NCERT Solutions for Class 10 Science Chapter 8: Heredity

These Class 10 Science Chapter 8 solutions cover Heredity from the NCERT textbook (session 2026–27). Every in-text “Questions” set and the end-of-chapter “Exercises” are reproduced verbatim and solved in clear, exam-ready language — including Mendel’s experiments, dominant and recessive traits, independent inheritance, how traits get expressed and sex determination in human beings.

Class: 10 Subject: Science Chapter: 8 — Heredity Unit: How do Organisms Reproduce / Heredity Type: In-text Questions + Exercises Session: 2026–27
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Class 10 Science Chapter 8 Solutions – Overview

Chapter 8, Heredity, explains how characteristics are passed from parents to offspring and how variation builds up over generations. It begins with the accumulation of variation during reproduction — small inaccuracies in DNA copying create minor differences in asexual reproduction, while sexual reproduction generates much greater diversity. The heart of the chapter is Mendel’s work on garden peas, which revealed dominant and recessive traits, the F1 and F2 patterns, the 3:1 ratio for one trait and the 9:3:3:1 ratio that proves traits are inherited independently. It then shows how genes (on chromosomes) control traits through proteins, and finally explains sex determination in human beings — women are XX, men are XY, and the child’s sex depends on whether it inherits an X or a Y chromosome from the father.

Key Concepts & Definitions

Heredity: the transmission of characters (traits) from parents to their offspring through genes.

Variation: the differences in characters among individuals of a species; sexual reproduction creates far more variation than asexual reproduction.

Gene: a section of DNA that carries information for making one protein; genes control traits.

Allele: the two versions of a gene a child carries — one from each parent — for a given trait (e.g. T and t).

Dominant trait: the trait that is expressed even when only one copy is present (e.g. T, tallness). Recessive trait: the trait expressed only when both copies are the recessive version (e.g. tt, shortness).

F1 and F2 generations: the first and second filial (offspring) generations in a cross.

Chromosome: an independent piece of DNA. Body cells have chromosomes in pairs (one from each parent); germ cells carry only one of each pair.

Sex chromosomes: the pair that decides sex — XX in females, XY in males.

Monohybrid cross (one trait): Tt × Tt → 1 TT : 2 Tt : 1 tt (genotype) = 3 tall : 1 short (phenotype) — the 3:1 ratio.

Dihybrid cross (two traits): RrYy × RrYy → phenotype ratio 9 : 3 : 3 : 1 — proof of independent inheritance.

Sex determination: Mother XX × Father XY → XX (girl) and XY (boy) in a 1 : 1 ratio; the father’s contribution (X or Y) decides the sex.

In-text Questions — Answers (Page 129)

1. 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?

ANSWER Trait B is likely to have arisen earlier. In an asexually reproducing species, variations are passed on faithfully from one generation to the next, so a trait that appeared earlier has had more generations to spread through the population. Since trait B is present in 60% of individuals while trait A is present in only 10%, B has had more time to accumulate and is therefore the older variation; A appeared more recently and has not yet spread widely.

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

ANSWER Variations allow a species to cope with changes in the environment. Different individuals carry different characteristics, so some of them may be suited to new conditions such as a heat wave, a new disease or a change in food supply. For example, bacteria that can withstand heat survive better during a heat wave. When the environment changes drastically, individuals with useful variations survive and reproduce, while those without them may die out. Thus variation gives a species a better chance of survival and forms the basis of evolution. (It need not always benefit the individual, but it helps the species persist.)

In-text Questions — Answers (Page 133)

1. How do Mendel’s experiments show that traits may be dominant or recessive?

ANSWER Mendel crossed a pure tall pea plant (TT) with a pure short pea plant (tt). All the F1 plants were tall — not medium height — showing that only one of the two parental traits (tallness) appeared, even though both traits were present in the plant. When these F1 tall plants (Tt) were self-pollinated, the F2 generation had both tall and short plants in a ratio of about 3 tall : 1 short. The shortness trait, which had disappeared in F1, reappeared in F2. This proves that the trait expressed in F1 (tallness) is dominant, while the trait that stayed hidden in F1 but reappeared in F2 (shortness) is recessive.

