Class 10 Science · Chapter 8

Heredity

120 Class 10 Science questions on Heredity from the NCERT textbook, NCERT Exemplar and CBSE board papers 2024–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 3 of 6

1. Q41 Exemplar · Short Answer Questions
   A very small population of a species faces ​a greater ​threat ​of extinction than ​a ​larger population. Provide ​a suitable genetic ​explanation.
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   A small gene pool means few variations to cope with change, strong genetic drift and inbreeding, so extinction is more likely.

   A very small population carries a small gene pool, so it holds few variations of each gene. Three things follow. First, when the environment changes, survival depends on some individuals already carrying a variation that lets them cope; in a small population the chance that such a variant exists at all is low, so a single change can destroy the whole population. Second, genetic drift acts strongly: the genes passed on each generation are a small chance sample, so variations are lost by accident and the gene pool shrinks further. Third, with few partners available, mating between close relatives becomes unavoidable, and inbreeding brings harmful recessive genes into the homozygous state where they are expressed, reducing fertility and survival. A large population keeps a large store of variation, drift is weak and inbreeding is rare, so it survives the same challenge.

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2. Q42 Exemplar · Short Answer Questions
   What ​are homologous structures? Give an example. ​Is it necessary that homologous structures ​always have ​a ​common ​ancestor?
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   Organs with the same basic design but different functions, as in vertebrate forelimbs; yes, they always trace back to a common ancestor.

   Homologous structures are organs of different organisms that have the same basic structural design and the same embryonic origin but may perform different functions, having been modified for different ways of life. Example: the forelimbs of vertebrates, where the arm of a human, the wing of a bat, the flipper of a whale and the wing of a bird all contain the same bones, humerus, radius and ulna, carpals, metacarpals and phalanges, in the same arrangement, although they are used for grasping, flying, swimming and flying respectively. Yes, it is necessary: homologous structures always trace back to a common ancestor, because the shared plan is inherited from an ancestor that already had it, and the differences arose later by divergent evolution. Structures that merely resemble each other but arose independently in unrelated lines are analogous, not homologous, as with the wing of a bird and the wing of an insect.

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3. Q43 Exemplar · Short Answer Questions
   Does the occurrence of ​diversity of animals on ​earth suggest ​their diverse ​ancestry also? Discuss this point in the ​light ​of ​evolution.
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   No: diversity arises by divergence from common ancestors, shown by homologous organs, shared DNA and genetic code, connecting-link fossils and similar embryos.

   No. The diversity of animals does not mean that their ancestry is diverse. Evolution explains diversity as the result of divergence from common ancestors: a single ancestral population becomes divided, gene flow between the parts stops, each part accumulates its own variations, and natural selection in different environments drives them apart until they can no longer interbreed. Repeated over long periods, this produces many species from one. The evidence that this is what happened is that animals of very different appearance still share deep similarities: homologous organs such as the forelimbs of a human, a bat, a whale and a bird, which contain the same bones in the same order; the same DNA and RNA chemistry and essentially the same genetic code in all of them, with the closest species having the most similar DNA and proteins; connecting-link fossils such as Archaeopteryx; and closely similar early embryos in very different vertebrates. Diversity is therefore the outcome of common ancestry, not evidence against it.

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4. Q44 Exemplar · Short Answer Questions
   Give the ​pair of ​contrasting traits of the ​following ​characters in pea ​plant ​and mention which is dominant ​and recessive

   1. (i)yellow seed ​
   2. (ii)round seed
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   Yellow (dominant) against green (recessive) seeds; round (dominant) against wrinkled (recessive) seeds.

   (i) Seed colour: the contrasting pair is yellow seed and green seed. Yellow is the dominant trait and green is the recessive trait. (ii) Seed shape: the contrasting pair is round seed and wrinkled seed. Round is the dominant trait and wrinkled is the recessive trait.

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5. Q45 Exemplar · Short Answer Questions
   Why did Mendel ​choose pea ​plant ​for ​his ​experiments?
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   It has clearly contrasting characters, self-pollinating bisexual flowers that can also be cross-pollinated at will, a short life cycle and many seeds.

