Heredity

1. What this chapter is about

Reproduction makes new individuals that are similar to their parents but subtly different. The study of how these similarities are passed on, and how the differences arise, is called heredity. The science of heredity is genetics.

• Heredity — transmission of characters (traits) from one generation to the next.
• Variation — the differences in characters among individuals of a species.
• Trait / character — a feature that can be inherited, e.g. plant height, seed shape, eye colour, earlobe type.

If you look at a field of sugarcane (grown asexually) the plants are almost identical — very little variation. But in animals and humans, which reproduce sexually, distinct variations are visible between individuals. This chapter explains the mechanism by which variations are created and inherited.

2. Accumulation of variation during reproduction (8.1)

Inheritance from the previous generation gives both a common basic body design and subtle changes in it. When the new generation reproduces in turn, it passes on the differences it inherited plus newly created ones — so variations accumulate generation after generation.

• Asexual reproduction (e.g. a bacterium dividing): the offspring are very similar. Only very minor differences arise, caused by small inaccuracies in DNA copying.
• Sexual reproduction: mixing of genetic material from two parents generates much greater diversity.

Do all variations have an equal chance of survival? No. Survival depends on the nature of the variation and the environment. For example, a variant bacterium able to withstand heat will survive a heat wave better than others. This selection of variants by environmental factors is the basis of evolutionary processes.

3. Heredity and inherited traits (8.2, 8.2.1)

The most obvious outcome of reproduction is the production of individuals of similar design. The rules of heredity determine how traits are reliably inherited.

A child carries all the basic features of a human being, yet does not look exactly like either parent — human populations show a great deal of variation. Both father and mother contribute practically equal amounts of genetic material to the child. This means a trait can be influenced by both paternal and maternal DNA, so for each trait there are two versions in every child.

4. Mendel and his pea experiments (8.2.2)

Gregor Johann Mendel (1822–1884) studied science and mathematics at Vienna, then grew pea plants (Pisum sativum) in his monastery garden. Others had studied inheritance before, but Mendel was the first to keep an exact count of how many individuals showed each trait in every generation — combining biology with mathematics to arrive at the laws of inheritance.

Why pea plants were a good choice:

• Many clearly contrasting visible characters — round / wrinkled seeds, tall / short plants, white / violet flowers, etc.
• Short life cycle and many offspring, so ratios could be counted.
• Normally self-pollinating, but can be cross-pollinated by hand when needed.

He took plants differing in one character (a tall plant and a short plant), crossed them, and counted the percentages of tall and short progeny in each generation.

5. The monohybrid cross — one trait, ratio 3 : 1 (Fig 8.3)

A monohybrid cross follows the inheritance of a single trait. Mendel crossed a pure Tall (TT) plant with a pure short (tt) plant.

• F₁ generation (first filial): all plants were Tall — there were no halfway "medium-height" plants. So only one parental trait showed; the two did not blend.
• When these F₁ tall plants were allowed to self-pollinate, the F₂ generation was not all tall. About one quarter were short.

This proved that both the tallness and shortness factors were present in the F₁ plants, but only the tallness trait was expressed. Mendel concluded that two copies of a factor (now called a gene) control each trait. The two copies may be identical or different, depending on parentage.

Here both TT and Tt are tall, while only tt is short. A single copy of 'T' is enough to make the plant tall, so 'T' is dominant; 't' shows only when both copies are present, so it is recessive.

• Dominant trait — the version that is expressed even when only one copy is present (T, tall).
• Recessive trait — the version that is hidden unless both copies are present (t, short).
• Homozygous = two identical copies (TT or tt); Heterozygous = two different copies (Tt).
• Genotype = the gene make-up (TT, Tt, tt); Phenotype = the visible feature (tall / short).

6. Punnett square for the monohybrid cross

A Punnett square shows all the ways gametes can combine. F₁ tall plants are Tt; each makes two kinds of gamete, T and t. Crossing Tt × Tt:

Counting the four boxes: 1 TT : 2 Tt : 1 tt → 3 tall : 1 short. This is why a recessive trait can skip a generation and reappear in F₂.

