Class 10 Science · Chapter 8

Heredity

285 Class 10 Science questions on Heredity from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 7 of 15

1. Q121 CBSE 2024
   Sex ​determination ​depends upon ​the environment ​in

   1. (a)Birds
   2. (b)Amphibians
   3. (c)Reptiles
   4. (d)Fishes
   Show answer

   (c) Reptiles

   Step-by-step solution →
2. Q122 CBSE 2024
   In a ​cross between two ​tall pea plants a few dwarf pea ​plants were also ​obtained in ${\text{F}}_1$ generation. It ​is possible ​only if ​the gene combination of the parental plants is :

   1. (a)TT and Tt
   2. (b)TT and tt
   3. (c)Tt and tt
   4. (d)Tt and Tt
   Show answer

   (d) Tt and Tt

   Step-by-step solution →
3. Q123 CBSE 2024
   Two pea plants, one round green ​(RRyy) seeds and another with wrinkled ​yellow ​(rrYY) seeds produce ${\text{F}}_1$ ​progeny that has round yellow (RrYy) seeds. If the ${\text{F}}_1$ progeny plants are selfed, then the minimum ​number of plants of the ${\text{F}}_2$ progeny will have the ​following ​observable characters :

   1. (a)Wrinkled green
   2. (b)Wrinkled yellow
   3. (c)Round green
   4. (d)Round yellow
   Show answer

   (a) Wrinkled green

   Step-by-step solution →
4. Q124 CBSE 2024
   These ​questions consist of ​two statements – Assertion (A) and ​Reason (R). ​Answer these questions ​selecting the appropriate option given below :

   Assertion (A): Human female has a perfect pair of sex chromosome.

   Reason (R): Sex chromosome contributed by the human male in the zygote decides the sex of a child.

   1. (a)Both (A) and (R) are true and (R) is the correct explanation of (A).
   2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of (A).
   3. (c)(A) is true, but (R) is false.
   4. (d)(A) is false, but (R) is true.
   Show answer

   (b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

   Step-by-step solution →
5. Q125 CBSE 2024
   Two ​statements are given ​— one ​labelled as Assertion (A) and ​the other labelled ​as Reason (R). Select the correct ​answer to this question from the codes (a), (b), (c) and (d) ​as given ​below.

   Assertion (A): Sex of the children will be determined by what they inherit from their mother.

   Reason (R): Women have XX sex chromosomes.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
   2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
   3. (c)Assertion (A) is true, but Reason (R) is false.
   4. (d)Assertion (A) is false, but Reason (R) is true.
   Show answer

   (d) Assertion (A) is false, but Reason (R) is true.

   Step-by-step solution →
6. Q126 CBSE 2024
   Two statements are ​given — one ​labelled Assertion ​(A) ​and the ​other labelled Reason (R). ​Select ​the correct answer from the ​codes (a), (b), (c) and (d) as given below.

   Assertion (A): In humans, males play an important role in determining the sex of a child.

   Reason (R): Males have two ‘X’ chromosomes.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
   2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
   3. (c)Assertion (A) is true, but Reason (R) is false.
   4. (d)Assertion (A) is false, but Reason (R) is true.
   Show answer

   (c) Assertion (A) is true, but Reason (R) is false.

   Step-by-step solution →
7. Q127 CBSE 2024
   Q. 17 to 20 consists ​of two statements – Assertion (A) ​and Reason (R), answer these questions selecting ​the appropriate ​option given ​below ​:

   Assertion (A): In human beings, males have ‘XX’ sex chromosomes and females have ‘XY’ sex chromosomes.

   Reason (R): Sex of the child is determined at the time of fertilisation when male and female gamete fuse to form a zygote.

   1. (a)Both (A) and (R) are true and (R) is the correct explanation of (A).
   2. (b)Both (A) and (R) are true and (R) is not correct explanation of (A).
   3. (c)(A) is true, but (R) is false.
   4. (d)(A) is false, but (R) is true.
   Show answer

   (d) (A) is false, but (R) is true.

