Heredity

This is the eighth chapter of NCERT's Class 10 Science textbook (pages 128–133 of the 2026-27 reprint; the contents page and the PDF agree). The rationalised edition keeps two parts: accumulation of variation during reproduction (8.1) and heredity (8.2: inherited traits, Mendel's rules, how traits are expressed, and sex determination). The older material on evolution, speciation and human evolution is no longer in this chapter, so this page does not teach it. The chapter has 2 activities, Figures 8.1–8.6, a box on Gregor Mendel, two in-text ‘Questions’ boxes (6 questions), a summary and 4 exercise questions. It is short — this page has 7 concept sections, not padded.

• Neighbouring chapters: How do Organisms Reproduce? (Chapter 7) explained DNA copying, variation and why germ cells carry half the chromosomes; this chapter uses all three. In Class 12, Principles of Inheritance and Variation takes Mendel much further (test cross, incomplete dominance, co-dominance and blood groups, linkage, sex-linked disorders) and Human Reproduction covers fertilisation and the sperm's role in sex; Molecular Basis of Inheritance explains how genes make proteins. This page links to them rather than re-teaching them.
• Prelims — science: UPSC has asked who decides a child's sex (1981, 1984, 2000) and how genes control the body — through protein synthesis (1986). Most other genetics questions (sex-linked diseases, blood groups, DNA technology) sit on the Class 12 chapters.
• GS1 — society: the biology of sex determination answers the practice of blaming mothers for daughters, one root of son preference and an adverse sex ratio (treated on lebo102). GS3 — science and agriculture: plant and animal breeding rest on Mendel's rules; genetic disorders and genetic testing build on them.

• Variation accumulates: each generation inherits its parents' differences and adds new ones; asexual reproduction makes only minor differences (DNA-copying errors), sexual reproduction makes more. Not all variants survive equally — heat-tolerant bacteria in a heat wave.
• Mendel (1822–1884) used clear-cut pea characters — round/wrinkled seeds, tall/short plants, violet/white flowers — and was the first to count individuals with each trait in every generation.
• Tall TT × short tt → F1 all tall (no medium) → F1 selfed → F2 3 tall : 1 short; combinations 1 TT : 2 Tt : 1 tt. One copy of T is enough: T is dominant, t recessive. Violet flowers are dominant over white (Fig. 8.4).
• Two traits: round-green RRyy × wrinkled-yellow rrYY → F1 round-yellow → F2 9 : 3 : 3 : 1 (Mendel: 315 : 108 : 101 : 32 of 556), including new combinations — traits are inherited independently.
• Expression: a gene is the DNA for one protein; e.g. an efficient enzyme makes much growth hormone → tall; an altered gene → less hormone → short. Each cell has two copies of each chromosome (one per parent); each germ cell takes one from each pair.
• Sex determination: environment in some reptiles (egg temperature), sex change in snails; in humans 22 pairs + XX (female) or XY (male). Every child gets X from the mother; the father's X or Y decides — half boys, half girls.

Inheritance gives the next generation both a common basic body design and subtle changes in it. When that generation reproduces, its offspring carry the differences they inherit plus newly created ones (Fig. 8.1). With a single parent, as in asexual reproduction — one bacterium dividing into two, and those into four — the individuals are very similar, with only minor differences from small inaccuracies in DNA copying. Sexual reproduction generates even greater diversity. Do all variants have equal chances of surviving? Obviously not: different variations give different advantages — bacteria that withstand heat survive better in a heat wave. Selection of variants by environmental factors forms the basis for evolutionary processes.

The rules of heredity decide how traits are reliably inherited. A child has all the basic features of a human being, yet does not look exactly like its parents, and human populations vary a great deal (8.2.1). Earlobes are an example: in some people the lowest part of the ear is free, in others attached to the side of the head (Fig. 8.2, Activity 8.1). In humans, father and mother contribute practically equal amounts of genetic material to the child, so each trait can be influenced by both paternal and maternal DNA, and each child has two versions of each trait. Which one shows? Gregor Johann Mendel (1822–1884) worked out the main rules more than a century ago. Educated in a monastery and at the University of Vienna, he failed the examinations for a teaching certificate, returned to the monastery and grew peas; others had studied inheritance before, but he blended science and mathematics and was the first to keep count of individuals showing a trait in each generation. He used contrasting visible characters of garden peas — round/wrinkled seeds, tall/short plants, white/violet flowers and so on.

Mendel crossed a tall pea plant with a short one and counted the progeny. The first generation, F1, had no halfway plants — no medium height: all were tall, so only one parental trait was seen, not a mixture. Were these F1 tall plants the same as the tall parents? Both were allowed to self-pollinate. The parents' progeny were all tall; but in the F2 from the F1 tall plants, one quarter were short. So both tallness and shortness had been inherited by the F1, but only tallness was expressed. Mendel proposed that two copies of a factor (now called genes) control each trait in sexually reproducing organisms; they may be identical or different (Fig. 8.3). Both TT and Tt plants are tall; only tt is short. A single T is enough for tallness — T is dominant, t is recessive.

