 44 are autosomes, and in both cases they add up to 46 chromosomes in every body cell.
One line of that table needs a caution. Saying that the sex chromosomes decide sex does not mean that they do nothing else. The X chromosome is a large chromosome and carries a great many genes that have no connection with sex at all, including genes for colour vision and for the clotting of blood. Section 3.10 follows what happens to those genes.
3.5 Sex Determination In Human Beings
A woman’s body cells carry two X chromosomes. A man’s carry one X and one Y. Follow what that difference does when the sex cells are made, and the whole question of son or daughter is settled in a single step.
The sex cells — the sperm and the egg — are called gametes. They are made by meiosis, the special division that halves the chromosome number and sends the two members of every pair into different cells.
In a woman, the twenty-third pair is XX. Whichever member of that pair a particular egg receives, it receives an X. Every egg a woman produces therefore carries one X chromosome, and in this respect all eggs are alike.
In a man, the twenty-third pair is XY. When it separates, half the sperms receive the X and the other half receive the Y. Sperms are therefore of two kinds, produced in equal numbers, and this is the only place in the process where there is a choice to be made.
At fertilisation, one sperm out of millions reaches the egg first. Which kind it is, is a matter of chance. The two possible outcomes are traced in FC 3.1 and drawn in full in Fig. 3.5-1.
Flowchart
FC 3.1 — How The Sex Of A Child Is Decided Cell in the testis (XY) → meiosis → two kinds of sperm in equal numbers Branch 1: X-bearing sperm → fuses with the egg (X) → zygote XX → daughter Branch 2: Y-bearing sperm → fuses with the egg (X) → zygote XY → son (Cell in the ovary (XX) → meiosis → every egg carries X, so the egg contributes the same chromosome in both branches)
Diagram
Fig. 3.5-1 — Sex Determination In Human Beings Line diagram in two columns joined at the bottom. Left column: a cell in the testis labelled XY, an arrow marked meiosis leading to two sperms drawn side by side, one labelled X and one labelled Y. Right column: a cell in the ovary labelled XX, an arrow marked meiosis leading to a single egg labelled X. Draw two crossing arrows from the sperms to the egg, leading to two zygotes at the bottom: one labelled XX and ‘daughter’, the other labelled XY and ‘son’. Mark each of the two crossing arrows ‘50 per cent chance’.
Because the two kinds of sperm are equally numerous, the chance of a daughter and the chance of a son at any one fertilisation are equal, a ratio of \(1:1\). The mother supplies an X in every single case. The sex of the child is therefore settled by the sperm, and by nothing else.
3.6 Chromosomes As Carriers Of Genes
Every species has its own fixed number of chromosomes, and Table 3.2 gave you a column of those numbers. But look again at three rows of it. The lion, the tiger and the domestic cat all have 38 chromosomes, and no one would confuse one of them with another.
So the number of packets cannot be what makes a lion a lion. The difference must lie in what is written inside them. Chromosomes do not carry characters directly; they carry the units that control characters, and those units are called genes.
A gene is a short length of the long DNA molecule that runs through a chromosome, and DNA is the substance in which the instructions of inheritance are written. Each chromosome carries thousands of genes, arranged along its length in a fixed order, rather like beads strung on a thread. Fig. 3.6-1 shows the arrangement.
Diagram
Fig. 3.6-1 — Genes Along A Chromosome Line drawing of a single chromosome with a series of short segments marked off along one chromatid and labelled gene A, gene B and gene C. From one of these segments draw a magnified view of a short length of the DNA molecule as a twisted two-stranded ladder. Label chromosome, DNA and gene, and add an arrow showing that a gene is one length of the DNA of that chromosome.
The word itself is older than the knowledge behind it. It was coined in 1909 by the Danish biologist Wilhelm Johannsen, simply as a name for whatever it was that parents passed on to their offspring. What a gene was actually made of was worked out only decades later.
One consequence follows immediately from the last section, and the rest of this chapter rests on it. Chromosomes occur in homologous pairs, and the same genes lie at the same positions on both members of a pair. So genes, too, come in twos — one from the mother and one from the father, for every character.
3.6.1 Genes And The Genome
A human being has roughly twenty thousand of these protein-coding genes altogether, spread over the 23 pairs of chromosomes. They are not shared out evenly. Chromosome 1, the largest, carries the greatest number of them, while the small Y chromosome carries the fewest of all.
Genes, however, are not the whole of the DNA. Long stretches lie between one gene and the next, and all of it — the genes and the stretches in between — is inherited together. The complete set of DNA of an organism is called its genome.
3.7 Genes And Their Alleles
Genes come in twos, because chromosomes come in pairs. The gene for a given character sits at a particular point on one chromosome, and the same gene sits at the matching point on its homologous partner.
But the two need not be identical copies. A gene usually exists in two alternative forms, which affect the same character in different ways. These alternative forms are called alleles.
Take the character of tongue rolling — the ability to fold the tongue into a tube when you put it out of your mouth. There are only two possibilities: either a person can do it or cannot. So the gene for tongue rolling exists in two forms, one for rolling and one for non-rolling, and these two are the alleles of that gene.
The fixed position that a gene occupies on a chromosome is called its locus. Both alleles of a gene occupy the same locus, one on the chromosome received from the mother and one on the chromosome received from the father, as Fig. 3.7-1 shows.
Diagram
Fig. 3.7-1 — Alleles At Their Locus On A Homologous Pair Line drawing of two homologous chromosomes side by side, one labelled ‘chromosome from the mother’ and the other ‘chromosome from the father’. Mark three loci at the same levels on both chromosomes, joined across by dotted guide lines. At the topmost locus write R on one chromosome and r on the other; label the two lower loci as the loci of two other genes. Label locus, and mark the pair of letters at the top locus as ‘the two alleles of the tongue-rolling gene’.
An individual, then, carries two alleles for every character. They may be two of the same kind, or one of each kind. What happens when they are of different kinds is the question the next subsection settles.
