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 12 of 15

1. Q221 CBSE 2019
   Define genetics. Why is decrease in ​the ​number of surviving ​tigers a cause ​of concern from the point ​of view ​of genetics ? Explain ​briefly.
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   Genetics studies heredity and variation; fewer tigers mean inbreeding and less variation, so a change in the environment could make the species extinct.

   Genetics is the branch of biology that deals with the study of heredity and variation. When the number of surviving tigers decreases, the few that remain have to breed among themselves (inbreeding), so little variation appears among their offspring. Variation is what allows some individuals of a species to survive when the environment changes; with little variation, a change in the environment (for example a new disease) may leave no tiger able to cope, and the species may become extinct.

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2. Q222 CBSE 2019
   Name ​the ​plant Mendel used for ​his experiment. What type of ​progeny was obtained by Mendel in ${\text{F}}_1$ and ${\text{F}}_2$ ​generations when he crossed the ​tall and short plants? Write the ​ratio ​he obtained in ${\text{F}}_2$ generation plants.
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   Garden pea; ${\text{F}}_1$ all tall; ${\text{F}}_2$ tall and short plants in the ratio $3:1$.

   Mendel used the garden pea plant (Pisum sativum). ${\text{F}}_1$: all the plants were tall (none short or of medium height). ${\text{F}}_2$ (from self-pollinating the ${\text{F}}_1$): both tall and short plants. Ratio in ${\text{F}}_2$: tall : short $=3:1$ (in terms of factors, $\text{TT}:\text{Tt}:\text{tt}=1:2:1$).

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3. Q223 CBSE 2019
   Explain the ​following :

   1. (a)Speciation
   2. (b)​Natural ​Selection
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   Speciation is the origin of a new species from an existing one after isolated groups diverge; natural selection spreads variations that help survival.

   (a) Speciation: It is the process by which new species arise from a pre-existing species. It happens when a population is split into groups that cannot interbreed, for example by geographical isolation (a large river or a mountain range), which stops the flow of genes between them. In each isolated group, genetic drift (random changes in the frequencies of genes, marked in small populations) and natural selection (different surroundings favouring different variations) bring about different changes over generations. When the differences become large, for example changes in the DNA or in the number of chromosomes, members of the two groups cannot reproduce with each other even if they meet, and a new species has formed. (b) Natural selection: It is the process by which, among the variations present in a population, those that give an advantage in a particular environment help their bearers survive and reproduce more, so that these variations become more common in the population over generations. For example, in a population of red beetles living on green bushes, a green variant is hidden from crows, survives and reproduces more, and over generations most of the beetles become green.

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4. Q224 CBSE 2019
   List ​two ​differences ​between acquired traits ​and ​inherited traits by giving an ​example ​of each.
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   Acquired traits (swimming skill) are not passed on, as germ-cell DNA is unchanged; inherited traits (eye colour) are controlled by germ-cell DNA.

   (1) Acquired traits are not passed on from one generation to the next; inherited traits are passed from parents to their offspring. (2) Acquired traits bring about no change in the DNA of the germ cells (they change only body cells); inherited traits are controlled by the DNA (genes) of the germ cells. Examples: acquired trait, a skill such as swimming or cycling; inherited trait, eye colour (or free or attached earlobes). Any other correct difference and example is equally acceptable.

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5. Q225 CBSE 2019
   Mendel, in one of his experiments with pea ​plants, crossed a ​variety ​of pea plant having round seeds with one having ​wrinkled ​seeds. State Mendel’s observations giving ​reasons of ${\text{F}}_1$ and ${\text{F}}_2$ progeny of this cross. Also, ​list any two contrasting characters, other than round seeds of pea plants that Mendel used in ​his experiments.
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   ${\text{F}}_1$ was all round ($Rr$) as round is dominant; ${\text{F}}_2$ gave round and wrinkled in about $3:1$ because the factors separate in gametes.

