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 1 of 6

1. Q1 NCERT · Exercises
   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

   1. (a)TTWW
   2. (b)TTww
   3. (c)TtWW
   4. (d)TtWw
   Show answer

   (c) TtWW

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2. Q2 NCERT · Intext Questions (Page 129)
   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?
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   Trait B arose earlier, because at $60\%$ it has had more generations to spread than trait A at $10\%$.

   Trait B is likely to have arisen earlier. In an asexually reproducing species a new trait appears in a single individual as a small inaccuracy in DNA copying, and it can spread only as that individual's descendants go on reproducing. The longer a trait has been present, the more generations it has had to spread and the larger the share of the population that carries it. Trait B is carried by $60\%$ of the population and trait A by only $10\%$, so B has passed through many more generations of spreading and must have appeared first; trait A is the more recent variation.

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3. Q3 NCERT · Exercises
   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?
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   No: light-eyed children of light-eyed parents are expected whether light is recessive or dominant, so the observation cannot decide.

   No, nothing can be concluded about dominance from this observation. The finding shows only that eye colour is inherited — that a child's eye colour is related to the parents' — which was not in question. Dominance is decided by crossing parents with contrasting forms of a trait: the form that appears in all the first-generation progeny is dominant, and the form that disappears and then reappears in about a quarter of the second generation is recessive. The study reports no such cross. Worse, the observation is exactly what both possibilities predict. If light eye colour is recessive, a light-eyed person is $ll$, so two light-eyed parents can pass on only $l$ and every child of theirs is light-eyed. If light eye colour is dominant, a light-eyed parent carries at least one $L$, so two light-eyed parents would still have light-eyed children in most cases — all of them if either parent is $LL$, and about three in four if both are $Ll$. Since both hypotheses predict that light-eyed children usually have light-eyed parents, the observation cannot separate them. What would settle it is a family in which the parents have contrasting eye colours, and in particular a light-eyed child born to two dark-eyed parents: a trait that appears in a child but in neither parent must have been carried hidden by both, and only a recessive trait can be carried hidden.

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4. Q4 NCERT · Intext Questions (Page 129)
   How ​does ​the creation ​of variations ​in ​a species promote survival?
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   Some individuals can survive a drastic change, reproduce and keep the species going; for example, heat-resistant bacteria survive a heat wave.

   Variation makes the individuals of a species different from one another, so they do not all react in the same way to a change in their surroundings. If a population were completely uniform, a drastic change such as a heat wave, a shortage of water or a new disease could destroy every member of it at once, because what one individual could not survive none could survive. In a varied population some individuals happen to carry a variation that suits the changed conditions; the chapter's example is bacteria that can withstand heat surviving a heat wave. These individuals live through the change, reproduce, and pass the useful variation on, so the population is rebuilt and the species continues even though many individuals are lost. Variation does not protect every individual, because different variations carry different kinds of advantage and some are of no use at all; what it does is give the species as a whole a far better chance of surviving a change in its environment. This selection of variants by environmental factors is also the basis of the evolutionary process.

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5. Q5 NCERT · Exercises
   Outline ​a ​project which ​aims to find ​the ​dominant coat ​colour ​in ​dogs.
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   Cross pure black and pure white dogs: the colour every first-generation puppy shows is dominant, and the second generation gives about $3:1$.

   The project follows Mendel's method, transferred from pea plants to dogs. (i) Choose one coat colour character with two clearly contrasting forms that cannot be confused — say black coat against white coat — and one known to be controlled by a single pair of genes, since several genes affect coat colour in dogs. (ii) Obtain dogs that breed true for each colour. A dog is taken as pure-breeding for black if, over several generations of matings within the black group, all its offspring have been black; breeders' and kennel-club records can establish this. (iii) Mate a pure-breeding black dog with a pure-breeding white one, and record the coat colour of every puppy of the first generation, over several pairs and several litters. (iv) Read the answer off the first generation: all the puppies will show one of the two colours and none the other, and the colour that every puppy shows is the **dominant** one, while the colour that has disappeared is recessive. (v) Confirm the result by mating the first-generation dogs among themselves. In the second generation the missing colour reappears, in about one puppy in four, giving a ratio of about $3:1$ of the dominant colour to the recessive one — which shows that the recessive allele was carried through the first generation unchanged and merely masked. Throughout, keep all the dogs in the same conditions, use healthy unrelated animals, record every puppy of every litter, and pool the counts from many litters before comparing them with the expected ratio, because a ratio only shows itself in large numbers.

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6. Q6 NCERT · Exercises
   How ​is ​the equal ​genetic ​contribution ​of male and female ​parents ​ensured in the ​progeny?
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   Sperm and egg each get one chromosome of every pair, and fertilisation restores the pairs, so each parent gives the same number.