3.7.1 Dominant And Recessive Alleles
Try it in your own classroom. Ask everyone to put out the tongue and fold it into a tube. Most of the class will manage it easily; a few will not, however hard they try.
Now notice what does not happen. There is no third group whose tongues fold halfway. When a person carries one allele of each kind, the two do not blend into an intermediate result — one of them shows itself completely and the other does not appear at all.
The allele that shows itself whenever it is present is called the dominant allele. The one that stays hidden in its presence is called the recessive allele. A recessive allele can express itself only when the other allele is recessive too.
Genetics writes this down with a simple lettering rule. The dominant allele is given a capital letter, and the recessive allele is given the same letter in small form. For tongue rolling, rolling is dominant, so the alleles are written R for rolling and r for non-rolling.
One caution before we go further. Dominance belongs to an allele, never to a chromosome. It makes no sense to call a whole chromosome dominant or recessive, because the same chromosome carries thousands of genes, some of whose alleles are dominant and some recessive.
3.8 Genotype And Phenotype
Two students in your class roll their tongues equally well. From the outside they are identical in this character. Inside their cells, they may not be.
Since each person carries two alleles for the character, and each allele is either R or r, there are exactly three combinations possible for tongue rolling.
- RR — both alleles dominant. The person is a roller.
- Rr — one dominant allele and one recessive. The dominant allele shows, so the person is again a roller.
- rr — both alleles recessive. There is no dominant allele to hide them, so the person is a non-roller.
Read that list twice, because the important point is not in any one line but in the pattern across them. Rollers have two possible constitutions, RR and Rr. Non-rollers have only one, rr.
Biology needs two different words to keep these apart. The combination of alleles an individual carries is its genotype. What that individual actually shows is its phenotype.
The two genotypes of a roller have names of their own, taken from Greek. A pair of like alleles is called homozygous, and a pair of unlike alleles is called heterozygous.
So RR is homozygous dominant, Rr is heterozygous, and rr is homozygous recessive. All three are set out in Table 3.4 and drawn in Fig. 3.8-1.
Diagram
Fig. 3.8-1 — The Three Genotypes For Tongue Rolling Three panels in a row. In each panel draw one pair of homologous chromosomes side by side with the tongue-rolling locus marked on both. Panel 1: R on both chromosomes, labelled RR. Panel 2: R on one and r on the other, labelled Rr. Panel 3: r on both, labelled rr. Below each panel draw a simple front view of a mouth with the tongue put out — rolled into a tube under panels 1 and 2, and lying flat under panel 3. Label the top row ‘genotype’ and the bottom row ‘phenotype’.
That answer works in one direction only, and the asymmetry is worth holding on to. An individual showing the recessive character has a genotype you can write down at once. An individual showing the dominant character may be RR or Rr, and looking at the person will never tell you which. To find out, you have to look at the family — which is exactly what the next section does.
3.9 From Parents To Children — Tongue Rolling As An Example
A genotype cannot be seen. But it can very often be worked out, and there are two ways of doing it. You can look backwards at a family that already exists and deduce what the parents must be carrying. Or you can look forwards from parents whose genotypes are known and predict what their children may be.
Biology has a standard tool for each. The first is the pedigree chart, the second the Punnett square, and this section takes them in that order.
3.9.1 Reading A Pedigree Chart
A pedigree chart is a family tree drawn for one character. It uses a small set of symbols, and once you know them the chart reads itself.
- A circle stands for a female, a square for a male.
- A solid symbol means the character is present in that person; a hollow symbol means it is absent.
- A horizontal line joins the two parents, and a vertical line drops from it to the row of their children.
Fig. 3.9-1 is such a chart, drawn for tongue rolling in one family.
Diagram
Fig. 3.9-1 — A Pedigree Chart For Tongue Rolling Line drawing of a two-generation pedigree. Top row: a solid circle joined by a short horizontal line to a solid square, both labelled ‘roller’. A vertical line drops from the middle of that line to a horizontal sibling line. From the sibling line drop three short vertical lines to the second row: a solid square labelled ‘roller’, a hollow circle labelled ‘non-roller’, and a solid circle labelled ‘roller’. Add a key at the side: circle = female, square = male, solid symbol = character present, hollow symbol = character absent.
Look at what the chart states plainly. Both parents can roll their tongues. Of their three children, two can and one cannot.
That one hollow symbol is enough to fix the genotypes of everybody in the top row, as long as you reason in the right order.
Notice how much the hidden allele explains. The non-rolling allele had been carried, unseen and unsuspected, by both parents for their whole lives. It became visible only when two copies of it came together in one child.
3.9.2 The Punnett Square
Deduction works backwards. To work forwards — from known parents to their possible children — genetics uses a small grid called the Punnett square.
Everything in it follows from one fact you have already met. When gametes are formed, the two members of every pair of alleles separate, so a gamete carries only one allele of the pair, never both. A parent who is Rr therefore makes two kinds of gametes, half carrying R and half carrying r. A parent who is RR makes only one kind, all carrying R.
Take a couple who are both tongue rollers but are not alike underneath: the father RR and the mother Rr. Their square is drawn in Fig. 3.9-2.
Diagram
Fig. 3.9-2 — Punnett Square For The Cross RR × Rr Draw a four-celled square grid. Along the top, outside the grid, write the two gametes of the father, both labelled R, one above each column, and label that side ‘gametes of the father (RR)’. Along the left, outside the grid, write the gametes of the mother, R against the upper row and r against the lower row, and label that side ‘gametes of the mother (Rr)’. Fill the four inner cells, reading across: RR and RR in the upper row, Rr and Rr in the lower row. Write ‘tongue roller’ below the genotype in every cell, and print under the grid: all children rollers — half RR, half Rr.
Read the four cells. Two of them are RR and two are Rr, so the two genotypes are expected in the ratio \(1:1\). But every one of the four carries at least one R, so every child is a tongue roller. A homozygous dominant parent cannot have a child showing the recessive character, whatever the other parent carries.