   Step-by-step solution →
8. Q128 CBSE 2024
   For Questions number 17 and 20, ​two statements are given — one labelled as Assertion (A) ​and ​the other labelled as Reason ​(R). Select the correct answer to these questions from the codes ​(a), ​(b), (c) and ​(d) as given ​below.

   Assertion (A): Chromosomes inherited from the parents decide the sex of a child.

   Reason (R): Y chromosome in a male child is inherited from the father.

   1. (a)Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
   2. (b)Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
   3. (c)Assertion (A) is true and Reason (R) is false.
   4. (d)Assertion (A) is false and Reason (R) is true.
   Show answer

   (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).

   Step-by-step solution →
9. Q129 CBSE 2024
   List ​two ways due ​to ​which variations arise ​in a population. ​State the ​importance of variation.
   Show answer

   Variation arises from DNA copying errors and sexual reproduction; it helps a species survive when its environment changes.

   Variations arise (1) from errors in copying DNA during reproduction, which give small differences even in asexual reproduction, and (2) from sexual reproduction, in which each offspring receives a new combination of DNA from two parents. Importance: variation helps a species survive when its environment changes. If conditions change drastically, individuals with a suitable variation (for example, bacteria that can withstand heat during a heat wave) survive and reproduce, so the species is not wiped out; the selection of such variants over generations is also the basis of evolution.

   Step-by-step solution →
10. Q130 CBSE 2024
    Mendel crossed ​a round and yellow seeded pea plant with a ​wrinkled and ​green seeded ​pea ​plant. What did the plants of ${\text{F}}_1$ generation ​look like in terms of shape and colour of seed ​? On self-pollinating ${\text{F}}_1$ generation plants, plants with four types of combinations of characters were seen in ${\text{F}}_2$ generation. Write the combinations along with their ratios.
    Show answer

    All ${\text{F}}_1$ seeds are round and yellow; ${\text{F}}_2$ shows four combinations in the ratio 9 : 3 : 3 : 1.

    All the ${\text{F}}_1$ plants had round and yellow seeds. In the ${\text{F}}_2$ the four combinations appeared in the ratio round yellow : round green : wrinkled yellow : wrinkled green $=9:3:3:1$.

    Step-by-step solution →
11. Q131 CBSE 2024
    List any two pairs of visible contrasting ​characters of ​garden pea plants used ​by Mendel for ​his experiments stating the ​dominant ​and recessive characters ​in each pair.
    Show answer

    Tall (dominant) and short (recessive) plants; round (dominant) and wrinkled (recessive) seeds.

    Pair 1, height of the plant: tall is dominant and short (dwarf) is recessive. Pair 2, shape of the seed: round is dominant and wrinkled is recessive. Other pairs are equally correct, for example seed colour, yellow (dominant) and green (recessive), or flower colour, violet (dominant) and white (recessive).

    Step-by-step solution →
12. Q132 CBSE 2024
    In ​human beings, the probability ​of getting ​a male or a female child is ​50%. Explain with the help ​of a flow diagram only.
    Show answer

    All eggs carry X and sperms are half X, half Y, so a girl or a boy is equally probable (50% each).

    Flow diagram: Father (XY) → sperms, 50% carrying X and 50% carrying Y. Mother (XX) → eggs, every one carrying X. Fertilisation: egg (X) + sperm (X) → zygote XX → girl; egg (X) + sperm (Y) → zygote XY → boy. The two kinds of sperm are equal in number and either can fertilise the egg, so the probability of a girl and of a boy is 50% each.

    Step-by-step solution →
13. Q133 CBSE 2024
    Mendel crossed pea plants ​with ​two pairs of ​contrasting characters. $$\begin{array}{ccc}\text{RRYY}& \times & \text{rryy}\\ \text{Round, Yellow}& & \text{Wrinkled, Green}\end{array}$$ He observed 4 types of ​combinations ​in ${\text{F}}_2$ generation. Which ​of the combinations were new ? Write the conclusion drawn by this experiment.
    Show answer

    Round green and wrinkled yellow are the new combinations; seed shape and seed colour are inherited independently.