   ${\text{F}}_1$ progeny: All the ${\text{F}}_1$ plants had round seeds; no wrinkled seeds appeared. Reason: each ${\text{F}}_1$ plant received a factor for round seeds ($\text{R}$) from one parent and a factor for wrinkled seeds ($\text{r}$) from the other, so it was $\text{Rr}$. Round is the dominant trait, so it was expressed, while wrinkled is recessive and remained hidden. ${\text{F}}_2$ progeny: When the ${\text{F}}_1$ plants were self-pollinated, both round and wrinkled seeds appeared, in the ratio of about $3$ round to $1$ wrinkled. Reason: the two factors $\text{R}$ and $\text{r}$ separate when the gametes are formed, each gamete getting only one. Random fusion of the gametes gives $\text{RR}$, $\text{Rr}$, $\text{Rr}$ and $\text{rr}$ in the ratio $1:2:1$; $\text{RR}$ and $\text{Rr}$ give round seeds and $\text{rr}$ gives wrinkled seeds, so the recessive trait reappears in one quarter of the plants. Two other contrasting characters Mendel used (any two): tall and dwarf plants; yellow and green seeds; violet (purple) and white flowers; green and yellow pods; full (inflated) and constricted pods; axial and terminal flowers.

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6. Q226 CBSE 2019
   1. (a)What ​are dominant and recessive traits ?
   2. (b)“Is it possible that a ​trait ​is inherited ​but may ​not ​be expressed in the next generation ​?” Give a suitable example to justify this statement.
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   Yes: a recessive trait is inherited but hidden by the dominant one in $F_1$, and reappears in $F_2$, as shortness does in pea plants.

   (a) Dominant trait: of a pair of contrasting traits, the one that is expressed in the ${\text{F}}_1$ (first) generation after a cross; it shows even when only one factor for it is present. Recessive trait: the contrasting trait that is not expressed in the ${\text{F}}_1$ generation although it is inherited; it shows only when both factors for it are recessive. (b) Yes. Example: Mendel crossed pure tall pea plants ($\text{TT}$) with pure short ones ($\text{tt}$). All the ${\text{F}}_1$ plants were tall ($\text{Tt}$): each had inherited the factor for shortness ($\text{t}$) from its short parent, but it was not expressed because tallness is dominant. When the ${\text{F}}_1$ plants were self-pollinated, the ${\text{F}}_2$ generation had tall and short plants in the ratio $3:1$ ($\text{TT}$, $\text{Tt}$, $\text{Tt}$ : $\text{tt}$). The short trait reappeared, showing that it had been inherited by the ${\text{F}}_1$ plants without being expressed in them.

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7. Q227 CBSE 2019
   (a) If we cross pure-bred tall (dominant) pea plants with pure-bred dwarf (recessive) pea plants we get pea plants of ${\text{F}}_1$ generation. If we now self-cross ​the pea plants ​of ${\text{F}}_1$ generation, then we obtain pea plants of ${\text{F}}_2$ generation.

   1. (i)​What do the plants of ${\text{F}}_1$ generation look like ​?
   2. (ii)What is the ratio of tall plants to dwarf plants ​in ${\text{F}}_2$ generation ?
   3. (iii)State the type of plants ​not found in ${\text{F}}_1$ generation but appeared in ${\text{F}}_2$ generation, mentioning the reason ​for the same. (b) What are homologous structures ? Give an example. Is it necessary that homologous structures always have common ancestors ?
   Show answer

   $F_1$ plants are all tall and $F_2$ is $3:1$ tall to dwarf; homologous structures share a basic design and indicate a common ancestor.

   (a) (i) All the plants of the ${\text{F}}_1$ generation are tall (genotype Tt). (ii) Tall : dwarf = $3:1$ in the ${\text{F}}_2$ generation. (iii) Dwarf plants are not found in ${\text{F}}_1$ but appear in ${\text{F}}_2$. Reason: tallness is dominant and dwarfness is recessive. The ${\text{F}}_1$ plants (Tt) carry the factor for dwarfness, but it is masked by the dominant factor. During gamete formation the two factors (genes) segregate, so on self-pollination one-fourth of the ${\text{F}}_2$ plants receive the dwarf factor through both gametes, the pollen and the egg (tt), and are dwarf. (b) Homologous structures are organs that have the same basic structure (structural plan) and origin but perform different functions. Example: the forelimbs of humans and the wings of birds. Yes: homologous structures are inherited from a common ancestor, and their common basic design is evidence of that common ancestry.