7. The dihybrid cross — two traits, ratio 9 : 3 : 3 : 1 (Figs 8.4, 8.5)

Mendel next bred plants differing in two characters at once — for example a plant with round, yellow seeds (RRYY) crossed with one having wrinkled, green seeds (rryy).

• F₁: all plants had round, yellow seeds — so round (R) and yellow (Y) are the dominant traits; wrinkled (r) and green (y) are recessive.
• F₂: on self-pollinating the F₁ (RrYy × RrYy), four kinds of seed appeared — including new combinations not seen in the parents (round-green and wrinkled-yellow).

The appearance of round-green and wrinkled-yellow combinations proves that the seed-shape gene and the seed-colour gene are inherited independently of each other. This is Mendel's Law of Independent Assortment: during gamete formation, the alleles of one gene separate independently of the alleles of another gene. (Each F₁ RrYy plant makes four kinds of gamete — RY, Ry, rY, ry — so a 4×4 Punnett square gives 16 boxes in the 9 : 3 : 3 : 1 pattern.)

8. How do these traits get expressed? (8.2.3)

Cellular DNA is the information source for making proteins. A section of DNA that carries the information for one protein is the gene for that protein. Genes control characters through the proteins (often enzymes) they make. Mendel's "factors" are these genes.

Tallness example: plant height depends on a plant hormone. The amount of hormone made depends on the efficiency of an enzyme in that process.

• If the gene makes an efficient enzyme → lots of hormone → plant is tall.
• If the gene is altered so the enzyme is less efficient → less hormone → plant is short.

Thus genes control traits by controlling the proteins that carry out body processes.

Why two separate gene sets — chromosomes: if both parents contribute equally, each individual must carry two copies of every gene, one from each parent, and each germ cell (gamete) must carry only one copy. But if a gamete simply carried one whole long thread of DNA, two traits (like R and Y) would always travel together and could not assort independently. They can because the gene set is not one thread but many separate pieces called chromosomes.

• Every body cell has two copies of each chromosome — one maternal, one paternal.
• During gamete formation, each germ cell takes one chromosome of each pair.
• When two germ cells fuse at fertilisation, the normal chromosome number is restored in the offspring, keeping the DNA of the species stable.

This chromosome mechanism explains Mendel's results and is used by all sexually reproducing organisms.

9. Sex determination in humans (8.2.4)

How is the sex of a newborn decided? Different species use different strategies:

• Environmental: in some reptiles, the temperature at which the fertilised eggs are kept decides whether the young are male or female.
• Changeable: some animals such as snails can change sex — sex is not genetically fixed.
• Genetic (humans): in human beings sex is largely genetically determined — the chromosomes inherited from the parents decide it.

Humans have 23 pairs of chromosomes. 22 pairs (autosomes) are perfectly matched. The 23rd pair are the sex chromosomes:

• Females are XX — a perfect pair, both X.
• Males are XY — a mismatched pair: one normal-sized X and one short Y.

So the mother always gives an X chromosome to every child. The father gives either an X or a Y. Therefore the father's contribution decides the child's sex:

Social note: since the sex of the child is decided by what the father contributes, blaming the mother for the child's sex has no scientific basis.

10. NCERT in-text questions — fully answered (page 129)

Q1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?

In asexual reproduction, variations are passed on as the population grows. A trait present in more individuals (60%) has had more time to spread through successive generations, so trait B is likely to have arisen earlier than trait A.

Q2. How does the creation of variations in a species promote survival?

Variations let different individuals cope with different environmental conditions. If the environment changes drastically (heat, cold, new disease), individuals with a useful variation can survive and reproduce, while others may not. Variation therefore protects the species from being wiped out and is the raw material for natural selection and evolution.

11. NCERT in-text questions — fully answered (page 133)

Q1. How do Mendel's experiments show that traits may be dominant or recessive?

When a pure tall plant (TT) was crossed with a pure short plant (tt), all F₁ plants were tall — the short trait disappeared. But on self-pollinating the F₁, the short trait reappeared in one-fourth of the F₂ plants. So the short factor was present in F₁ but not expressed. The trait that appears in F₁ is dominant (tall, T); the one that is hidden and reappears in F₂ is recessive (short, t).