   Mendel chose the pea plant because it has several characters that occur in two sharply contrasting and easily distinguishable forms, such as tall and dwarf plants, round and wrinkled seeds, yellow and green seeds and violet and white flowers, so the offspring could be sorted into classes and counted. Its flowers are bisexual and normally self-pollinating, which produces pure-breeding lines and gave him parents of known constant type. At the same time the flower is large enough for the stamens to be removed and for pollen of his own choosing to be applied, so he could carry out controlled cross-pollination. The plant is an annual that is easy to grow and needs little care, so many generations could be raised in a short time, each cross yielded a large number of seeds, and the hybrids were fully fertile, so an $F_2$ generation could always be obtained and reliable ratios calculated.

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6. Q46 Exemplar · Short Answer Questions
   A woman ​has only ​daughters. Analyse the situation genetically and ​provide a ​suitable ​explanation.
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   The mother gives only $\text{X}$, so the father's sperm decides each child's sex; having only daughters is a chance outcome.

   The woman is not responsible for having only daughters. Every human female is $\text{XX}$, so each of her eggs carries an $\text{X}$ chromosome and her contribution is the same at every conception. The father is $\text{XY}$ and produces $\text{X}$-bearing and $\text{Y}$-bearing sperms in equal numbers, so the sex of each child was decided by whether an $\text{X}$ sperm or a $\text{Y}$ sperm fertilised the egg: $\text{X}$ gives an $\text{XX}$ zygote, a girl, and $\text{Y}$ gives an $\text{XY}$ zygote, a boy. Each conception is an independent event with a probability of one half for each sex, so a run of daughters is simply a run of chance outcomes. For four children the probability of all being daughters is ${\left(\frac{1}{2}\right)}^4=\frac{1}{16}$, which is far from unusual. There is nothing in the woman's genetic make-up that could favour daughters, since she has no $\text{Y}$ chromosome to give.

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7. Q47 Exemplar · Long Answer Questions
   Does ​geographical ​isolation of ​individuals of a species ​lead ​to formation of a new species? Provide ​a ​suitable ​explanation.
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   Yes: a barrier stops gene flow, each group accumulates different variations under selection, and after many generations they become reproductively isolated species.

   Yes, in sexually reproducing organisms geographical isolation can lead to the formation of a new species. A physical barrier such as a river, a mountain range or a sea divides one interbreeding population into sub-populations that live apart. Gene flow between them stops, so a variation arising in one group can no longer spread to the other. Each group then accumulates its own genetic changes: new mutations arise independently in each, and genetic drift alters gene frequencies by chance, strongly so in a small group. Because the two habitats differ, natural selection favours different variations on the two sides, and the two gene pools move steadily apart. After a large number of generations the differences become so great that even if the barrier is removed the members of one group can no longer interbreed with the other, or their offspring are not fertile. At that point they are reproductively isolated and are two separate species. Two qualifications apply: isolation alone is not enough, since the genetic changes must have time to accumulate; and geographical isolation is not an important factor for self-pollinating plants or for asexually reproducing organisms, whose reproduction does not depend on reaching a partner.

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8. Q48 Exemplar · Long Answer Questions
   Bacteria have ​a simpler ​body ​plan ​when compared with human beings. Does it ​mean that human beings are more evolved than bacteria? Provide ​a suitable explanation.
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   No: body complexity is not degree of evolution; bacteria have evolved as long as humans and succeed in a wider range of habitats.

   No. Human beings have a more complex body design than bacteria, but that does not make them more evolved. Complexity of body plan and degree of evolution are different things. Bacteria and human beings are both alive today and both descend from the earliest forms of life, so the bacterial lineage has been evolving for exactly as long as ours. Evolution has no goal and no direction; changes are kept when they help an organism survive and reproduce in its own surroundings, and a simple, small, fast-reproducing body is as good a solution as a complex one. Judged by success, bacteria are outstanding: they live in hot springs, deep sea vents, ice, acidic and saline waters, soil, air and inside the bodies of other organisms, including ours; they occupy a far wider range of habitats than human beings and far outnumber all other organisms. Many of their habitats are places where no complex organism could survive at all. So human beings are more complex, not more evolved; bacteria are simply adapted in a different and highly successful way.

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9. Q49 Exemplar · Long Answer Questions
   All the human races ​like Africans, ​Asians, Europeans, Americans ​and others ​might ​have evolved from a common ancestor. Provide a few evidences ​in support of this ​view.
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   Fertile interbreeding, very small DNA differences, oldest fossils from Africa and shared anatomy all point to one common ancestor.