Compare that with the family in Fig. 3.9-1, where both parents were Rr. There, a non-roller child was possible, and one appeared. The parents look the same in both families; what differs is the genotype underneath, and that is what changes the range of possibilities in the children.
3.9.3 Dominant And Recessive Traits In Human Beings
Tongue rolling is not a special case. A good many human characters come in two contrasting forms, one of which behaves as dominant and the other as recessive. Some of them are listed in Table 3.5.
Two entries in that table — eye colour and hand use — were flagged in section 3.1 as being controlled by more than one gene. They are listed here in the simplified form in which such tables are always drawn.
A few less common characters follow the same rules. Polydactyly, in which a person has extra fingers or toes, is due to a dominant allele. Camptodactyly, in which the little finger cannot be straightened fully, is dominant as well. Albinism, in which the skin, hair and eyes lack pigment altogether, is due to a recessive allele — which is why a child with albinism may be born to two parents who have normal pigment.
That last line is worth pausing on. It is the tongue-rolling family again, in a different character: a recessive allele can be carried quietly through generations and shows itself only when two copies meet.
3.10 Sex-Linked Inheritance
Almost everything in this chapter so far has concerned genes on the autosomes. For those genes, a boy and a girl are in exactly the same position: each of them has two copies, one from each parent.
The twenty-third pair breaks that symmetry. The X chromosome is large and carries a great many genes; the Y is short and carries very few. So for most genes that lie on the X, a male has only one copy and no partner allele at all. A female has two.
That single structural fact produces a pattern of inheritance of its own, and it is called sex-linked inheritance.
Two well-known human conditions are inherited in this way, and both are due to recessive alleles on the X chromosome. Because the allele is recessive, we need a way of writing it: \(X^{c}\) will stand for an X chromosome carrying the recessive allele for colour blindness, and a plain \(X\) for one carrying the normal allele.
3.10.1 Colour Blindness And Haemophilia
A person with red-green colour blindness cannot tell certain colours apart — most often red from green. Nothing is wrong with the eye as a whole; the pigments in some of the cone cells of the retina do not work as they should, so the two colours look alike.
A person with haemophilia has blood that clots very slowly, because one of the proteins needed for clotting is missing or faulty. An injury that would stop bleeding in minutes goes on bleeding for far longer, and bleeding can also occur inside joints. The condition is managed today by giving the missing clotting factor.
The allele responsible in each case is recessive, and in each case it sits on the X chromosome. Now follow what that means for the two sexes.
A female has two X chromosomes. For the character to appear in her, both of her X chromosomes must carry the recessive allele. If only one does, the normal allele on the other X does the work, her vision or her clotting is normal, and she is called a carrier — she does not show the character but can pass the allele on.
A male has one X and one Y. The Y does not carry a matching allele, so there is nothing to mask a recessive allele on his single X. One copy is enough for the condition to appear.
Count the rows. A male needs one copy of the allele to be affected; a female needs two. That is why far more boys than girls are colour blind, and why haemophilia in girls is very rare indeed.
3.10.2 Inheritance Of Colour Blindness — Three Cases
Everything about sex-linked inheritance follows from one rule of gamete formation: a father passes his X to his daughters and his Y to his sons, while a mother passes one of her two X chromosomes to every child. Three crosses are enough to show what that produces.
Case 1 — Normal Mother, Colour-Blind Father
The mother is \(XX\) and the father \(X^{c}Y\). His gametes are of two kinds, \(X^{c}\) and \(Y\); all her eggs carry a plain \(X\).
Diagram
Fig. 3.10-1 — Punnett Square: Normal Mother × Colour-Blind Father Draw a four-celled grid. Along the top write the father’s two kinds of sperm, X with a superscript c above the first column and Y above the second; label that side ‘sperms of the father, genotype X-superscript-c Y’. Along the left write the mother’s eggs, X against both rows; label that side ‘eggs of the mother, genotype XX’. Fill the cells, reading across: upper row X-superscript-c X and XY; lower row X-superscript-c X and XY. Under each genotype write its phenotype: ‘daughter, carrier, normal vision’ for the first column and ‘son, normal vision’ for the second.
Not one child is colour blind. Every daughter has received her father’s \(X^{c}\), so every daughter is a carrier. Every son has received his father’s Y, and his single X came from his mother, so his colour vision is normal. A father cannot pass an X-linked allele to his son at all.
Case 2 — Carrier Mother, Normal Father
The mother is \(X^{c}X\) and the father \(XY\). Now it is the mother who makes two kinds of gametes.
Diagram
Fig. 3.10-2 — Punnett Square: Carrier Mother × Normal Father Draw a four-celled grid. Along the top write the father’s two kinds of sperm, X above the first column and Y above the second; label that side ‘sperms of the father, genotype XY’. Along the left write the mother’s two kinds of eggs, X with a superscript c against the upper row and plain X against the lower row; label that side ‘eggs of the mother, genotype X-superscript-c X’. Fill the cells, reading across: upper row X-superscript-c X and X-superscript-c Y; lower row XX and XY. Under each genotype write its phenotype: ‘daughter, carrier’, ‘son, colour blind’, ‘daughter, normal vision’, ‘son, normal vision’.
Here a colour-blind child appears, and it is a son. Half the sons are expected to be colour blind and half to have normal vision. No daughter is colour blind, because each of them receives a normal X from the father, but half of the daughters are carriers.
Case 3 — Carrier Mother, Colour-Blind Father
This is the only one of the three crosses in which a colour-blind daughter is possible. Both parents make two kinds of gametes.
Diagram
Fig. 3.10-3 — Punnett Square: Carrier Mother × Colour-Blind Father Draw a four-celled grid. Along the top write the father’s two kinds of sperm, X with a superscript c above the first column and Y above the second; label that side ‘sperms of the father, genotype X-superscript-c Y’. Along the left write the mother’s two kinds of eggs, X with a superscript c against the upper row and plain X against the lower row; label that side ‘eggs of the mother, genotype X-superscript-c X’. Fill the cells, reading across: upper row X-superscript-c X-superscript-c and X-superscript-c Y; lower row X-superscript-c X and XY. Under each genotype write its phenotype: ‘daughter, colour blind’, ‘son, colour blind’, ‘daughter, carrier’, ‘son, normal vision’.