2. How do Mendel’s experiments show that traits are inherited independently?

ANSWER Mendel made a dihybrid cross — he crossed plants differing in two traits, e.g. a tall plant with round seeds and a short plant with wrinkled seeds. All F1 plants were tall with round seeds (both dominant). When the F1 plants were self-pollinated, the F2 generation showed the parental combinations (tall–round and short–wrinkled) as well as new combinations — tall plants with wrinkled seeds and short plants with round seeds — in a phenotype ratio of about 9 : 3 : 3 : 1. The appearance of these new combinations shows that the gene for height and the gene for seed shape are passed on independently of each other, so the two traits are inherited independently.

3. 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?

ANSWER No, this single piece of information is not enough to decide which trait is dominant. The daughter has blood group O, which means she has received an O-allele (recessive) from each parent. The father, though he shows blood group A, must therefore carry one A-allele and one O-allele (genotype AO); the mother is OO. From only one child we cannot establish dominance — we would need to know the proportion of offspring of each blood group across many such crosses. (In fact, from other evidence A is known to be dominant over O, but this particular family information alone does not prove it.)

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

ANSWER Human beings have 23 pairs of chromosomes. Of these, one pair is the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The mother always contributes an X chromosome to the egg. The father’s sperm may carry either an X or a Y chromosome. If a sperm carrying an X fertilises the egg, the child is XX (a girl); if a sperm carrying a Y fertilises the egg, the child is XY (a boy). Thus the sex of the child is determined by the chromosome inherited from the father, and roughly half the children are boys and half are girls.

NCERT Exercises — Solutions

1. 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 that the genetic make-up of the tall parent can be depicted as (a) TTWW (b) TTww (c) TtWW (d) TtWw

ANSWER (c) TtWW. All progeny bore violet flowers, so violet is dominant and the tall parent must carry only the violet allele (WW) so that no white-flowered plant appears — this rules out (d) TtWw, which would give some white flowers. Almost half of the progeny were short. Short is recessive (tt), so to obtain short offspring the tall parent must be heterozygous (Tt) and the short parent tt: Tt × tt gives 1 tall : 1 short (about half short). Therefore the tall parent is TtWW.

2. 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?

ANSWER No, this observation alone does not tell us whether light eye colour is dominant or recessive. The fact that light-eyed children usually have light-eyed parents only shows that the trait is inherited — it is passed from parents to children. It does not reveal the pattern of inheritance. To decide dominance we would need data on the offspring of many different crosses (for example, light-eyed × dark-eyed parents) and the proportions of each eye colour in their children across two generations. Without knowing whether the trait skips a generation or the F2 ratios, we cannot label it dominant or recessive.

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

ANSWER Aim: to find out which coat colour in dogs is dominant, e.g. black coat versus brown (or white) coat. Step 1 – Choose pure-breeding parents: select dogs that are pure (homozygous) for each coat colour — one pure black and one pure brown. Step 2 – First cross (F1): cross the pure black dog with the pure brown dog and record the coat colour of all the puppies. The colour that appears in every F1 puppy is likely the dominant colour; the colour that disappears is likely recessive. Step 3 – Second cross (F2): mate the F1 dogs with one another. If a roughly 3:1 ratio of the two coat colours appears in the F2 puppies, it confirms that the colour present in three-quarters is dominant and the one in one-quarter is recessive. Conclusion: the coat colour seen in all F1 puppies and in about three-fourths of the F2 puppies is the dominant coat colour. Repeating the crosses with many dogs makes the result reliable.

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

ANSWER Every body cell of an organism contains two sets of chromosomes — one set inherited from the mother and one set from the father — so each cell has two copies of every gene. During the formation of gametes (germ cells), the chromosome number is halved: each gamete (egg or sperm) receives only one chromosome from each pair, i.e. a single set of genes. At fertilisation, the egg (with one set) fuses with the sperm (with one set), restoring the normal two sets in the zygote. Because the mother contributes exactly one set through the egg and the father exactly one set through the sperm, both parents make an equal genetic contribution to the progeny.