What happens when plants differing in two characters are crossed? A tall plant with round seeds × a short plant with wrinkled seeds gives F1 plants that are all tall with round seeds — tallness and round seeds are dominant. When the F1 are self-pollinated, some F2 plants are tall with round seeds and some short with wrinkled seeds, but there are also new combinations: tall with wrinkled seeds, and short with round seeds. Figure 8.5 shows the same idea with seed shape and seed colour: round-green (RRyy) × wrinkled-yellow (rrYY) gives round-yellow F1 (RrYy); the F2 includes all four types. So the factors for the two traits recombine at fertilisation: the traits are independently inherited.

Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides information for one protein is the gene for that protein. How do proteins control traits? Take tallness. Plants have hormones that trigger growth, so height can depend on the amount of a particular plant hormone, which depends on how efficiently it is made. Consider an enzyme important for making it: if the enzyme works efficiently, a lot of hormone is made and the plant is tall; if the gene for that enzyme has an alteration that makes it less efficient, less hormone is made and the plant is short. Thus genes control characteristics, or traits.

If Mendel's interpretation is right, both parents contribute equally to the progeny's DNA: both must contribute a copy of the same gene, so each pea plant has two sets of all genes, one from each parent, and each germ cell must have only one gene set. How? If a germ cell carried a single whole gene set as one unit, Figure 8.5's result could not happen: ‘R’ and ‘y’ would stay linked and could not be inherited independently. The answer is that each gene set is present not as one long thread of DNA but as separate, independent pieces — chromosomes. Each cell has two copies of each chromosome, one from each parent; every germ cell takes one chromosome from each pair, of either maternal or paternal origin. When two germ cells combine, the normal number is restored, keeping the species' DNA stable. All sexually reproducing organisms use this mechanism; asexually reproducing organisms follow similar rules too.

How is the sex of a newborn decided? Species use very different strategies. Some rely entirely on environmental cues: in a few reptiles the temperature at which the fertilised eggs are kept decides whether the young will be male or female. In snails, individuals can change sex, so sex is not genetically fixed. In human beings, sex is largely genetically determined. But if similar gene sets come from both parents, how? Because not all human chromosomes are paired perfectly: most have a maternal and a paternal copy — 22 such pairs — but one pair, the sex chromosomes, is not always a perfect pair. Women have a perfect pair, both X; men have a mismatched pair, a normal-sized X and a short Y. So women are XX and men XY. As Figure 8.6 shows, half the children will be boys and half girls; all children get an X from the mother, so the child's sex is decided by what it inherits from the father: X → girl, Y → boy.

Short outlines in page order for the two ‘Questions’ boxes (6 questions), three questions in the running text, and the two activities.

Numbers

One-liners

• Variation accumulates each generation; sexual reproduction creates more of it.
• The environment selects variants — heat-tolerant bacteria in a heat wave.
• Both parents contribute practically equal genetic material.
• Mendel's novelty was counting individuals with each trait in every generation.
• F1 shows no blending: tall × short gives all tall.
• Dominant: one copy is enough; recessive: shows only as two copies (tt).
• New combinations in F2 show traits are inherited independently.
• A gene is the DNA for one protein; genes act through proteins.
• Efficient enzyme → more growth hormone → tall plant.
• Genes sit on separate chromosomes; each germ cell takes one of each pair.
• Egg always X; sperm X or Y; the father's chromosome decides the sex.
• Some reptiles: egg temperature decides sex; snails can change sex.

Traps to avoid

• 3:1 is appearance; 1:2:1 is combinations (TT : Tt : tt).
• A tall plant may be TT or Tt; a short one is always tt.
• 9:3:3:1 is appearance; there are 9 genotypes in the 16 squares.
• Females do not decide the sex of the child; the father's X or Y does.
• Exercise 1 is TtWW (c): all violet needs WW; half short needs Tt.
• One family (A × O → O) cannot show which blood group is dominant.
• Evolution is not in this rationalised chapter — taught in Class 12.
• NCERT slips: Figure 8.4's ‘fertilisation’ for pollination; text pairs height with shape but Figure 8.5 pairs shape with colour; summary says variation helps ‘individuals’; leftover ‘later sections’ on evolution; QR label 1064CH09.

Prelims — what UPSC asks

• Sex determination: the sex is decided when the sperm fertilises the ovum (1981), by the father's chromosomes (1984), and the assertion that females play the major role is false while ‘women have two X chromosomes’ is true (2000).
• Genes and proteins: genes control the activity of organisms through protein synthesis (1986).
• Likely next: dominance and recessive carriers, 3:1 and 9:3:3:1 ratios, why a trait can skip a generation, temperature-dependent sex determination in reptiles (with climate change), gene editing and genetic testing (Class 12 links).
• Blood-group inheritance (1994), sex-linked disorders (1985, 1993, 1994, 1995), pleiotropy and genetic engineering sit on Principles of Inheritance and Variation and the biotechnology chapters; this chapter gives only the first rung. All stored keys of the four questions kept here stand.

Mains — where it appears and how to answer

• No GS Mains question in the bank so far is built on this chapter's core content. It supplies the first line for GS1 answers on son preference and the sex ratio (the father's chromosome decides the sex) and the base for GS3 answers on plant and animal breeding, genetic disorders and genetic technologies.
• Answer frame: the rule in one line (XX/XY; dominant and recessive; independent inheritance) → the social or farm consequence → the law, programme or technology (PCPNDT; breeding programmes; genetic screening) → positions on open questions, attributed, no verdict.
• A 2 × 2 Punnett square (XX × XY, or Tt × Tt) fits in a 150-word answer.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

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