Half the daughters are expected to be colour blind and half to be carriers; half the sons colour blind and half with normal vision. A daughter is affected only when she receives a defective allele from both parents at once — which is exactly why affected females are so much rarer than affected males.
3.10.3 Criss-Cross Inheritance
Put the three cases side by side and a zig-zag pattern comes out of them.
A father gives his only X to every one of his daughters and to none of his sons. A mother gives an X to every child, so a son’s single X can only have come from his mother. An X-linked allele therefore travels from father to daughter, and from mother to son, generation after generation. This is called criss-cross inheritance.
It also explains why such a condition often seems to skip a generation, as FC 3.2 shows.
Flowchart
FC 3.2 — Path Of An X-Linked Allele Through Three Generations Colour-blind grandfather (\(X^{c}Y\)) → passes his \(X^{c}\) to every daughter → daughters are carriers with normal vision (the allele is now hidden, and the generation appears unaffected) → each carrier daughter passes \(X^{c}\) to half her children → a grandson who receives it is colour blind (\(X^{c}Y\)), while a granddaughter who receives it becomes a carrier in her turn
3.11 Mendel’s Experiments On Inheritance
Everything in the last four sections — dominant and recessive alleles, genotypes, the \(3:1\) kind of reasoning behind a pedigree — had to be discovered before it could be taught. It was discovered in a monastery garden, by a man who did something no naturalist before him had bothered to do. He counted.
Gregor Mendel began his breeding experiments in the middle of the nineteenth century, at Brno, and worked at them for about eight years. He chose one plant, the garden pea, Pisum sativum, crossed it in a controlled way, and then counted the offspring of every cross, class by class, in thousands. The numbers were what gave the answer.
3.11.1 Why Mendel Chose The Garden Pea
The choice of plant did as much work as the counting. Mendel picked the garden pea for four reasons, and each of them removes a difficulty that would otherwise have wrecked the experiment.
- Many varieties in clearly alternative forms. Pea varieties differed in features that were either one thing or the other — a flower was purple or it was white — with nothing in between to argue about.
- Varieties that bred true. A pure-breeding variety is one that produces the same type generation after generation. Tall plants gave only tall offspring, year after year. Mendel could therefore start from a known, unmixed stock.
- Self-pollination that could be prevented. In a pea flower the stamens and the carpel are enclosed by the petals, so the flower normally pollinates itself. Mendel could leave it to do so when he wanted a self-pollinated generation, or he could open the bud, cut away the immature stamens, and dust pollen from another plant on to the stigma himself. In other words, he controlled who the parents were.
- A short life span. A pea plant completes its life cycle in a season, so several generations could be raised and studied in a few years.
3.11.2 The Seven Pairs Of Contrasting Features
Out of everything the pea plant offers, Mendel worked with seven characters. Each of them appeared in exactly two contrasting forms, which is what made counting possible. They are drawn in Fig. 3.11-1 and listed with their dominant and recessive forms in Table 3.7.
Diagram
Fig. 3.11-1 — The Seven Pairs Of Contrasting Features Studied By Mendel Seven labelled rows, each showing two small line drawings side by side. Row 1, flower colour: a purple pea flower and a white one. Row 2, seed colour: a yellow seed and a green seed. Row 3, seed shape: a smooth round seed and a wrinkled seed. Row 4, pod colour: a green pod and a yellow pod. Row 5, pod shape: a full inflated pod and a pod constricted between the seeds. Row 6, flower position: a stem bearing flowers in the axils of the leaves and a stem bearing flowers at its tip. Row 7, plant height: a tall pea plant and a dwarf one, drawn to the same scale. Label the character at the left of each row and each of the two forms beneath its drawing.
Mendel did not take these seven together. He crossed pure-breeding varieties first for one character at a time, and later for two characters at a time. A cross that follows a single pair of contrasting characters is called a monohybrid cross; one that follows two pairs at once is a dihybrid cross.
3.11.3 The Monohybrid Cross
Take Mendel’s cross for flower position. He crossed a pure-breeding plant bearing axillary flowers with a pure-breeding plant bearing terminal flowers, dusting pollen from one on to the stigma of the other by hand.
The offspring of that cross make up the first filial generation, written \(F_1\). The word filial simply means ‘of the children’. When the \(F_1\) plants are allowed to self-pollinate, their offspring form the second filial generation, \(F_2\).
The \(F_1\) result was startling. Every single plant bore axillary flowers. The terminal form had disappeared completely — not diluted, not intermediate, simply absent.
Mendel then let the \(F_1\) plants self-pollinate. In the \(F_2\) generation, terminal flowers came back, in a definite proportion: three plants with axillary flowers to every one with terminal flowers.
Write the letters in and the whole thing becomes readable. Let A stand for the dominant allele for axillary flowers and a for the recessive allele for terminal flowers. The pure axillary parent is AA, the pure terminal parent is aa, and every \(F_1\) plant, having received one allele from each, is Aa — axillary in appearance, but carrying the hidden recessive allele. Fig. 3.11-2 crosses two such \(F_1\) plants.
Diagram
Fig. 3.11-2 — The Monohybrid Cross Through Two Generations Upper part: at the left a plant labelled ‘pure axillary, AA’ and at the right a plant labelled ‘pure terminal, aa’, joined by a cross sign. Arrows from each to a row of gametes, A from the left parent and a from the right. These meet at a single plant labelled ‘F1 — all axillary, Aa’, with a note ‘the terminal form has disappeared’. Lower part: the F1 plant self-pollinated, shown by a curved arrow returning to itself, its gametes A and a written out. Below it draw a four-celled Punnett square with A and a along the top and A and a down the side, the cells filled AA, Aa, Aa, aa. Beneath the square write the phenotypes: three axillary to one terminal, and mark the aa cell ‘terminal form reappears’.