   Several independent lines of evidence support a single common ancestor. (i) All human races interbreed freely and their children are fully fertile, which is the definition of one species; all human beings therefore belong to Homo sapiens and share one line of descent. (ii) Studies of human DNA show that the genetic differences between the races are very small, and smaller than the differences between individuals within a single race; comparisons along the maternal and paternal lines trace all living people to one small ancestral population, and the greatest genetic diversity is found in Africa. (iii) The earliest fossils of modern human beings come from Africa and are older than modern human fossils found anywhere else, and human fossils and tools appear at progressively later dates through Asia, Europe, Australia and the Americas, recording a spread outwards from one region. (iv) All races share the same forty-six chromosomes, the same anatomy and physiology, the same blood group systems and the same biochemistry, while the features that differ, such as skin colour, hair form and body build, are superficial adaptations to climate controlled by a few genes. Taken together these show that the races are branches of one species that spread from a common ancestral population and diverged only in surface characters.

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10. Q50 Exemplar · Long Answer Questions
    Differentiate between ​inherited ​and acquired ​characters. ​Give ​one ​example for ​each type.
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    Inherited characters, such as eye colour, pass to offspring through gametes; acquired ones, such as low weight from starvation, affect only body cells.

    Inherited character: it is received from the parents through their gametes and is present in the DNA from the moment of fertilisation; it is controlled by genes and is present in the germ cells as well as the body cells; it is transmitted to the offspring; it cannot be removed by a change in the surroundings and lasts for life; and because it accumulates over generations it provides the raw material for evolution. Acquired character: it develops during the lifetime of the individual as a result of use or disuse, food, training, injury, disease or climate; it changes only the body or somatic cells and leaves the DNA of the germ cells unaltered; it is therefore not transmitted to the offspring; it may appear and disappear within one lifetime and dies with the individual; and it causes no evolution. Examples: colour of the eyes is an inherited character, and low body weight caused by starvation is an acquired character.

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11. Q51 Exemplar · Long Answer Questions
    Give ​reasons ​why ​acquired characters ​are ​not ​inherited.
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    Acquired changes affect only body cells, not germ-cell DNA, so the gametes carry no record of them to the offspring.

    The only physical link between a parent and its offspring is the gamete, so a character can be inherited only if the information for it is written in the DNA carried by the sperm or the egg. An acquired character develops during the life of the individual through use or disuse, diet, training, injury, disease or climate, and such an influence changes only the body or somatic cells and the tissues concerned. It does not alter the DNA of the germ cells in the reproductive organs. Since the germ-cell DNA is unchanged, the gametes carry no record of the acquired change and the offspring is born without it. The altered body cells themselves cannot help, because they are never handed on and die with the individual. Weismann tested this directly by removing the tails of mice for many successive generations, and every generation was still born with a normal tail. Because acquired characters are not inherited they cannot accumulate over generations and cannot cause evolution, which is why Lamarck's theory of the inheritance of acquired characters is not accepted.

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12. Q52 Exemplar · Long Answer Questions
    Evolution has exhibited a greater stability of ​molecular structure when compared ​with morphological ​structures. Comment ​on the statement and ​justify your ​opinion.
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    Correct: all life shares DNA, RNA and the genetic code, but body forms diversified, because almost any change in core molecules is harmful.

    The statement is correct. Every organism, from a bacterium to a human being, uses DNA and RNA of the same chemistry, essentially the same genetic code so that the same triplet specifies the same amino acid throughout life, the same twenty amino acids, ATP as the energy currency, ribosomes for protein synthesis and core pathways such as glycolysis; even individual proteins such as cytochrome c are recognisably the same molecule in a yeast cell and in a human being. Over the very same period the external form has changed beyond recognition, so that a bacterium, a fern, a fish, a bird and a mammal share almost nothing in body plan, organs, size, nutrition or locomotion. The reason is that the core molecules are indispensable and every part of the cell depends on them, so almost any change in them is lethal or seriously harmful and is removed by natural selection as soon as it appears, whereas morphological characters can vary without killing the organism and different environments actively favour different forms. The same selection therefore conserves the molecules and diversifies the bodies. Greater stability does not mean no change: molecules do change, but slowly and steadily, and it is precisely this that allows the number of differences in DNA or protein between two species to measure how long ago they separated, which makes molecular phylogeny the most reliable modern evidence of evolutionary relationship.