    The new combinations were round seeds with green colour (round, green) and wrinkled seeds with yellow colour (wrinkled, yellow); the other two ${\text{F}}_2$ types, round-yellow and wrinkled-green, are the parental combinations. The four types appear in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. Conclusion: the traits for seed shape and seed colour are inherited independently of each other. The alleles of the two genes are sorted into the gametes independently, which is why new combinations of traits appear in the ${\text{F}}_2$ generation.

    Step-by-step solution →
14. Q134 CBSE 2024
    Explain ​how equal ​genetic contribution of male ​and ​female ​parents is ​ensured ​in the ​progeny.
    Show answer

    Sperm and egg each carry one chromosome of every pair, so the zygote gets half its chromosomes from each parent.

    Every body cell carries its chromosomes in pairs ($46$ in humans, as $23$ pairs), one member of each pair having come from each parent. When germ cells are formed, the chromosome number is halved, so each sperm and each egg receives only one chromosome of every pair, $23$ in all. At fertilisation a sperm fuses with an egg, and the zygote gets $23$ chromosomes from the father and $23$ from the mother, restoring $46$ ($23$ pairs). Every pair in the child therefore has one paternal and one maternal chromosome, so both parents contribute equally to the progeny.

    Step-by-step solution →
15. Q135 CBSE 2024
    Explain how ​the ​original number of chromosomes present in the parents are restored in the progeny. Name the cell division by which ​chromosome number is ​maintained in the ​progeny.
    Show answer

    Meiosis halves the chromosome number in germ cells and fertilisation joins the two halves, restoring the original number.

    Germ cells (sperm and egg) are formed by meiosis, so each carries only half the chromosome number of a body cell, one chromosome of each pair ($23$ instead of $46$ in humans). At fertilisation a male and a female germ cell fuse, and the zygote receives one half set from each parent, so the original number is restored in the progeny ($23+23=46$ in humans). The cell division is meiosis.

    Step-by-step solution →
16. Q136 CBSE 2024
    List ​two differences between dominant traits and recessive ​traits. What percentage of ​pea plants ​in the ${\text{F}}_2$ generation were with yellow seeds in Mendel's cross between the pea plants having yellow (YY) ​and green coloured (yy) ​seeds ?
    Show answer

    Dominant traits show with one gene copy and appear in ${\text{F}}_1$, recessive ones need two copies; $75\%$ of ${\text{F}}_2$ plants are yellow-seeded.

    Differences: (1) A dominant trait is expressed even when only one copy of its gene is present (Yy plants are yellow), whereas a recessive trait is expressed only when both copies of the gene are recessive (yy plants are green). (2) A dominant trait appears in the ${\text{F}}_1$ generation, whereas a recessive trait remains hidden in ${\text{F}}_1$ and reappears in the ${\text{F}}_2$ generation. Percentage: the ${\text{F}}_1$ plants are all Yy; on self-pollination they give $\text{YY}:\text{Yy}:\text{yy}=1:2:1$ in ${\text{F}}_2$, so the plants with yellow seeds (YY and Yy) are $\frac{3}{4}$ of the total, i.e. $75\%$.

    Step-by-step solution →
17. Q137 CBSE 2024
    How ​is the sex of a newborn individual ​determined in ​different species of animals ​? ​Give ​three examples ​to support ​your answer.
    Show answer

    Sex is set by egg temperature in some reptiles, changes in snails, and depends on sex chromosomes in humans ($XX$ girl, $XY$ boy).

    Different species determine the sex of the newborn in different ways. (1) In some animals, such as some reptiles (for example, many turtles), the temperature at which the fertilised eggs are kept decides whether the young developing in them will be male or female. (2) In animals such as snails, individuals can change sex, which shows that sex is not genetically determined. (3) In human beings, sex is largely genetically determined by the sex chromosomes: women are XX and men are XY. Every egg carries an X chromosome, while half the sperms carry X and half carry Y. An egg fertilised by a sperm carrying X gives a girl (XX), and one fertilised by a sperm carrying Y gives a boy (XY). So the sex of a human child depends on the chromosome inherited from the father, and a boy or a girl is equally likely.

    Step-by-step solution →
18. Q138 CBSE 2024
    (a) Name any two pairs of contrasting characters other than ​height used by Mendel in his experiments. (b) ​On ​crossing a tall and a short ​pea plant, ​Mendel obtained ${\text{F}}_1$ generation in which all ​pea plants were tall.

