Heredity: Class 10 Science · Chapter 8

At a family wedding, look along a row of cousins. Several have their grandmother's sharp nose or their grandfather's curly hair, yet no two faces are the same. Now picture a banana plantation in which every plant was raised from a sucker cut from one parent plant: the plants look almost like copies of one another. Reproduction that involves two parents produces visible differences; growing new plants from parts of one plant produces very few.

In the last chapter you saw that reproduction copies DNA, and that the copies are never perfect. This chapter asks two questions. How do differences, or variations, arise and build up over the generations? And by what rules are features handed from parents to their children?

8.1 Accumulation of Variation during Reproduction

Think of a family recipe handed down from cook to cook. Each cook copies it faithfully but adds a small tweak, a pinch more salt or a little less sugar, and the next cook copies the tweaked version and adds a tweak of their own. Living things hand on their design in much the same way. Each generation inherits a plan that works, along with its parents' small changes, and adds a few new ones, so differences build up over the generations.

DNA copying is remarkably accurate, but over millions of copies a few slips creep in. Picture the Lactobacillus bacteria that set milk into curd overnight, doubling again and again from a few cells. A daughter cell that inherits a copying slip passes it to all its descendants, and any of them may add a slip of its own. Follow one family line for three generations (Fig. 8.1) and you find cells that are almost, but not quite, identical, each carrying the slips of its own ancestors.

Sexual reproduction creates far more variety. Every offspring receives DNA from two different parents, so it begins life with a fresh mixture of features that neither parent had in exactly that form. You will see how this mixing works when we study Mendel's experiments in Section 8.2.2.

Now add a change in the surroundings. When a patient takes an antibiotic, the rare bacteria that happen to carry a change letting them resist the medicine survive and multiply, while the rest die. In a year of poor rain, the millet plants whose roots happen to reach a little deeper are the likeliest to set seed. A difference that seemed useless in ordinary times can decide who lives. So variants are not equal: the surroundings favour some over others, and this sorting is where the story of evolution begins, which you will study in higher classes.

8.2 Heredity

Variation is only half the story. Look at what does not change: a mango seed always grows into a mango tree, never a neem, and a cat's kittens are always cats. Heredity is this passing on of features from parents to offspring, and its rules explain why it is so dependable, and why it still leaves room for differences.

8.2.1 Inherited Traits

Stand on a busy railway platform and look around. Height, skin tone, hair and the shape of the eyes vary endlessly. Yet everyone there shares the same human plan of two eyes, a backbone and five-fingered hands, and each person resembles their own parents more than any stranger. Features handed from parents to children are called inherited traits. The easiest traits to study have two clear forms, such as the shape of the earlobe.

8.2.2 Rules for the Inheritance of Traits – Mendel's Contributions

Suppose your mother has free earlobes and your father attached ones. You carry instructions from both of them, because each parent gave you almost exactly half of your DNA. So for every trait you hold two instructions. When they disagree, which one wins, and what happens to the loser? Gregor Mendel found the answer in the 1860s by breeding garden peas and counting the results.

Peas suited Mendel well. They grow quickly, a single plant gives many seeds, and each flower normally pollinates itself, yet pollen can also be carried by hand from one plant to another. Above all, peas show clear either-or forms of many characters, with nothing in between: round or wrinkled seeds, violet or white flowers, tall or short stems, and four more pairs listed in Table 8.2.

Mendel began with pure-breeding plants: tall plants whose offspring, generation after generation, were always tall, and short plants that only ever gave short offspring. He crossed the two kinds by dusting pollen from one onto the flowers of the other. These parents are called the P generation, and their offspring the first filial, or F1, generation.

If traits blended like milk stirred into tea, tall × short should give plants of medium height. Mendel found nothing of the sort: every F1 plant was as tall as its tall parent. Had shortness been destroyed? He let the F1 plants pollinate themselves and grew their seeds into the second generation, the F2. Short plants reappeared, about one in every four; in one experiment he counted 787 tall plants and 277 short ones. So the F1 plants had been carrying shortness all along, without showing it.

Mendel's explanation was simple and bold: a plant holds a pair of hereditary factors for height. Today we call these factors genes. Write the tall version as T and the short version as t. A pure tall plant is then TT and a pure short plant is tt. Each pollen grain or egg cell carries just one copy. So every F1 plant received T from one parent and t from the other: it is Tt, and it is tall because one T is all it takes.

A Tt plant makes two kinds of germ cells in equal numbers: half carry T and half carry t. When it pollinates itself, any pollen grain can meet any egg, so four pairings are equally likely. A grid called a Punnett square lists them all (Fig. 8.3): TT, Tt, tT and tt. Three of the four contain at least one T and are tall; only tt is short.

An experiment like this, which follows one pair of contrasting traits, is a monohybrid cross. Notice that two plants can look alike yet carry different genes: TT and Tt are both tall. So we separate an organism's phenotype, the form of the trait that we can see, from its genotype, the pair of gene copies it carries.