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8. Q228 CBSE 2019
   1. (a)​“Sexual reproduction gives rise ​to more ​viable variations than asexual reproduction.” Justify this statement and explain how the viable ​variations affect the evolution of those ​organisms that reproduce sexually.
   2. (b)Does genetic ​combination ​of mothers play a significant role in determining the sex of ​a newborn ? Give reason to justify your answer.
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   Sexual reproduction gives more viable variations, helping organisms survive change and evolve; the mother does not decide sex, as all her eggs carry X.

   (a) In asexual reproduction one parent's DNA is copied, so the offspring are almost identical; the only variations come from small errors in DNA copying. In sexual reproduction two parents form gametes, each with half of its DNA, and fertilisation combines DNA from two different individuals, so every offspring gets a new combination of the variations present in both parents. These variations have already been carried by living, reproducing parents, so they are not seriously harmful: they are viable. Hence sexual reproduction gives rise to more viable variations. Effect on evolution: in a population with many variations, some individuals are better suited to a changed environment. They survive and reproduce more (natural selection) and pass their useful variations to the next generation. Over many generations these variations accumulate, the population changes and new species may arise. So viable variations help sexually reproducing organisms to survive changes and to evolve. (b) No. The mother has the sex chromosomes XX, so every egg carries an X chromosome. The father has XY, so half of his sperms carry X and half carry Y. An X-carrying sperm gives an XX zygote (girl) and a Y-carrying sperm an XY zygote (boy). The sex of the newborn therefore depends on the sperm from the father, by chance, and not on the genetic make-up of the mother.

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9. Q229 CBSE 2019
   1. (a)List in tabular form two ​differences between ​acquired ​traits and inherited traits.
   2. (b)Give an example of ​body characteristics ​used ​to determine how close ​two species are in ​terms of evolution and explain it.
   Show answer

   Acquired traits are not passed on, inherited traits are; the more homologous organs two species share, such as forelimbs, the closer their evolutionary relationship.

   (a) Differences between acquired and inherited traits (any two; set out as a table in step 3): 1. Acquired traits are developed by an individual during its lifetime (e.g. loss of weight of a starving beetle); inherited traits are received from the parents (e.g. eye colour). 2. Acquired traits do not change the DNA of the germ cells, so they are not passed on to the next generation; inherited traits are coded in the DNA of the germ cells and are passed on. 3. Acquired traits cannot direct evolution; inherited traits can direct evolution. (b) Homologous organs, for example the forelimbs of a human being, a cat, a whale and a bat. They have the same basic structure (humerus, radius and ulna, wrist bones, hand and finger bones) but are modified to perform different functions: grasping, walking, swimming and flying. This common design has been inherited from a common ancestor, so homologous characteristics show how closely species are related: the more of them two species share, the closer they are in evolution.

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10. Q230 CBSE 2019
    1. (a)How is equal ​genetic contribution ​of ​male ​and female parents ensured in the progeny ? Explain.
    2. (b)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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    Gametes carry half the chromosomes, so each parent contributes equally; animal diversity arose by branching from common ancestors, not from diverse ancestry.

    (a) In sexual reproduction the progeny develops from a zygote formed by the fusion of two gametes, a sperm from the father and an egg from the mother. Gametes are formed by a special cell division that halves the chromosome number, so each carries only one chromosome of every pair: a human body cell has $46$ chromosomes ($23$ pairs), while a sperm and an egg have $23$ each. At fertilisation $23+23=46$ restores the normal number, and every pair in the zygote has one chromosome from each parent. So each parent contributes exactly half of the chromosomes, and of the DNA, of the progeny. (b) No. In the light of evolution, the diversity of animals is the result of divergence from common ancestors, not evidence of diverse ancestry. Variations arise and accumulate, natural selection and genetic drift act on them, and when a population is split, for example by geographical isolation, the separated groups become new species. Repeated over millions of years, this branching gave today's variety. Homologous organs, such as the forelimbs of a frog, a lizard, a bird and a human, and fossils such as Archaeopteryx show that very different animals share common ancestors.

