Heredity

Chapter 7 ended with an observation that this chapter picks up: reproduction makes copies, but the copies are never perfect. A child looks like its parents, and yet you can tell the child apart from them. Both halves of that sentence matter. The passing of traits from parents to offspring is heredity. The differences between parents and offspring, and between the offspring themselves, are variation. NCERT opens with a simple example: a human population has a huge amount of variation, yet everyone is recognisably human, because the basic body design is inherited reliably.

The chapter asks two questions and answers them in order. First, where does variation come from and how much of it builds up? That is section 8.1. Second, what are the rules by which traits are inherited? That is section 8.2, which is where Mendel, genes, chromosomes and sex determination live.

One distinction runs through every year's paper, so settle it now. An inherited trait is one carried in the genes (in the DNA of the germ cells) and passed to the next generation: eye colour, blood group, attached or free earlobes. An acquired trait is one an individual picks up during its own lifetime by use, practice or environment: a wrestler's muscles, a scar, a language you learnt. Acquired traits change the body but not the DNA of the germ cells, so they are not inherited. The examiner phrases this as 'we cannot pass on to our progeny the experiences and qualifications earned during our lifetime', and wants exactly that reason. One honest note: this distinction sat in the evolution part of the older textbook, which the rationalised syllabus removed, so the current NCERT chapter doesn't spell it out. The board asked it repeatedly up to 2020 and question banks still carry it, and it is a one-line reason, so keep it.

Reproduction hands the offspring a copy of the parent's DNA. Even in asexual reproduction that copying isn't exact, so a bacterium dividing produces two cells that are very similar but not identical. Let each of those divide again and you get four, each carrying its parent's small differences plus one or two new ones of its own. NCERT draws this as a branching tree over three generations: the further you go down, the more the individuals differ, because variation from earlier generations is inherited and new variation is added on top. That is what 'accumulation' means in the section title.

Two kinds of reproduction give two amounts of variation. Asexual reproduction produces only minor differences, the ones that arise from small inaccuracies in DNA copying. Sexual reproduction produces far more, because two parents contribute and their genes are shuffled together in the offspring (Chapter 7 covered the mechanics). This is why NCERT calls sexually reproducing populations much more variable.

Now the exam angle. Is variation good for the individual? Not necessarily. NCERT's example is a population of bacteria in temperate water. If a heat wave hits, most die, but the few that happen to carry a heat-resistant variation survive and multiply. The variation was useless to the others, and it was useless to the resistant ones too until the environment changed. So the answer to 'how does variation help?' is: it gives the species a better chance of surviving a change in its surroundings. Nature selects which variants persist; the individual doesn't choose.

The in-text question everyone gets asked: in an asexually reproducing population, trait A is in 10% of individuals and trait B in 60%. Which arose earlier? B, because in asexual reproduction variation is passed on faithfully, so a trait that has had more generations to spread is present in a larger fraction of the population.

Heredity is the reliable transmission of traits from one generation to the next. NCERT makes a point that is easy to miss: the very fact that a trait is inherited reliably, generation after generation, tells you that a rule is operating. If inheritance were random, you couldn't have said 'children look like their parents'. Section 8.2 is about finding those rules.

The first example is human earlobes. Look at the lower part of the ear: in some people the lobe hangs free below the point where it joins the head, in others it is attached along its whole length. Free and attached earlobes are two forms of one trait, and which form you have runs in families. NCERT suggests a small survey: note the earlobe type of students and of their parents and see whether children with attached lobes tend to have parents with attached lobes. The board has turned exactly this into a 1-mark question (two parents with attached earlobes, a recessive trait: what is the chance of a child with attached earlobes? 100%, because both parents can pass on only the recessive form).

What you should take from 8.2.1 is the shape of the reasoning, not the earlobes. To decide how a trait is inherited, you look at parents and children together, over more than one generation, and you count. That is what Mendel did with pea plants, with one advantage: he could set up the crosses himself, choose pure-breeding parents, and grow hundreds of offspring. Humans don't give you that control, which is why the chapter moves to peas in the next section, and why the NCERT exercise on light-coloured eyes says you cannot decide dominance from the observation that children with light eyes tend to have light-eyed parents.