Extra Practice Questions

Short Answer Type Questions

Q1. Define heredity and variation.

ANSWERHeredity is the transmission of characters (traits) from parents to offspring through genes. Variation refers to the differences in characters among the individuals of a species.

Q2. Why did Mendel choose the garden pea (Pisum sativum) for his experiments?

ANSWERThe garden pea has several clear contrasting characters (round/wrinkled seeds, tall/short plants, violet/white flowers), it has a short life cycle, produces many offspring, and is normally self-pollinating but can also be cross-pollinated easily — making it ideal for counting traits across generations.

Q3. Distinguish between dominant and recessive traits with an example.

ANSWERA dominant trait is expressed even when only one copy of its allele is present (e.g. tallness, T, expressed in TT and Tt plants). A recessive trait is expressed only when both alleles are recessive (e.g. shortness, expressed only in tt plants).

Q4. A woman is XX and a man is XY. Why is the father, and not the mother, said to determine the sex of the child?

ANSWERThe mother can pass on only an X chromosome, so every egg carries X. The father can pass on either an X or a Y. The child becomes a girl (XX) if it receives the father’s X and a boy (XY) if it receives the father’s Y. Hence the chromosome from the father decides the sex.

Q5. Why does asexual reproduction produce only minor variations?

ANSWERIn asexual reproduction a single parent passes on its DNA to the offspring. The only variations arise from small inaccuracies during DNA copying, so the offspring are almost identical to the parent and to each other, with only minor differences.

Long Answer Type Questions

Q1. Explain Mendel’s monohybrid cross using tallness and shortness in pea plants, with genotypes and ratios.

ANSWERMendel crossed a pure tall plant (TT) with a pure short plant (tt). The gametes were T and t, so all F1 plants were Tt — and all were tall, showing tallness is dominant and shortness recessive. When the F1 plants (Tt) were self-pollinated, each produced T and t gametes. Their combination gave F2 genotypes in the ratio 1 TT : 2 Tt : 1 tt. Since TT and Tt are both tall and only tt is short, the phenotype ratio was 3 tall : 1 short. This experiment established the ideas of dominant and recessive traits and the segregation of factors (genes) during gamete formation.

Q2. How do genes control characteristics (traits) in an organism? Explain with the example of plant height.

ANSWERA gene is a section of DNA that carries the information to make a particular protein (such as an enzyme). Enzymes control the chemical processes in the cell, and these processes decide the organism’s characteristics. Take plant height: plants make hormones that trigger growth, and the amount of hormone depends on the efficiency of an enzyme that makes it. If the gene for that enzyme works efficiently, plenty of hormone is produced and the plant grows tall. If the gene is altered so the enzyme is less efficient, less hormone is made and the plant remains short. Thus genes, by deciding which proteins are made and how well they work, control the traits of the organism.

Q3. Explain sex determination in human beings with the help of a cross.

ANSWERHumans have 23 pairs of chromosomes; one pair is the sex chromosomes. Females are XX and males are XY. The mother forms eggs that all carry an X chromosome. The father forms two kinds of sperm — half carry X and half carry Y. At fertilisation: egg (X) + sperm (X) → XX (girl); egg (X) + sperm (Y) → XY (boy). Since the two kinds of sperm are formed in equal numbers, boys and girls are produced in a roughly 1:1 ratio. The sex of the child therefore depends on whether the fertilising sperm from the father carries an X or a Y chromosome — the mother always contributes an X.