Q2. How do Mendel's experiments show that traits are inherited independently?

In the dihybrid cross (round-yellow RRYY × wrinkled-green rryy), the F₂ showed new combinations — round-green and wrinkled-yellow — in the ratio 9 : 3 : 3 : 1. The seed-shape gene and seed-colour gene recombined freely, showing each trait is inherited independently of the other.

Q3. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits — blood group A or O — is dominant? Why or why not?

No, this is not enough. The daughter is O, so she is genotype OO, meaning she received an O allele from each parent — so the father (group A) must be AO, carrying a hidden O. This tells us O can be masked by A, hinting A is dominant, but a single cross of one family cannot prove dominance — we would need data from many families / a large number of progeny to be sure which trait is dominant.

Q4. How is the sex of the child determined in human beings?

The mother is XX and always passes an X. The father is XY and passes either X or Y. If the child gets the father's X it is XX (girl); if it gets the father's Y it is XY (boy). So the child's sex is decided by the chromosome inherited from the father.

12. NCERT Exercises — fully answered (page 133)

Q1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests the genetic make-up of the tall parent is: (a) TTWW (b) TTww (c) TtWW (d) TtWw

Answer: (c) TtWW. All progeny had violet flowers → the violet parent must be WW (pure violet). But almost half the progeny were short → the tall parent must carry the short allele, i.e. it is Tt (a Tt × tt cross gives 1 tall : 1 short). So the tall parent is TtWW.

Q2. A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?

No, we cannot decide from this alone. The fact that light-eyed children tend to have light-eyed parents only shows the trait is inherited; it does not tell us whether it is dominant or recessive. To decide, we would need to know the trait in grandparents and several generations, and study a large number of families to see whether the trait can be masked (recessive) or always shows (dominant).

Q3. Outline a project which aims to find the dominant coat colour in dogs.

Choose two contrasting pure-breeding coat colours, say black and white. (1) Cross a pure black dog with a pure white dog and record the coat colour of all F₁ puppies. (2) The colour that appears in all the F₁ puppies is the dominant coat colour; the one that disappears is recessive. (3) Confirm by mating two F₁ dogs: the recessive colour should reappear in about one-fourth of the F₂ (a 3 : 1 ratio). Repeat over many litters to make the result reliable.

Q4. How is the equal genetic contribution of male and female parents ensured in the progeny?

Genes are carried on chromosomes, which exist in pairs in body cells — one of each pair from the mother and one from the father. During gamete formation each germ cell receives only one chromosome of each pair (so eggs and sperm carry one set each). At fertilisation the egg and sperm fuse, restoring the paired set — half from the mother and half from the father. This ensures both parents contribute equal genetic material to the child.

13. Common mistakes to avoid

• Confusing genotype (TT, Tt, tt — the genes) with phenotype (tall / short — the look). Tt and TT look the same but are different genotypes.
• Writing the monohybrid genotype ratio as 3 : 1 — that's the phenotype. The genotype ratio is 1 : 2 : 1.
• Thinking F₁ traits "blend". They do not — only the dominant trait shows; there are no medium-height plants.
• Saying the mother determines the child's sex. The father does (he carries X and Y).
• Forgetting that the dihybrid 9 : 3 : 3 : 1 ratio is what proves independent inheritance.
• Mixing up dominant and recessive symbols — dominant is the capital letter (T, R, Y), recessive is the small letter (t, r, y).
• Forgetting that variations during asexual reproduction come only from small DNA-copying errors, so they are few.

14. Quick revision checklist

• Heredity = inheritance of traits; variation = differences between individuals.
• Two copies of each gene per individual; gametes carry one copy each.
• Dominant = capital letter, shows in F₁; recessive = small letter, hides until F₂.
• Monohybrid F₂: phenotype 3 : 1, genotype 1 : 2 : 1.
• Dihybrid F₂: 9 : 3 : 3 : 1 → traits inherited independently.
• Genes control traits via the proteins / enzymes they make.
• Chromosomes are in pairs; gene sets are separate pieces, so traits assort independently.
• Sex determination: mother XX (gives X), father XY (gives X or Y); father decides sex.