   The equality is ensured by the way chromosomes are handled when germ cells are made and when they fuse. (i) In the body cells of a sexually reproducing organism the chromosomes occur in pairs, one member of each pair having come from the father and the other from the mother; in human beings there are $23$ such pairs, that is $46$ chromosomes. (ii) When germ cells are formed, a special kind of cell division halves this number: each germ cell receives only **one** chromosome of every pair, so a sperm carries a single complete set of $23$ and an egg carries a single complete set of $23$. Neither carries an arbitrary half of the DNA — each carries exactly one member of every pair. (iii) At fertilisation the sperm and the egg fuse, and the zygote again has the full $46$ arranged in $23$ pairs, with one member of every pair coming from the father and the other from the mother. So each parent contributes exactly the same number of chromosomes, and contributes to every pair, which is what makes the genetic contribution equal; and because the halving and the restoration happen in every generation, the chromosome number of the species stays constant. The one qualification is the sex chromosomes: the mother always contributes an X while the father contributes an X or a Y, and it is this that determines the sex of the child — but the number of chromosomes contributed by each parent is still the same.

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7. Q7 NCERT · Intext Questions (Page 133)
   How do Mendel's ​experiments ​show that ​traits may be ​dominant ​or recessive?
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   All $F_1$ plants were tall but one quarter of $F_2$ were short, so tall is dominant and the hidden short is recessive.

   Mendel crossed a pure-breeding tall pea plant with a pure-breeding short one. Every plant of the first generation, F1, was tall, and not one was of medium height, so the two parental traits had not blended and only one of them was seen. He then let these F1 tall plants self-pollinate and counted the second generation. F2 was not all tall: about one quarter of the plants were short, giving a ratio of three tall to one short. The short trait had therefore not been lost in F1. It had only been hidden, and it reappeared as soon as a plant received the short factor from both parents. This shows that a plant carries two copies of the factor for a character, and that when the two copies differ, one of them is expressed and the other is not. The version that is expressed, tallness $T$, is called the dominant trait, and the version that stays hidden in its presence, shortness $t$, is called the recessive trait. Both $TT$ and $Tt$ plants are tall while only $tt$ plants are short, which is exactly why a recessive trait can skip a generation and then return.

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8. Q8 NCERT · Intext Questions (Page 133)
   How do Mendel's ​experiments ​show that ​traits ​are ​inherited ​independently?
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   In a two-character cross, F2 showed new combinations (tall wrinkled, short round) in a $9:3:3:1$ ratio, so the traits separate independently.

   Mendel crossed pea plants that differed in two characters at once, a tall plant bearing round seeds with a short plant bearing wrinkled seeds. Every F1 plant was tall with round seeds, so tallness and round seed shape are the dominant traits and each F1 plant is $TtRr$. He then let the F1 plants self-pollinate. The F2 generation contained the two parental combinations, tall with round seeds and short with wrinkled seeds, but it also contained two combinations that neither parent had shown: tall plants with wrinkled seeds and short plants with round seeds. The four classes appeared in the ratio 9 : 3 : 3 : 1. If height and seed shape had been tied to each other and passed on as a package, these new combinations could never have appeared, because every offspring would have been either tall with round seeds or short with wrinkled seeds. Their appearance shows that the pair of factors controlling height separates without reference to the pair controlling seed shape when gametes are formed, so the two can recombine freely in the zygote. The same point can be read from the ratio one character at a time: 12 tall to 4 short is 3 : 1, and 12 round to 4 wrinkled is also 3 : 1, so the ratio for either character is unchanged by what the other character does. Mendel's count for the seed shape and seed colour version of this cross, 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green out of 556 seeds, is the same 9 : 3 : 3 : 1.

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9. Q9 NCERT · Intext Questions (Page 133)
   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?
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   No: the same family arises whether A is dominant over O or O over A, so it cannot show which is dominant.

   No, this information is not enough. Dominance can only be decided by seeing which version of a trait is expressed in a person who carries both versions, and this one family does not identify such a person with certainty, because both possibilities produce exactly the family that was observed. Suppose A is dominant over O. Then O is recessive, so the mother showing blood group O must be $OO$ and the daughter must be $OO$ as well; the daughter's second O must have come from her father, so the father is $AO$ and still shows blood group A. The family fits. Now suppose O is dominant over A. Then A is recessive, so a person showing blood group A can carry no O at all and the father must be $AA$; the mother showing blood group O may be $AO$; the father passes A to every child, so the daughter is $AO$, and because O is dominant she shows blood group O. The family fits equally well. Since the same father, mother and daughter arise on either assumption, and both even predict that about half the couple's children would be of each blood group, the observation cannot separate the two. To settle the matter one would need information that one of the possibilities forbids, from more children or from more generations: a child of blood group A born to two parents of blood group O would prove that A is recessive, and a child of blood group O born to two parents of blood group A would prove that O is recessive.