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13. Q53 Exemplar · Long Answer Questions
    In the following ​crosses write ​the characteristics of ​the progeny

    1. (a)$\text{RR YY}$ (Round, ​yellow) $\times$ $\text{RR YY}$ (Round, yellow)
    2. (b)$\text{Rr Yy}$ (Round, yellow) $\times$ $\text{Rr Yy}$ (Round, yellow)
    3. (c)$\text{rr yy}$ (wrinkled, green) $\times$ $\text{rr yy}$ (wrinkled, ​green)
    4. (d)$\text{RR YY}$ (Round, yellow) $\times$ $\text{rr yy}$ (wrinkled ​green)
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    (a) all $\text{RRYY}$; (b) round yellow, round green, wrinkled yellow, wrinkled green in the ratio $9:3:3:1$; (c) all $\text{rryy}$; (d) all $\text{RrYy}$.

    (a) $\text{RR YY}\times \text{RR YY}$: each parent forms only $\text{RY}$ gametes, so all the progeny are $\text{RRYY}$, that is $100\%$ round and yellow and true-breeding; only one genotype occurs. (b) $\text{Rr Yy}\times \text{Rr Yy}$: each parent forms four kinds of gamete, $\text{RY}$, $\text{Ry}$, $\text{rY}$ and $\text{ry}$, and the sixteen combinations give four kinds of progeny in the ratio $9$ round yellow $:3$ round green $:3$ wrinkled yellow $:1$ wrinkled green, with the genotypic ratio $1\text{RRYY}:2\text{RRYy}:2\text{RrYY}:4\text{RrYy}:1\text{RRyy}:2\text{Rryy}:1\text{rrYY}:2\text{rrYy}:1\text{rryy}$; round green and wrinkled yellow are new combinations. (c) $\text{rr yy}\times \text{rr yy}$: each parent forms only $\text{ry}$ gametes, so all the progeny are $\text{rryy}$, that is $100\%$ wrinkled and green, identical to the parents; only one genotype occurs. (d) $\text{RR YY}\times \text{rr yy}$: one parent gives only $\text{RY}$ and the other only $\text{ry}$, so every offspring is $\text{RrYy}$, that is $100\%$ round and yellow but heterozygous for both characters; only one genotype occurs.

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14. Q54 Exemplar · Long Answer Questions
    Study the following ​cross and showing self ​pollination ​in $F_1$, fill in the ​blank and answer the ​question that follows Parents: $\text{RRYY}$ (Round, yellow) $\times$ $\text{rryy}$ (wrinkled, green) $F_1$: $\text{Rr Yy}$ (Round, yellow) $\times$ ​?
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    The blank is $\text{Rr Yy}$, because self pollination crosses the $F_1$ plant with itself.

    The blank is $\text{Rr Yy}$, round and yellow. Self pollination means that the pollen and the ovule come from the same plant, so the second partner in the $F_1$ cross is the $F_1$ plant itself, whose genotype is $\text{Rr Yy}$. The $F_1$ cross is therefore $\text{Rr Yy}\times \text{Rr Yy}$, and it is this cross that gives rise to the $F_2$ generation.

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15. Q55 Exemplar · Long Answer Questions
    In question 44, ​what are the ​combinations of character in the $F_2$ ​progeny? ​What ​are their ratios?
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    Round yellow, round green, wrinkled yellow and wrinkled green in the ratio $9:3:3:1$.

    The $F_1$ plants of question $44$ are $\text{Rr Yy}$ and are self pollinated, so the cross is $\text{Rr Yy}\times \text{Rr Yy}$. Each plant forms four kinds of gamete, $\text{RY}$, $\text{Ry}$, $\text{rY}$ and $\text{ry}$, in equal numbers, and the sixteen combinations give four combinations of characters in the $F_2$: round and yellow, round and green, wrinkled and yellow, and wrinkled and green. Their phenotypic ratio is $9:3:3:1$. Two of these, round green and wrinkled yellow, are new combinations not present in either grandparent. The corresponding genotypic ratio is $1\text{RRYY}:2\text{RRYy}:2\text{RrYY}:4\text{RrYy}:1\text{RRyy}:2\text{Rryy}:1\text{rrYY}:2\text{rrYy}:1\text{rryy}$.