Gregor Johann Mendel (1822–1884) worked with the garden pea and chose characters that come in two clear-cut forms: round or wrinkled seeds, yellow or green seeds, tall or short plants, violet or white flowers. He took pure-breeding parents (tall plants that always gave tall offspring, short plants that always gave short) and crossed them. The generation from this cross is called F₁ (first filial). He then let the F₁ plants self-pollinate to get F₂.

Here is what he saw, and every marking scheme wants these three observations in order. First, all F₁ plants were tall. Not medium height, not a blend: tall. Second, when F₁ plants were selfed, F₂ had both tall and short plants, in the ratio of about three tall to one short. Third, the short plants that reappeared were as short as the original short parent. So the trait for shortness had not been lost in F₁; it had been carried along without being expressed.

The explanation. Each plant carries two copies of the factor (we now say gene) for height. Write the tall version as T and the short version as t. The pure tall parent is TT, the pure short parent is tt. Each gamete carries only one copy, so the F₁ plant gets T from one parent and t from the other and is Tt. It is tall because T is dominant: one copy is enough to show the trait. The t version is recessive: it shows only when both copies are t. When Tt is selfed, gametes T and t combine at random and give TT, Tt, tT and tt in equal numbers. That is the genotype ratio 1 TT : 2 Tt : 1 tt. Since TT and Tt both look tall, the phenotype ratio is 3 tall : 1 short.

Two things follow that the board loves to ask. A trait can be inherited but not expressed (t in the F₁ plant). And you cannot tell TT from Tt by looking; you have to cross and count. A cross of Tt with tt gives 1 tall : 1 short, and a cross of TT with anything gives all tall, which is how the 2018 question about two tall parents producing no short offspring is settled.

Mendel then asked whether two traits travel together or separately. He crossed pea plants that were tall with round seeds and plants that were short with wrinkled seeds. F₁ were all tall with round seeds, so tall and round are the dominant forms. If the two traits were tied together, F₂ should contain only the two parental types: tall-round and short-wrinkled. Instead F₂ contained tall-round, tall-wrinkled, short-round and short-wrinkled plants. Some tall plants had wrinkled seeds and some short plants had round seeds. New combinations appeared, so the two traits are inherited independently of each other.

The figure in NCERT uses seed shape and seed colour, and that is the version the 2024 paper asked. Parents RRYY (round, yellow) and rryy (wrinkled, green). Each parent gives one gamete type, RY or ry, so F₁ is RrYy: round and yellow, because R and Y are dominant. An F₁ plant makes four kinds of gamete in equal numbers, RY, Ry, rY and ry, because the R/r pair and the Y/y pair separate independently. A 4 × 4 Punnett square gives 16 combinations, and counting phenotypes gives 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green. The 9:3:3:1 ratio is the fingerprint of a dihybrid cross; the 9 and the 1 are the parental types, the two 3s are the new combinations.

Check the ratio by looking at one trait at a time. Round : wrinkled in F₂ is (9 + 3) : (3 + 1) = 12 : 4 = 3 : 1. Yellow : green is (9 + 3) : (3 + 1) = 3 : 1 as well. Each trait still follows the monohybrid rule; the dihybrid ratio is just the two 3:1 ratios multiplied together, and that multiplication is only possible if the traits are independent. That is the sentence to end a 3-mark answer with.

Fractions worth having ready: 9/16 show both dominant traits, 1/16 show both recessive traits, and only 1 in 16 is RRYY, the pure double dominant. If a question changes the parents (say RRYy × RRYy), don't reach for 9:3:3:1; work the square. Here every plant is RR, and Yy × Yy gives 3 yellow : 1 green, so 75% round-yellow and 25% round-green.

The next question is what a gene actually does to make a plant tall or short. Cellular DNA carries the information for making proteins, and a gene is the section of DNA that provides the information for one protein. Take height. A plant grows tall because a growth hormone is made in enough quantity, and how much hormone is made depends on how efficiently an enzyme works to produce it. The gene for this enzyme is what we have been calling the height gene. If the gene is in its normal form, the enzyme is efficient, a lot of hormone is made and the plant is tall. If the gene is altered, the enzyme is less efficient, the hormone is scarce and the plant is short. So a gene controls a trait by controlling a protein.