Mendel repeated the experiment with the other characters and got the same pattern every time. Crossing pure tall (TT) with pure dwarf (tt) gave an \(F_1\) of tall plants only, all Tt, and an \(F_2\) of three tall to one dwarf, as Fig. 3.11-3 sets out. Crossing plants grown from pure round seeds (RR) with plants grown from pure wrinkled seeds (rr) gave an \(F_1\) of round seeds only and an \(F_2\) of three round to one wrinkled.
Diagram
Fig. 3.11-3 — Mendel’s Tall × Dwarf Cross Three tiers connected by arrows. Top tier: a tall pea plant labelled ‘TT, pure tall’ and a dwarf pea plant labelled ‘tt, pure dwarf’, with a cross sign between them and a curved arrow marked ‘cross-pollination by hand’. Middle tier: four tall plants of equal height labelled ‘F1 — all tall, Tt’, with a curved self-pollination arrow. Bottom tier: four plants labelled TT, Tt, Tt and tt, the first three drawn tall and the fourth drawn dwarf, with a bracket beneath reading ‘3 tall : 1 dwarf’.
3.11.4 The Monohybrid Ratio
Read the four cells of the square in Fig. 3.11-2 in two different ways, and you get the two ratios that a monohybrid cross always produces.
Read them by appearance. Three of the four show the dominant character and one shows the recessive. The phenotypic ratio in \(F_2\) is \(3:1\).
Now read them by genotype. One cell is AA, two are Aa, and one is aa. The genotypic ratio in \(F_2\) is \(1:2:1\).
The two rows of the \(F_2\) that look identical are not identical. Of the three axillary plants, one is pure-breeding and two carry a hidden recessive allele — which is why, when Mendel grew a further generation, one of the three axillary plants gave only axillary offspring while the other two again gave three axillary to one terminal.
From results of this kind, Mendel drew five general conclusions. They are stated below in modern words, and every one of them has been used somewhere in this chapter already.
- Each pair of contrasting characters depends on a pair of genes.
- Each individual carries such genes in duplicate, one member of the pair from each parent.
- An individual produces gametes that carry only one member of each pair of genes.
- Fertilisation of the gametes restores the duplicate condition in the offspring.
- Sex cells fuse at random, so far as the genes they carry are concerned.
3.11.5 The Dihybrid Cross And Its Ratio
One character at a time answers only half the question. What happens when two characters are followed together — does the allele a gamete carries for seed shape have any bearing on the allele it carries for seed colour?
Mendel crossed a pure-breeding variety having round, yellow seeds with a pure-breeding variety having wrinkled, green seeds. Round is dominant to wrinkled and yellow is dominant to green, so the parents are RRYY and rryy.
Every \(F_1\) plant received RY from one parent and ry from the other, making it RrYy, and every \(F_1\) seed was round and yellow, as expected.
The \(F_1\) plants were then self-pollinated, and here the second character earns its place. An RrYy plant makes four kinds of gametes, not two: RY, Ry, rY and ry. The two pairs separate independently of each other, so the allele for shape that a gamete receives does not decide which allele for colour it receives. Four kinds of gametes on each side give the sixteen-celled square of Fig. 3.11-4.
Diagram
Fig. 3.11-4 — Punnett Square For The Dihybrid Cross RrYy × RrYy Draw a sixteen-celled square grid, four columns by four rows. Along the top write the four kinds of male gametes RY, Ry, rY and ry, one above each column; along the left write the same four kinds of female gametes, one against each row. Fill each of the sixteen cells with the genotype formed by combining the gamete of its row and its column. Shade the cells in four tones according to phenotype and add a key: round and yellow (9 cells), round and green (3 cells), wrinkled and yellow (3 cells), wrinkled and green (1 cell). Print the ratio 9 : 3 : 3 : 1 below the grid.
Count the cells by appearance and the \(F_2\) falls into four classes: nine round and yellow, three round and green, three wrinkled and yellow, one wrinkled and green. The phenotypic ratio of a dihybrid cross is \(9:3:3:1\).
Two of those four classes deserve a second look. Round-and-green and wrinkled-and-yellow are combinations that neither parent had. The two characters have been shuffled and dealt out afresh, which is only possible if they are inherited independently of one another.
The genotypic ratio of a dihybrid cross also exists, but with nine different genotypes among the sixteen cells it is complicated, and it is not required at this level. Table 3.8 collects the ratios you do need.
3.12 Mendel’s Laws Of Inheritance
Mendel’s conclusions were drawn from counted results, not from theory. When those conclusions are stated in their most general form, they come to three laws, and every one of them can be traced back to something in the last section.
The Law Of Dominance
The evidence is the F1. Whenever a pure tall plant was crossed with a pure dwarf one, every plant of the F1 was tall, and dwarfness could not be found anywhere in that generation. Yet it had not been destroyed, because it came back in the F2.
The Law Of Segregation
The evidence is the F2. If the two alleles carried by an F1 plant had mixed or diluted each other, the F2 would have shown a blend, and each generation after it would have been more blended still. Instead the F2 threw up clean dwarf plants that bred true. Whatever the F1 carried, it had kept the two alleles separate and intact.
That is possible only if each gamete receives one allele and one alone. This is why the law has a second name — the law of purity of gametes. A gamete is never a mixture; it is pure for the character concerned.
The Law Of Independent Assortment
The evidence is the dihybrid cross. Round green and wrinkled yellow seeds appeared in the F2 although no parent and no F1 plant had ever shown those combinations. Seed shape and seed colour had clearly not travelled as a package.
3.12.1 Applications Of Mendel’s Laws
Mendel’s laws are not merely an explanation of what happened in one garden. They turned breeding from guesswork into something that can be planned.
First, they allow prediction. Knowing the genotypes of two parents, a breeder can say which combinations will appear among their offspring and in what proportion — the whole point of a Punnett square.
Second, that prediction is of direct use to plant and animal breeders. Desirable characters can be brought together deliberately, and the number of offspring likely to carry them can be worked out in advance instead of being waited for.