MCQs & Assertion–Reason

1. The transmission of characters from parents to offspring is called:

(a) variation    (b) heredity    (c) reproduction    (d) evolution

2. Mendel carried out his classic experiments on:

(a) garden pea    (b) maize    (c) sweet pea    (d) bean

3. In a cross between Tt × Tt, the phenotype ratio in the offspring is:

(a) 1:1    (b) 9:3:3:1    (c) 3:1    (d) 1:2:1

4. The genotype ratio in the F2 generation of a monohybrid cross is:

(a) 3:1    (b) 1:2:1    (c) 1:1    (d) 9:3:3:1

5. A 9:3:3:1 ratio in the F2 generation is obtained from a:

(a) monohybrid cross    (b) dihybrid cross    (c) test cross    (d) back cross

6. The sex chromosomes in a human male are:

(a) XX    (b) XY    (c) YY    (d) XO

7. The sex of a human child is determined by the chromosome inherited from the:

(a) mother    (b) father    (c) both equally    (d) environment

8. A section of DNA that provides information for one protein is called a:

(a) chromosome    (b) gene    (c) allele    (d) gamete

9. In pea plants, if T is for tallness and t for shortness, a short plant has the genotype:

(a) TT    (b) Tt    (c) tt    (d) TtTt

10. Greater variation among individuals is produced by:

(a) asexual reproduction    (b) sexual reproduction    (c) DNA copying alone    (d) cloning

Answer key: 1-(b), 2-(a), 3-(c), 4-(b), 5-(b), 6-(b), 7-(b), 8-(b), 9-(c), 10-(b).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: All F1 plants in Mendel’s cross between tall and short pea plants were tall.

Reason: Tallness is a dominant trait and is expressed even when only one copy of its allele is present.

A-R 2. Assertion: The sex of a human child is decided by the mother.

Reason: The mother contributes an X chromosome to every egg she produces.

A-R 3. Assertion: Sexual reproduction produces greater variation than asexual reproduction.

Reason: In sexual reproduction, genetic material from two different parents is combined in the offspring.

A-R 4. Assertion: A 9:3:3:1 ratio in the F2 generation shows that two traits are inherited independently.

Reason: The genes for the two traits assort independently during gamete formation.

A-R 5. Assertion: Each germ cell carries only one set of genes.

Reason: Halving the gene set in gametes restores the normal chromosome number when two germ cells combine at fertilisation.

Answer key: 1-(A), 2-(D), 3-(A), 4-(A), 5-(A).

Common Mistakes & Exam Tips

Common mistakes to avoid

  • Confusing genotype ratio (1:2:1) with phenotype ratio (3:1) in a monohybrid cross.
  • Writing that the mother determines the child’s sex — it is the chromosome (X or Y) from the father.
  • Saying a trait is dominant or recessive from a single child or one family — dominance is judged from ratios over many offspring.
  • Mixing up gene (a piece of DNA coding for a protein) with chromosome (a whole DNA thread carrying many genes).
  • Forgetting to show gametes and a Punnett-square layout when explaining a cross — marks are given for working, not just the answer.

How to score full marks in this chapter

For every cross question, write the parental genotypes, the gametes, the cross (Punnett square if asked), and then the genotype and phenotype ratios separately. Learn the three key ratios — 3:1 (monohybrid phenotype), 1:2:1 (monohybrid genotype) and 9:3:3:1 (dihybrid). For sex determination, always state that the mother gives X and the father gives X or Y. Use exact terms — dominant, recessive, allele, gene, chromosome — and keep your definitions precise.

Frequently Asked Questions

What is Class 10 Science Chapter 8 Heredity about?

Chapter 8, Heredity, explains how traits are passed from parents to offspring. It covers the accumulation of variation, Mendel’s experiments on pea plants, dominant and recessive traits, independent inheritance (the 9:3:3:1 ratio), how genes control traits through proteins, and sex determination in human beings.

What are the three important ratios in this chapter?

The monohybrid cross gives a 3:1 phenotype ratio and a 1:2:1 genotype ratio in the F2 generation, while a dihybrid cross gives a 9:3:3:1 phenotype ratio, which proves that two traits are inherited independently.

How is the sex of a child determined in humans?

Females are XX and males are XY. The mother always gives an X chromosome. If the father’s sperm carries an X, the child is a girl (XX); if it carries a Y, the child is a boy (XY). So the father’s chromosome decides the child’s sex.

Are these Class 10 Science Chapter 8 solutions free?

Yes. All ClearStudy NCERT Solutions for Class 10 Science are free and follow the official NCERT textbook for session 2026–27, with every in-text and exercise question solved.

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