    1. (i)Are these plants exactly the same as tall plants of the parent generation ? Write ​their gene combination.
    2. (ii)Give the percentage of short plants obtained in ${\text{F}}_2$ generation when ${\text{F}}_1$ plants are self-pollinated.
    3. (iii)In what ratio would you find TT and Tt in ${\text{F}}_2$ ​generation ?
    Show answer

    ${\text{F}}_1$ plants are $Tt$, unlike the pure $TT$ parent; $25\%$ of ${\text{F}}_2$ are short, and $TT:Tt=1:2$.

    (a) Any two of: round / wrinkled seeds; violet / white flowers; yellow / green seeds. (b)(i) No. They look the same (tall), but the tall parent was pure tall (TT), whereas the ${\text{F}}_1$ plants carry the gene for shortness too; their gene combination is Tt. (ii) $25\%$ of the ${\text{F}}_2$ plants are short (tt). (iii) $\text{TT}:\text{Tt}=1:2$.

    Step-by-step solution →
19. Q139 CBSE 2024
    Mendel crossed ​pure tall pea plants ​(TT) with pure short pea ​plants (tt) and obtained ${\text{F}}_1$ progeny. When the plants of ${\text{F}}_1$ progeny were self-pollinated, ​plants ​of ${\text{F}}_2$ progeny were obtained.

    1. (a)What did the ​plants of ${\text{F}}_1$ progeny look like ? Give their gene combination.
    2. (b)Why could the gene for shortness not be expressed in plants of ${\text{F}}_1$ progeny ?
    3. (c)Write the ratio of the plants obtained in ${\text{F}}_2$ progeny and state the ​conclusion that can be drawn ​from this experiment.
    Show answer

    ${\text{F}}_1$ plants are all tall ($\text{Tt}$) because tallness is dominant; ${\text{F}}_2$ gives tall : short $=3:1$, so the recessive trait reappears.

    (a) All the ${\text{F}}_1$ plants were tall; their gene combination is Tt. (b) The factor for tallness (T) is dominant over the factor for shortness (t), which is recessive. A single copy of T is enough to make the plant tall, so in Tt plants the gene for shortness is present but is not expressed. (c) In ${\text{F}}_2$, tall : short $=3:1$ (gene combinations TT : Tt : tt $=1:2:1$). Conclusion: both the tallness and the shortness traits were inherited by the ${\text{F}}_1$ plants, but only tallness was expressed; each plant carries two copies of the trait, one from each parent, and the recessive trait reappears in ${\text{F}}_2$ when two t copies come together.

    Step-by-step solution →
20. Q140 CBSE 2024
    A plant with violet flowers ​(VV) was crossed with a plant with white flowers (vv).

    1. (a)What colour of flowers was obtained in the ​plants of ${\text{F}}_1$ generation ​and why ?
    2. (b)Write ​the percentage of plants with white flowers in ${\text{F}}_2$ generation plants, if ${\text{F}}_1$ plants were self-pollinated. Give reason why this trait was not expressed in ${\text{F}}_1$ generation.
    3. (c)In what ratio did we get the plants ​with (VV) and (Vv) gene ​combination in the ${\text{F}}_2$ generation ​?
    Show answer

    ${\text{F}}_1$ plants are violet ($\text{Vv}$, violet dominant); 25% of ${\text{F}}_2$ plants are white, as white is recessive; $\text{VV}:\text{Vv}=1:2$.

    (a) All the ${\text{F}}_1$ plants have violet flowers. Each receives V from the violet parent and v from the white parent, so it is Vv, and V (violet) is dominant over v (white). (b) In ${\text{F}}_2$, $\text{VV}:\text{Vv}:\text{vv}=1:2:1$, so plants with white flowers (vv) are 1 in 4, that is 25%. White was not expressed in ${\text{F}}_1$ because it is a recessive trait: in Vv plants it is masked by the dominant V, and it shows only when both copies are v. (c) $\text{VV}:\text{Vv}=1:2$.

    Step-by-step solution →