Third, hybridisation can produce genuinely new combinations. A variety that yields well but falls to a particular disease can be crossed with a variety that resists that disease, and among the progeny will be plants carrying both useful characters together — a combination that existed in neither parent, exactly as round green seeds did in the F2.
3.13 Mutation
Everything in this chapter so far has been a reshuffling. Alleles are dealt out into gametes, brought together again at fertilisation, and hidden or shown — but the alleles themselves have been passed on unchanged, generation after generation.
Occasionally that is not what happens. The hereditary material itself changes, and something appears that was in neither parent and in no ancestor. Such a sudden change is called a mutation.
The definition has three parts because a change can happen at three different scales. A single gene may be altered, so that one instruction now reads differently. The number of chromosomes may change, so that a cell carries one too many or one too few. Or the structure of a chromosome may change, with a piece lost, doubled or turned around.
Whichever it is, the hereditary material of the cell is no longer what it was, and the character controlled by it may be altered.
Where the change happens decides how far it travels. A mutation in an ordinary body cell affects only that individual and goes no further. A mutation in a cell of the reproductive organs, one that gives rise to gametes, can be passed on to the next generation and to every generation after it.
Diagram
Fig. 3.13-1 — Normal And Sickle-Shaped Red Blood Cells Line drawing of a short length of a blood capillary shown in section. In the upper half draw several normal red blood cells as rounded discs dipped in at the centre, flowing freely along the vessel. In the lower half draw several stiff crescent-shaped cells, some of them jammed against one another where the vessel narrows. Label normal red blood cell, sickle-shaped red blood cell and capillary.
Many mutations arise on their own, as rare copying mistakes when DNA is duplicated before a cell divides. Others are caused from outside, by certain chemicals, by ultraviolet light, and above all by radiation — X-rays and the radiations given off by radioactive substances. That radiation can do this was not guessed; it was shown by experiment.
Most mutations are either harmful or make no difference at all. A few are useful. But mutation matters out of all proportion to how often it happens, because it is the only process that makes a genuinely new allele. Every allele that Mendel shuffled in his garden began, at some point in the past, as a mutation.
3.14 Summary
- Genetics is the branch of biology that studies heredity and the laws that govern it.
- Heredity is the transmission of genetically based characteristics from parents to offspring.
- Like begets like: every organism produces its own kind. Yet offspring are never exact copies of their parents.
- A character is any inheritable feature of an organism; the alternative forms of a character are its traits.
- Variations are the small differences found between individuals of the same species, and they occur in every population and within every family.
- Chromosomes, in the nucleus, are the carriers of heredity. They are visible only when the nucleus is about to divide, when each appears as two chromatids joined at a centromere.
- A karyotype is the chromosomes of a cell arranged in order of their size and shape.
- The chromosome number is constant for a species, and every body cell of an individual carries it. A human being has 46 chromosomes.
- The chromosome number does not measure how complex an organism is, and different species may share the same number.
- Chromosomes occur in homologous pairs, matched in size and shape, with one member of each pair received from each parent. The 46 human chromosomes form 23 pairs.
- Twenty-two pairs are autosomes, which carry genes for general body features. The twenty-third pair are the sex chromosomes: XX in a female and XY in a male, the Y being much shorter than the X.
- Every egg carries an X chromosome, while sperms are of two kinds in equal numbers, X-bearing and Y-bearing.
- The sex of a child is therefore decided by the kind of sperm that fertilises the egg, and the chance of a son and of a daughter at each fertilisation is \(1:1\).
- Genes are segments of the DNA of a chromosome that determine hereditary characters. The differences between species come from the genes carried, not from the number of chromosomes.
- The genome is the full complement of DNA of an organism, including its genes and the regions between them.
- Alleles are the alternative forms of a gene, occupying the same locus on the two chromosomes of a homologous pair.
- A dominant allele expresses itself whenever it is present; a recessive allele expresses itself only when both alleles are recessive. The dominant allele is written as a capital letter and the recessive as the same letter in small form.
- The genotype is the set of alleles an organism carries; the phenotype is the character it shows.
- A pair of like alleles is homozygous and a pair of unlike alleles is heterozygous. One phenotype may arise from two genotypes — RR and Rr look alike — but the recessive phenotype has only one possible genotype, rr.
- A pedigree chart traces a character through a family using squares for males, circles for females, and solid or hollow symbols for the presence or absence of the character. Two parents showing a dominant character who have a child showing the recessive character must both be heterozygous.
- A Punnett square sets the gametes of one parent along one side and those of the other along the adjacent side, so that every box shows one possible genotype of the offspring.
- Sex-linked inheritance is the appearance of a character due to an allele carried exclusively on the X or the Y chromosome.
- Red-green colour blindness and haemophilia are caused by recessive alleles on the X chromosome. They appear far more often in males, who have a single X and so no second allele to mask them.
- A female carrying such an allele on one X only is a carrier: she does not show the condition but can pass the allele on.
- X-linked alleles show criss-cross inheritance. A father passes his X to every daughter and to no son, so a son’s X-linked allele always comes from his mother.
- Mendel chose the garden pea (Pisum sativum) because it had many pure-breeding contrasting varieties, could be self-pollinated or cross-pollinated at will, and grew quickly. He studied seven pairs of contrasting features.
- In a monohybrid cross, which follows one character, the F1 is uniform and shows the dominant character, and the F2 gives a phenotypic ratio of \(3:1\) and a genotypic ratio of \(1:2:1\).
- In a dihybrid cross, which follows two characters together, the F2 gives a phenotypic ratio of \(9:3:3:1\), and includes combinations present in neither parent.
- Mendel’s law of dominance: of a pair of contrasting characters present together, only one expresses itself, and the recessive shows only in the homozygous recessive condition.
- Mendel’s law of segregation, or law of purity of gametes: the two members of a pair of factors separate during gamete formation, so each gamete carries only one of them, and they combine again by random fusion at fertilisation.