Being dominant does not make a trait stronger, better or more common. It says only this: when the two copies differ, the dominant one decides what we see. A recessive trait is not lost when it is hidden; it waits in Tt plants and shows again whenever two t copies meet.

Two traits at once

Mendel's next step was bolder: follow two characters through the same cross and see whether they stay together or go their separate ways. He crossed a pure-breeding plant that had round, yellow seeds (RRYY) with one that had wrinkled, green seeds (rryy). All the F1 seeds were round and yellow (RrYy), so round and yellow are the dominant forms.

The surprise came in the F2. Besides the two kinds seen in the parents, round yellow and wrinkled green, there were two new combinations: round green seeds and wrinkled yellow seeds. The four kinds came in a ratio close to 9 : 3 : 3 : 1.

The Punnett square shows why (Fig. 8.5). An RrYy plant makes four kinds of germ cells, RY, Ry, rY and ry, in equal numbers, because the shape copy that a germ cell receives has no effect on the colour copy it receives. Four kinds of pollen meeting four kinds of eggs give 16 equally likely pairings. A cross like this, which follows two pairs of traits together, is a dihybrid cross.

Look at one trait at a time and the old pattern returns. Round seeds fill 9 + 3 = 12 of the 16 squares and wrinkled seeds 3 + 1 = 4, a ratio of 3 : 1. Yellow to green is also 12 : 4, or 3 : 1. Each trait follows its own 3 : 1 rule as if the other were not there, which is why the probabilities simply multiply in (8.3). Seed shape and seed colour are inherited independently.

8.2.3 How do these Traits get Expressed?

So far genes have been letters in a grid. What are they inside a living cell? Every cell carries DNA, the store of instructions for building proteins, and a gene is simply one stretch of that DNA.

Proteins are how genes act. Follow one chain in a pea plant, starting from the end you can see. A stem grows long because its cells respond to a growth hormone, a gibberellin. The plant makes that hormone in several steps, and one of the steps needs a particular enzyme, which is a protein. The instructions for that enzyme are a gene. So the chain runs gene → enzyme → hormone → height (Fig. 8.7). If a gene copy is the working version, its enzyme does the job briskly, hormone is plentiful and the stem grows tall. A copy with a small change in its DNA gives a sluggish enzyme, little hormone and a short plant.

The same chain explains dominance. A Tt plant has one working copy and one faulty copy. The single working copy makes enough enzyme to keep the hormone level up, so the plant grows as tall as a TT plant. Only a tt plant, with no working copy, stays short.

Now do some arithmetic. A pea plant holds two copies of each gene, one from each parent, as Mendel's results require. If pollen and eggs also carried two copies, their offspring would have four, the next generation eight, and so on. So each germ cell must carry just one copy of every gene: a single complete set.

How does a germ cell end up with exactly one set? Suppose each parent handed on all its genes as a single package. In our dihybrid cross, R and Y arrived together from one parent and r and y from the other. If they stayed packed together, an F1 plant could make only RY and ry germ cells, and round green or wrinkled yellow seeds would never appear. They do appear, so the genes cannot be travelling as one package.

Think of a cell's genes as an encyclopaedia printed in several volumes rather than one giant book. Each volume is a chromosome. A body cell keeps two copies of every volume, one inherited from the mother and one from the father. A germ cell receives one copy of each volume, and for every volume chance decides whether it is the mother's copy or the father's (Fig. 8.8). Genes in different volumes are therefore sorted independently, just as Mendel's results demand.

At fertilisation a male and a female germ cell fuse. Each brings one member of every pair, so the zygote again has complete pairs and the same number of chromosomes as its parents. A human body cell, for instance, has 23 pairs (46 chromosomes), while a human egg or sperm has 23. In this way the amount of DNA stays steady from one generation to the next.11The special cell division that halves the number of chromosomes while germ cells form is called meiosis. You will study it in Class 11.

8.2.4 Sex Determination

Why is one baby a boy and another a girl? Nature has more than one way of settling this, as the comparison below shows.

Humans are in the last group: the chromosomes received at fertilisation settle whether a baby develops as a boy or a girl. But we have just seen that chromosomes come in matching pairs, one member from each parent. How, then, can they make two kinds of children?

Count the chromosomes in a human body cell and you find 46, arranged as 23 pairs. Line the pairs up by size, and 22 of them look the same in men and in women. The last pair, the sex chromosomes, gives the answer away. In a woman its two members are alike, both X chromosomes (XX). In a man one member is an X and the other is a much smaller chromosome called Y (XY).

Now follow the germ cells (Fig. 8.9). Every egg receives an X, because a woman has nothing else to give. A man's sperm are of two kinds in equal numbers: half carry an X and half carry a Y. If a sperm with an X fertilises the egg, the child is XX, a girl. If a sperm with a Y fertilises it, the child is XY, a boy. Each pregnancy is like the toss of a fair coin, so roughly half the babies born are girls and half are boys.