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11. Q231 CBSE 2019
    1. (a)​What are homologous structures ? Give ​an example.
    2. (b)​“The sex ​of a newborn child is a ​matter ​of chance and none of the parents may ​be considered responsible for it.” Justify this statement with the help of a ​flow chart showing sex-determination in human beings.
    Show answer

    Homologous structures share a basic design but differ in function; the fertilising sperm (X or Y) decides sex by chance, not either parent.

    (a) Homologous structures are organs of different organisms that have the same basic structure (and a similar origin) but perform different functions. Example: the forelimbs of a frog, a lizard, a bird and a human being (for instance, the wing of a bird and the arm of a human being) have the same basic design of bones but are used for different functions. (b) Flow chart of sex determination in human beings: Parents: Father (XY) × Mother (XX) Gametes: sperms with X and sperms with Y (in equal numbers); eggs, all with X Zygotes: X sperm + X egg → XX (girl); Y sperm + X egg → XY (boy) Justification: the mother always contributes an X chromosome, so the sex of the child depends only on whether the sperm that fertilises the egg carries X or Y. Both kinds of sperm are formed in equal numbers, and which one fertilises the egg is purely a matter of chance, so a girl and a boy are equally likely. Hence the sex of a newborn child is a matter of chance, and neither parent can be held responsible for it.

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12. Q232 CBSE 2019
    (a) ​With the help of Mendel’s experiments show that

    1. (i)​traits may be dominant or recessive, and
    2. (ii)traits are inherited ​independently. (b) Birds and ​bats are more closely related to ​each other ​than ​to squirrels or lizards. Comment.
    Show answer

    Mendel's $3:1$ and $9:3:3:1$ ratios show dominance and independent inheritance; bird and bat wings are analogous, so the statement is wrong.

    (a) (i) Mendel crossed pure tall pea plants ($\text{TT}$) with pure short plants ($\text{tt}$). All the ${\text{F}}_1$ plants were tall; none was short or of medium height. When the ${\text{F}}_1$ plants were self-pollinated, the ${\text{F}}_2$ had tall and short plants in the ratio $3:1$. The short trait hidden in the ${\text{F}}_1$ reappeared, so the ${\text{F}}_1$ plants carried it without showing it: the trait that is expressed (tallness) is dominant and the one that is hidden (shortness) is recessive. (ii) Mendel crossed pure plants having round, yellow seeds ($\text{RRYY}$) with pure plants having wrinkled, green seeds ($\text{rryy}$). All the ${\text{F}}_1$ plants had round, yellow seeds. On self-pollination of the ${\text{F}}_1$, the ${\text{F}}_2$ had round yellow, round green, wrinkled yellow and wrinkled green seeds in the ratio $9:3:3:1$. New combinations (round green and wrinkled yellow) not present in either parent appeared, so seed shape and seed colour are inherited independently of each other. (b) The statement is not correct. The wings of birds and bats are analogous organs: they have the same use, flying, but a different design, since a bat's wing is a fold of skin stretched between elongated fingers while a bird's wing is a feathery covering all along the arm. A common use does not show close relationship. Bats share far more basic features with squirrels (both are mammals with hair that give birth to young ones and feed them milk), and birds with lizards (both lay eggs with shells, and birds evolved from reptiles). So bats are more closely related to squirrels, and birds to lizards.

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13. Q233 CBSE 2019
    1. (a)List two visible traits of garden pea that Mendel considered in his experiments. How ​do ​Mendel’s experiments ​show that traits may be dominant or recessive ?
    2. (b)With the help of a flow ​diagram, how would you ​establish ​that ​in human beings the sex of a newborn is purely a matter of chance and none of the parents may be considered responsible for a ​particular sex of a newborn child ?
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    Tall and dwarf pea crosses show dominant and recessive traits; sex depends on whether an X or Y sperm fertilises the egg, by chance.