- Mendel’s law of independent assortment: when two pairs of characters are considered together, the alleles of one character are distributed into the gametes independently of the alleles of the other.
- These laws let breeders predict which combinations will appear among offspring and in what proportion, and let new combinations of useful characters be produced by hybridisation.
- A mutation is a sudden change in one or more genes, or in the number or structure of chromosomes.
- A mutation is inherited only if it occurs in a cell that gives rise to gametes. Mutations arise as copying errors and are caused by agents such as radiation, and they are the only source of genuinely new alleles.
Flowchart
FC 3.3 — From Chromosome To Character, And Back To The Next Generation Homologous pair of chromosomes, one from each parent → a gene at its fixed locus on both → two alleles of that gene, one on each chromosome → genotype (RR, Rr or rr) → dominance decides which allele expresses → phenotype, the character seen → at gamete formation the two alleles segregate, one to each gamete → random fusion at fertilisation → a new genotype in the offspring, and the chain begins again in the next generation
3.15 Key Terms
- Allele — one of the alternative forms of a gene, affecting the same character in a different way.
- Autosomes — the chromosomes other than the sex chromosomes; the 22 pairs that carry genes for general body features.
- Carrier — an individual who carries a recessive allele without showing the character it controls, and can pass it to the offspring.
- Centromere — the narrow constriction at which the two chromatids of a chromosome are joined.
- Character — any inheritable feature of an organism, such as eye colour or ear lobe shape.
- Chromatid — one of the two identical threads of a chromosome that has already been copied, before the cell divides.
- Chromosome — a thread-like structure in the nucleus that carries the genes and so carries heredity.
- Criss-cross inheritance — the pattern in which an X-linked allele passes from a mother to her son and from a father to his daughter.
- Dihybrid cross — a cross in which two pairs of contrasting characters are followed at the same time.
- Dominant allele — the allele that expresses itself whenever it is present, whether the other allele is like it or not.
- Gamete — a sex cell, the sperm or the egg, carrying only one member of each pair of genes.
- Gene — a segment of the DNA of a chromosome that determines a particular hereditary character.
- Genetics — the branch of biology that studies heredity and the laws that govern it.
- Genome — the full complement of DNA of an organism, including all its genes and the regions between them.
- Genotype — the set of alleles an organism carries for a character, such as RR, Rr or rr.
- Genotypic ratio — the proportion in which the different genotypes appear among the offspring of a cross.
- Heredity — the transmission of genetically based characteristics from parents to offspring.
- Heterozygous — carrying two unlike alleles of a gene, as in Rr.
- Homologous chromosomes — a pair of chromosomes of the same size and shape, one received from each parent.
- Homozygous — carrying two like alleles of a gene, as in RR or rr.
- Karyotype — the chromosomes of a cell arranged in order of their size and shape for study.
- Locus — the fixed position that a particular gene occupies on a chromosome.
- Monohybrid cross — a cross in which a single pair of contrasting characters is followed.
- Mutation — a sudden change in one or more genes, or in the number or structure of the chromosomes.
- Pedigree chart — a family chart drawn with standard symbols to trace one character through the generations.
- Phenotype — the character an organism actually shows, as against the alleles it carries.
- Phenotypic ratio — the proportion in which the different visible forms of a character appear among the offspring of a cross.
- Population — all the individuals of one species living together in an area.
- Punnett square — a grid in which the gametes of the two parents are combined so that every possible genotype of the offspring is shown.
- Pure-breeding — describing a variety that produces the same type generation after generation.
- Recessive allele — the allele whose expression is suppressed by a dominant allele, and which shows only when both alleles are recessive.
- Sex chromosomes — the pair of chromosomes that decides the sex of the individual, XX in a human female and XY in a human male.
- Sex-linked inheritance — the inheritance of a character controlled by an allele carried exclusively on the X or the Y chromosome.
- Trait — one of the alternative forms in which a character appears, such as brown or blue for eye colour.
- Variation — a small difference in features between individuals belonging to the same species.
3.16 Points To Ponder
- A recessive allele is hidden in every heterozygous individual who carries it, and shows only in the few who inherit it twice. Why, then, does the dominant allele not slowly replace it and drive it out of a population altogether?
- Mendel never saw a chromosome, a gene or an allele, and had no word for any of them. What was it about his method that allowed him to reach correct conclusions about things he could not observe — and what would have become of his ratios if he had raised ten plants instead of thousands?
- The X and the Y are counted as one pair, yet they differ in size and carry largely different genes. Weigh the case for and against calling them a homologous pair, given how the chapter defines that term.
- Mutations are described as the only source of genuinely new alleles, and also as changes that are usually harmful or of no use. How can a process that goes wrong far more often than it goes right be necessary to a species?
- Alleles segregate cleanly and never blend, and yet children often seem to fall midway between their parents in features such as height. How can both of these statements be true at the same time?
- Suppose a character were controlled by an allele carried on the Y chromosome instead of the X. Work out the pattern it would show as it passed down a family, and say how a pedigree chart of it would look different from one drawn for colour blindness.