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16. Q56 Exemplar · Long Answer Questions
    Give the basic ​features ​of ​the ​mechanism ​of inheritance.
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    Genes come in two copies, one from each parent; gametes get one copy, fertilisation restores two, and different genes are inherited independently.

    The basic features are these. Every character is controlled by a gene, a section of the DNA of a chromosome that carries the information for one protein. In a sexually reproducing organism the chromosomes are in pairs, so each gene is present in two copies, one inherited from the father and one from the mother, situated at the same place on the two chromosomes of a pair. The two copies may be identical or may be different versions of the gene, and when they differ the dominant copy is expressed while the recessive copy is carried without being expressed and can reappear in a later generation. During gamete formation the two chromosomes of every pair separate, so each gamete receives only one copy of each gene and the number is halved. At fertilisation the two gametes fuse, restoring two copies in the progeny, one from each parent, so the chromosome number of the species stays constant. Finally, genes for different characters lying on different chromosomes are inherited independently of one another, which is why the progeny can show combinations of characters that neither parent had.

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17. Q57 Exemplar · Long Answer Questions
    Give reasons ​for the ​appearance of new combinations of ​characters ​in the $F_2$ progeny.
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    Independent assortment makes new gamete combinations and random fertilisation brings them together, giving round green and wrinkled yellow, 6 in 16 of $F_2$.

    New combinations appear for two reasons acting in turn. First, independent assortment during gamete formation. The $F_1$ plant is $\text{Rr Yy}$, heterozygous for both characters, and the two gene pairs lie on different pairs of chromosomes, so when the pairs separate in meiosis the separation of $\text{R}$ from $\text{r}$ does not decide whether $\text{Y}$ or $\text{y}$ goes with it. The $F_1$ therefore makes four kinds of gamete in equal numbers, $\text{RY}$, $\text{Ry}$, $\text{rY}$ and $\text{ry}$, and two of these, $\text{Ry}$ and $\text{rY}$, carry combinations that neither grandparent could produce. Second, random fusion at fertilisation. Any male gamete may fuse with any female gamete, giving sixteen equally likely combinations, so the recombined gametes are brought together in every possible way. Dominance then decides when the result is seen: in the $F_1$ every plant looks round and yellow and the new combinations are hidden, while in the $F_2$ the recessive copies can meet and the new appearances become visible. Sorting the sixteen by appearance gives $9$ round yellow $:3$ round green $:3$ wrinkled yellow $:1$ wrinkled green, in which round green and wrinkled yellow, six seeds in every sixteen, are the new combinations. No new gene has been created; the existing genes have simply been redistributed.

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18. Q58 CBSE 2026
    Choose ​the correct ​statements with reference ​to chromosomes :

    1. (i)carry hereditary information from parents to ​next generation.
    2. (ii)are thread-like structures located inside the ​nucleus of an animal cell.
    3. (iii)always exist in pairs ​in human gametes.
    4. (iv)are involved in ​the process of cell division.

    1. (a)(i) and (ii)
    2. (b)(iii) and (iv)
    3. (c)(i), (ii) and (iv)
    4. (d)(ii), (iii) and (iv)
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    (c) (i), (ii) and (iv)

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19. Q59 CBSE 2026
    The genotype of ​green stemmed tomato plant is denoted by ‘GG’ ​and that of ​purple stemmed tomato ​plant as ‘gg’. Hybrid of a ​cross between these two ​were self-pollinated to obtain ${\text{F}}_2$ progeny. What will be the ratio of GG, Gg and gg ​in this ${\text{F}}_2$ ​progeny ?

    1. (a)$2:1:1$
    2. (b)$3:1:0$
    3. (c)$1:1:2$
    4. (d)$1:2:1$
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    (d) $1:2:1$

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20. Q60 CBSE 2026
    In a cross carried ​out by Mendel, when he crossed tall pea plant ​(TT) ​with a ​short pea plant ​(tt) in ${\text{F}}_1$ ​progeny (first generation), all the plants ​were found to ​be tall. Select the correct reason from the given options.

    1. (a)Tallness is the recessive trait
    2. (b)Shortness is the dominant trait
    3. (c)Tallness is the dominant trait
    4. (d)Height of pea plant is not governed by traits like tallness ‘T’ or shortness ‘t’
    Show answer

    (c) Tallness is the dominant trait

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