    (a) Two visible traits: height of the plant (tall or dwarf) and shape of the seed (round or wrinkled); others are seed colour (yellow or green) and flower colour (violet or white). Mendel crossed pure tall plants (TT) with pure dwarf plants (tt). All the ${\text{F}}_1$ plants were tall (Tt), although each carried a factor for dwarfness; on self-pollination, the ${\text{F}}_2$ had tall and dwarf plants in the ratio $3:1$. The trait expressed in ${\text{F}}_1$ (tallness) is dominant; the trait that remained hidden in ${\text{F}}_1$ and reappeared in ${\text{F}}_2$ (dwarfness) is recessive. A single copy of T makes a plant tall (TT and Tt are tall), while only tt plants are dwarf. (b) Flow diagram: Father XY $\times$ Mother XX $\to$ gametes: sperms X or Y (in equal numbers), eggs X only $\to$ egg X + sperm X gives XX (girl); egg X + sperm Y gives XY (boy). The mother gives only X, so she cannot decide the sex; whether an X-sperm or a Y-sperm fuses with the egg is purely a matter of chance, so a girl and a boy are equally likely ($1:1$) and neither parent is responsible for the sex of the newborn.

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14. Q234 CBSE 2019
    (a) How do ​the ​following provide evidences in ​favour ​of evolution in organisms ? Explain with an example for each.

    1. (i)​Homologous organs
    2. (ii)Analogous organs
    3. (iii)Fossils (b) Explain two ​methods ​to determine the age of fossils.
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    Homologous and analogous organs and fossils support evolution; fossil age comes from the depth of the layer and from carbon-isotope ratios.

    (a)(i) Homologous organs have the same basic structure (and origin) but perform different functions, e.g. the forelimbs of a human (grasping) and of a lizard (walking): both have one upper arm bone, two forearm bones, wrist bones and finger bones. The common plan shows that these animals descended from a common ancestor; the differences arose by adaptation. (ii) Analogous organs have different basic structures (and origin) but perform the same function, e.g. the wings of an insect (a thin membrane, no bones) and the wings of a bat (a forelimb with skin stretched between the fingers). They show that unrelated organisms develop similar organs when they adapt to the same way of life - the similarity comes from adaptation, not from a common ancestor. (iii) Fossils are the remains or impressions of organisms that lived in the past, preserved in rocks. They show what extinct organisms were like and how organisms changed over time, e.g. Archaeopteryx, with feathers and wings like a bird but teeth and a long bony tail like a reptile, links reptiles and birds. (b) Method 1 - relative dating: fossils found in layers closer to the surface are more recent than those found in deeper layers, because newer layers settle on top of older ones. Method 2 - isotope (carbon) dating: the ratio of different isotopes of the same element, such as carbon-14 to carbon-12, in the fossil is measured; carbon-14 decays at a known rate after death, so the ratio gives the age of the fossil.

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15. Q235 CBSE 2019
    1. (a)List two reasons in favour ​of Mendel’s choice of pea plants for his experiments.
    2. (b)“The geographical isolation of ​individuals of a ​species can lead to ​formation of a ​new species.” Provide suitable explanation ​to ​justify this statement.
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    Mendel chose peas for contrasting traits and pure-breeding lines; a geographical barrier stops gene flow, so isolated groups accumulate differences and become new species.

    (a) (i) The pea plant has many clearly contrasting visible traits, such as tall and dwarf plants, round and wrinkled seeds, and violet and white flowers, which are easy to observe and count. (ii) Its flowers are bisexual and normally self-pollinate, so pure-breeding varieties are available, and cross-pollination can be carried out by hand when needed. (Other correct reasons: a short life cycle; many seeds from each cross.) (b) When a geographical barrier such as a river or a mountain range splits a population of a species into two, the individuals of the two groups can no longer interbreed, so gene flow between them stops. Over many generations, each group accumulates different variations: genetic drift changes gene frequencies at random, and natural selection favours different variations in the two different environments. Eventually the groups become so different (for example, in their DNA or number of chromosomes) that even if they meet again they cannot interbreed to produce fertile offspring. They have become separate species, so geographical isolation can lead to the formation of a new species.