3.17 Objective Questions
- The genotypic ratio obtained in the \(F_2\) generation of a monohybrid cross is: (1 mark)
- \(3:1\)
- \(1:2:1\)
- \(9:3:3:1\)
- \(1:1\)
- Each body cell of a human being contains: (1 mark)
- 23 pairs of autosomes and no sex chromosomes
- 22 pairs of sex chromosomes and 1 pair of autosomes
- 22 pairs of autosomes and 1 pair of sex chromosomes
- 21 pairs of autosomes and 2 pairs of sex chromosomes
- The fixed position occupied by a particular gene on a chromosome is called its: (1 mark)
- centromere
- chromatid
- genome
- locus
- A woman with normal colour vision who carries the allele for colour blindness on one of her X chromosomes is described as: (1 mark)
- colour blind
- a carrier
- homozygous recessive
- unable to pass the allele to her children
- Which one of the following characters of the garden pea was not studied by Mendel? (1 mark)
- Seed shape
- Pod colour
- Length of the root
- Position of the flower
- The number of kinds of gametes that a plant of genotype RrYy can produce is: (1 mark)
- one
- two
- four
- sixteen
The following are assertion–reason questions. Choose the answer from the codes given below. (1) A is true and R is false (2) A is false and R is true (3) Both A and R are true (4) Both A and R are false
- Assertion (A): A boy who is colour blind has received the allele for it from his mother. Reason (R): A boy receives his single X chromosome from his mother and his Y chromosome from his father. (1 mark)
- Assertion (A): Two parents who can both roll their tongue can never have a child who cannot. Reason (R): Rolling is dominant over non-rolling. (1 mark)
- Assertion (A): The number of chromosomes in a body cell shows how complex an organism is. Reason (R): The lion, the tiger and the domestic cat all have 38 chromosomes in each body cell. (1 mark)
- Match the terms in Column I with their explanations in Column II. (5 marks)
| Column I | Column II |
|---|---|
| a. Karyotype | i. a sex cell carrying only one member of each pair of genes |
| b. Allele | ii. a chromosome other than a sex chromosome |
| c. Gamete | iii. the chromosomes of a cell arranged in order of size and shape |
| d. Mutation | iv. one of the alternative forms of a gene |
| e. Autosome | v. a sudden change in the hereditary material |
- Fill in the blanks. (4 marks)
- The alternative forms in which a character appears are called ____________.
- A pair of unlike alleles is described as ____________.
- The phenotypic ratio obtained in the \(F_2\) generation of a dihybrid cross is ____________.
- The full complement of DNA of an organism is called its ____________.
- State whether each of the following is true or false. (4 marks)
- Every egg produced by a woman carries an X chromosome.
- A recessive allele expresses itself in the heterozygous condition.
- The law of segregation is also called the law of purity of gametes.
- A mutation occurring in a cell of the skin can be passed on to a person’s children.
- Name the following. (4 marks)
- The narrow constriction that joins the two chromatids of a chromosome.
- The grid used to set out every possible genotype among the offspring of a cross.
- The scientific name of the plant on which Mendel carried out his breeding experiments.
- The condition in which blood clots very slowly because a clotting protein is missing.
- Pick the odd one out in each group and give a reason for your choice. (2 marks)
- Brown eyes, free ear lobes, rolling tongue, blue eyes
- Flower colour, pod shape, seed shape, leaf shape
3.18 Check Your Concepts
Define: (a) heredity (b) allele. (2 marks)
Give a reason why a single copy of the allele for red-green colour blindness is enough to make a boy colour blind. (2 marks)
State Mendel’s law of dominance. (2 marks)
Differentiate between genotype and phenotype, stating the basis of the difference. (3 marks)
Differentiate between autosomes and sex chromosomes, stating the basis of the difference. (3 marks)
State any three reasons for which Mendel chose the garden pea for his breeding experiments. (3 marks)
Explain, with one example, why two individuals showing the same character need not carry the same pair of alleles for it. (3 marks)
Why does the \(F_2\) generation of a monohybrid cross show a phenotypic ratio of \(3:1\) but a genotypic ratio of \(1:2:1\)? Explain. (4 marks)
What would be the consequence of a mutation occurring in (a) a cell of the skin, and (b) a cell of the reproductive organs that gives rise to gametes? Give a reason in each case. (4 marks)
A couple who can both roll their tongue have a child who cannot. Work out the genotype of each parent, setting out your reasoning step by step. (5 marks)
Explain why red-green colour blindness appears far more often in males than in females, and state the conditions under which a female can be colour blind. (5 marks)
3.19 Figure-Based Questions
Study Fig. 3.3-1. (a) Name the two identical threads shown and the structure that joins them. (b) Explain why there are two of them at this stage. (c) State why chromosomes can be seen at this stage but not in a resting cell. (3 marks)
Study Fig. 3.5-1. (a) Name the kind of cell shown at the top of each of the two columns. (b) How many kinds of sperm are formed, and why? (c) Why is only one kind of egg formed? (d) State the chance of a daughter being born at any one fertilisation. (4 marks)
Fig. 3.9-1 is a pedigree chart drawn for tongue rolling. (a) State what a square, a circle and a solid symbol each represent. (b) Give the genotype of the child at the left of the chart, with a reason. (c) Give the genotype of the father, with a reason. (3 marks)
Redraw the Punnett square of Fig. 3.10-2 for a cross between a carrier mother and a father with normal colour vision. Write the genotype in each of the four cells and the phenotype below each cell. (4 marks)
Study Fig. 3.11-4. (a) Name the kind of cross it represents. (b) Give the four kinds of gametes formed by each parent plant. (c) Name the two phenotypes appearing in the \(F_2\) that were present in neither parent. (3 marks)
Fig. 3.13-1 shows two kinds of red blood cells. (a) Name the change in the hereditary material that produces the cell shown below. (b) State one way in which this cell is less efficient than the normal one. (c) Name one further difficulty its shape can cause in the body. (3 marks)
3.20 Genetics & Numerical Problems
In pea plants, tall (T) is dominant over dwarf (t). A heterozygous tall plant is crossed with a dwarf plant. Draw the Punnett square for this cross and give the genotypic and phenotypic ratios of the offspring. (5 marks)
In a certain plant, red flower colour (R) is dominant over white (r). Two heterozygous red-flowered plants are crossed. What fraction of the offspring is expected to bear white flowers? Show your working. (3 marks)
A woman with normal colour vision, whose father was colour blind, marries a man with normal colour vision. Using a Punnett square, work out the chance that a son born to them will be colour blind, and state whether any of their daughters can be colour blind. (4 marks)
A pure-breeding pea plant bearing round yellow seeds (RRYY) is crossed with a pure-breeding plant bearing wrinkled green seeds (rryy). (a) Give the genotype and the phenotype of the \(F_1\) plants. (b) Name the four kinds of gametes an \(F_1\) plant can form. (c) State the phenotypic ratio expected in the \(F_2\) generation, and name the law that explains it. (4 marks)