This is also the real reason Tt plants are tall. A Tt plant has one efficient copy of the enzyme gene and one poor copy. The efficient copy alone makes enough hormone, so the plant reaches full height. Only a tt plant, with two poor copies, falls short. Dominance, then, isn't a magic property of the letter T; it is a consequence of one working copy being sufficient. A 2022 assertion-reason item was built on precisely this chain: height controlled by enzyme efficiency, and the enzyme controlled by the gene.

Notice the logic the section is setting up. Mendel's results need each plant to carry two copies of each gene. If both parents are to contribute one copy each, then a germ cell must carry only one copy of every gene, not two. How that single set is produced is the subject of the next part of 8.2.3, and it brings in chromosomes.

Genes sit on chromosomes. In a body cell each chromosome is present as a pair, so each gene is present in two copies, one on each member of the pair. The pea plant's Tt is one such pair: T on one chromosome, t on its partner. When germ cells are made, the two members of every pair separate and a gamete receives just one chromosome from each pair. So a gamete carries one complete set of chromosomes, and therefore one copy of every gene. Which member of a pair a gamete receives is a matter of chance, and it may be the copy that came from the plant's mother or the copy that came from its father. That randomness is what makes an Rr Yy plant produce RY, Ry, rY and ry gametes in equal numbers, and it is the physical basis of independent inheritance.

At fertilisation, a male gamete with one set meets a female gamete with one set, and the zygote has the normal two sets again. This is the answer to the NCERT exercise question on how the equal genetic contribution of both parents is ensured: every cell has two copies of each chromosome; germ cells carry only one; the offspring gets one full set from the egg and one from the sperm, so the mother and father contribute equally, one copy of every gene each.

NCERT adds a caution that examiners like. The mechanism described here works for organisms with two copies of each chromosome, but not all organisms are like that. Some do not reproduce sexually at all, and some have unusual arrangements. Keep the claim to what the chapter says: in organisms such as peas and humans, each germ cell gets one chromosome from every pair, so each parent contributes one set.

A quick numerical anchor for humans, which the next section needs: body cells have 46 chromosomes as 23 pairs, so a gamete carries 23 chromosomes, and a zygote has 23 + 23 = 46.

Species that reproduce sexually have two sexes, and different species decide the sex of a new individual in different ways. NCERT gives three. In some reptiles, the temperature at which the fertilised eggs are kept decides whether the animals developing in them will be male or female. In snails, an individual can change its sex, which shows that sex in these animals is not fixed by genes. In human beings the sex of a child is largely determined genetically, by the chromosomes it inherits.

Here is the human mechanism, and the flow chart the board asks for. Every body cell has 23 pairs of chromosomes. 22 pairs are the same in both sexes and are called autosomes. The 23rd pair is the pair of sex chromosomes. In a woman this pair is two X chromosomes, written XX; in a man it is one X and one much shorter Y, written XY. A woman's egg therefore always carries 22 + X. A man's sperm carries either 22 + X or 22 + Y, in equal numbers, because the X and Y separate when sperm are made. If an X-carrying sperm fertilises the egg, the zygote is XX and the child is a girl. If a Y-carrying sperm fertilises it, the zygote is XY and the child is a boy.

Three conclusions follow, and you should write all three. Every child receives an X from the mother, so the mother does not decide sex. The sex of the child is decided by which chromosome, X or Y, it receives from the father. And since the two kinds of sperm are produced in equal numbers and fertilisation is a matter of chance, the probability of a boy is 50% and of a girl is 50%, which is why roughly half the children born are boys and half girls.

The question that keeps returning: 'the sex of a newborn is a matter of chance and neither parent may be considered responsible for it'. Justify with a flow chart. Draw the parents (44 + XX and 44 + XY), their gametes (22 + X; 22 + X or 22 + Y), the two possible zygotes, and label them girl and boy in a 1:1 ratio. The chart is half the answer; the sentence about the father's sperm and chance is the other half.