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16. Q236 CBSE 2018
    A Mendelian experiment consisted of breeding pea plants ​bearing violet flowers ​with ​pea plants ​bearing white flowers. What will ​be the ​result in ${\text{F}}_1$ ​progeny ?
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    All ${\text{F}}_1$ plants bear violet flowers, because violet is dominant over white.

    All the ${\text{F}}_1$ progeny bear violet flowers. Violet is dominant over white: each ${\text{F}}_1$ plant receives a factor for violet from one parent and a factor for white from the other ($\text{Vv}$), and only the dominant violet is expressed.

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17. Q237 CBSE 2018
    Mendel took tall ​pea plants ​and ​short pea ​plants and produced ${\text{F}}_1$ progeny through cross-fertilisation. What did Mendel observe in the ${\text{F}}_1$ ​progeny ​?
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    All ${\text{F}}_1$ plants were tall; no short plant and no plant of medium height appeared.

    All the plants of the ${\text{F}}_1$ progeny were tall. No short plant, and no plant of medium (intermediate) height, appeared: only one of the two parental traits, tallness, was expressed.

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18. Q238 CBSE 2018
    What ​are acquired ​traits ​? ​Why are these ​traits generally not ​inherited over generations ​? Explain.
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    Acquired traits are developed during a lifetime and change only body tissues, not germ-cell DNA, so they are generally not inherited.

    Acquired traits are characteristics that an organism develops during its own lifetime, because of its environment, the use or disuse of its organs, training or accident; for example, the low weight of a beetle that has been starved, muscles built up by exercise, or a tail cut off by surgery. They are generally not inherited because they are changes in the non-reproductive (body) tissues only. They do not change the DNA of the germ cells (the reproductive cells), and only the DNA of the germ cells is copied into the gametes and passed on to the next generation. For example, if the tails of mice are cut off in every generation, their offspring are still born with tails, because the cut does not change the genes of the germ cells.

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19. Q239 CBSE 2018
    What ​is ​speciation ? ​List four ​factors ​responsible ​for ​this phenomenon.
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    Speciation is the formation of a new species from a pre-existing one; its factors are geographical isolation, genetic drift, natural selection and mutation.

    Speciation is the formation of a new species from a pre-existing species: groups of one population change so much that they can no longer interbreed. Four factors responsible for speciation: (i) Geographical isolation: a barrier such as a river or a mountain range divides the population, so gene flow between the sub-populations stops. (ii) Genetic drift: chance changes in the frequency of genes, strongest in small sub-populations. (iii) Natural selection: different surroundings favour different variations in each sub-population. (iv) Mutation: severe changes in DNA, such as a change in the number of chromosomes, so that the germ cells of the two groups cannot fuse. Variation is also accepted as a factor.

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20. Q240 CBSE 2018
    What are ​fossils ? ​How ​are these formed ​? List ​two methods of determining ​the age of fossils.
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    Fossils are preserved remains of past organisms, formed by burial in sediment that hardens; age is found from layer depth or carbon dating.

    Fossils: the preserved remains or traces of organisms that lived long ago, such as bones, teeth and shells, or impressions of bodies and leaves, found in rocks. Formation: usually a dead organism decomposes completely. Sometimes the body, or a part of it, is buried quickly in mud, silt or sand (often at the bottom of a lake or sea), where it cannot decompose completely. Over a long time the mud hardens into rock that keeps the impression of the body, and hard parts such as bones and shells are replaced by minerals and turn to stone. This preserved form is a fossil. Two methods of determining the age of fossils: (i) Depth of the layer (relative method): fossils found in layers closer to the surface of the earth are more recent than those found in deeper layers. (ii) Carbon dating (isotope dating): the ratio of different isotopes of the same element, such as carbon-14 to carbon-12, is detected in the fossil material; the less carbon-14 is left, the older the fossil.

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