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10. Q10 NCERT · Intext Questions (Page 133)
    How is ​the ​sex of ​the child ​determined in ​human beings?
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    The fertilising sperm decides the sex: an $\text{X}$ sperm gives a girl ($\text{XX}$) and a $\text{Y}$ sperm a boy ($\text{XY}$).

    The sex of the child is decided by the sex chromosomes it inherits, and in practice by the one it receives from its father. A human body cell carries 23 pairs of chromosomes. Twenty-two of these pairs are the same in men and in women; the twenty-third is the pair of sex chromosomes. A woman has two X chromosomes, XX, a perfect pair, while a man has a mismatched pair, one normal-sized X and one short Y, so he is XY. Every germ cell receives one chromosome of each pair, so every egg the mother makes carries an X, whereas the father makes two kinds of sperm in equal numbers, half carrying an X and half carrying a Y. If a sperm carrying X fertilises the egg the zygote is XX and the child is a girl; if a sperm carrying Y fertilises it the zygote is XY and the child is a boy. Since the mother can supply nothing but an X, she cannot influence the outcome: every child inherits an X from her whether it is a boy or a girl, and the sex depends entirely on whether the fertilising sperm carried the X or the Y. Because the two kinds of sperm are produced in equal numbers, about half the children born are girls and half are boys.

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11. Q11 Exemplar · Multiple Choice Questions
    Exchange ​of ​genetic ​material ​takes ​place ​in

    1. (a)vegetative reproduction
    2. (b)asexual reproduction
    3. (c)sexual reproduction
    4. (d)budding
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    (c) sexual reproduction

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12. Q12 Exemplar · Multiple Choice Questions
    Two ​pink coloured flowers on crossing resulted in 1 ​red, 2 pink and ​1 white ​flower progeny. The ​nature of ​the ​cross will be

    1. (a)double fertilisation
    2. (b)self pollination
    3. (c)cross fertilisation
    4. (d)no fertilisation
    Show answer

    (b) self pollination

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13. Q13 Exemplar · Multiple Choice Questions
    A cross between a tall plant ​(TT) and short ​pea plant (tt) resulted ​in progeny ​that were all ​tall ​plants ​because

    1. (a)tallness is the dominant trait
    2. (b)shortness is the dominant trait
    3. (c)tallness is the recessive trait
    4. (d)height of pea plant is not governed by gene ‘T’ or ‘t’
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    (a) tallness is the dominant trait

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14. Q14 Exemplar · Multiple Choice Questions
    Which ​of the ​following statement ​is ​incorrect?

    1. (a)For every hormone there is a gene.
    2. (b)For every protein there is a gene.
    3. (c)For production of every enzyme there is a gene.
    4. (d)For every molecule of fat there is a gene
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    (d) For every molecule of fat there is a gene

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15. Q15 Exemplar · Multiple Choice Questions
    If a round, green seeded pea ​plant (RR yy) is ​crossed with wrinkled, yellow ​seeded ​pea ​plant, (rr YY) the ​seeds produced in $F_1$ generation are

    1. (a)round and yellow
    2. (b)round and green
    3. (c)wrinkled and green
    4. (d)wrinkled and yellow
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    (a) round and yellow

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16. Q16 Exemplar · Multiple Choice Questions
    In human males all ​the chromosomes are ​paired ​perfectly except one. This/these unpaired ​chromosome ​is/are

    1. (i)large chromosome
    2. (ii)small chromosome
    3. (iii)Y-chromosome
    4. (iv)​X-chromosome

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

    (c) (iii) and (iv)

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17. Q17 Exemplar · Multiple Choice Questions
    The ​maleness ​of a ​child ​is determined by

    1. (a)the X chromosome in the zygote
    2. (b)the Y chromosome in zygote
    3. (c)the cytoplasm of germ cell which determines the sex
    4. (d)sex is determined by chance
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    (b) the Y chromosome in zygote

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18. Q18 Exemplar · Multiple Choice Questions
    A ​zygote ​which has ​an X-chromosome ​inherited ​from ​the father will ​develop into ​a

    1. (a)boy
    2. (b)girl
    3. (c)X- chromosome does not determine the sex of a child
    4. (d)either boy or girl
    Show answer

    (b) girl

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19. Q19 Exemplar · Multiple Choice Questions
    Select the incorrect statement

    1. (a)Frequency of certain genes in a population change over several generations resulting in evolution
    2. (b)Reduction in weight of the organism due to starvation is genetically controlled
    3. (c)Low weight parents can have heavy weight progeny
    4. (d)Traits which are not inherited over generations do not cause evolution
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    (b) Reduction in weight of the organism due to starvation is genetically controlled

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20. Q20 Exemplar · Multiple Choice Questions
    New species ​may be formed ​if

    1. (i)DNA undergoes ​significant changes ​in germ cells
    2. (ii)​chromosome number changes ​in the gamete
    3. (iii)there is no change in ​the genetic material
    4. (iv)mating does not take place

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

    (a) (i) and (ii)

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