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 13 of 15

1. Q241 CBSE 2018
   What ​are fossils ​? Write ​in brief any two methods of determining the ​age of ​fossils.
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   Fossils are preserved remains of past organisms; their age comes from the depth of the rock layer or carbon dating.

   Fossils are the preserved remains, impressions or traces of organisms that lived in the remote past, such as a dead insect or the impression of a leaf preserved in rock. Two methods of determining the age of fossils: (i) Relative method (by digging): when we dig into the earth, the fossils found closer to the surface are more recent than those found in the deeper layers, because newer layers settle over older ones. (ii) Carbon dating: the ratio of different isotopes of the same element (carbon-14 to carbon-12) in the fossil material is detected. Carbon-14 decays at a fixed rate after the organism dies, its amount halving every $5730$ years, so this ratio gives the age of the fossil.

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2. Q242 CBSE 2018
   What ​is ​speciation ? List ​four ​factors ​responsible for speciation.
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   Speciation is the formation of a new species from an existing one; four factors are geographical isolation, genetic drift, natural selection and mutation.

   Speciation: the process by which a new species arises from an existing (pre-existing) species, when a population splits into groups whose members can no longer reproduce with one another. Four factors responsible for speciation: (i) Geographical isolation: a barrier such as a river or a mountain range splits a population into sub-populations and stops the flow of genes (gene flow) between them. (ii) Genetic drift: random changes in the frequency of genes, marked in small sub-populations, make the isolated groups drift apart. (iii) Natural selection: different surroundings favour different variations, so each sub-population changes in a different way. (iv) Mutation (severe changes in the DNA), such as a change in the number of chromosomes, after which the germ cells of the two groups cannot fuse and the groups can no longer interbreed. (Variation within the population is also a correct factor, since drift and selection act on it.)

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3. Q243 CBSE 2017
   What ​is speciation ? State ​any ​two factors which ​could ​lead to ​speciation.
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   Speciation is the formation of a new species from an existing one; geographical isolation and genetic drift can lead to it.

   Speciation is the process by which a new species develops from an existing species: a population splits into groups that change until their members can no longer interbreed with each other. Two factors that could lead to speciation (any two): 1. Geographical isolation — a barrier such as a river or a mountain range divides a population, so little or no gene flow takes place between the two parts. 2. Genetic drift — random changes in the frequency of genes accumulate differently in the separated sub-populations over generations. (Also accepted: natural selection operating differently in the two places; severe changes in DNA, such as a change in the number of chromosomes, so that the germ cells of the two groups cannot fuse.)

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4. Q244 CBSE 2017
   What are analogous ​organs ? Why cannot the wing of ​a ​butterfly and the wing ​of a bat ​be considered homologous organs ? State ​one ​reason.
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   Analogous organs have different structure and origin but similar function; bat and butterfly wings differ in origin, so they are not homologous.

   Analogous organs are organs that have a different basic structure and a different origin but perform a similar function (and may look alike) - for example, the wings of an insect and the wings of a bird or a bat. The wing of a butterfly and the wing of a bat cannot be considered homologous organs because homologous organs must have the same basic structure and origin, and these two do not. Reason: the bat’s wing is a modified forelimb, a fold of skin stretched between the bones of the arm and the long fingers, whereas the butterfly’s wing is a thin membrane growing out of the body wall, supported by veins and with no bones. They only perform the same function, flying, so they are analogous organs.

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5. Q245 CBSE 2017
   List in ​tabular form any two distinguishing features between ​the ​acquired ​traits ​and ​the inherited traits.
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   Acquired traits develop in one's lifetime and are not passed on; inherited traits come from parents' genes and are passed on.

   Distinguishing features (the table is shown in step 2): 1. Origin — Acquired traits develop during the lifetime of an individual, through its experiences or surroundings (e.g. the low weight of a starved beetle, a skill that has been learnt). Inherited traits are received from the parents through their genes (e.g. eye colour). 2. Transmission — Acquired traits are changes in body (non-reproductive) tissues and are not passed on to the next generation. Inherited traits are coded in the DNA of the germ cells and are passed on from one generation to the next.

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6. Q246 CBSE 2017
   How did Mendel interpret the results of his experiments to ​show ​that ​“the traits ​may be dominant ​or recessive” ? Explain ​briefly.
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   All ${\text{F}}_1$ were tall and ${\text{F}}_2$ gave tall : short $\approx 3:1$, so the hidden trait (shortness) is recessive and the expressed trait (tallness) dominant.

   Mendel crossed pure-breeding tall pea plants with pure-breeding short ones. All the F₁ plants were tall; none was short or of medium height. When he let the F₁ plants self-pollinate, the F₂ generation had both tall and short plants, in the ratio of about 3 tall : 1 short. Interpretation: since short plants reappeared in F₂, the F₁ plants must have inherited both the tallness and the shortness factors, but only tallness was expressed in them. So each plant carries two copies of the factor (gene) for a trait, and the F₁ plants were Tt. The trait expressed when the two copies differ, tallness, is dominant; the trait that stays hidden in F₁ and shows only when both copies are t (tt), shortness, is recessive. TT and Tt plants are tall, and only tt plants are short.

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7. Q247 CBSE 2017
   Mention the total number of chromosomes ​along ​with the sex chromosomes that are present in a human ​female and a human male. Explain how in sexually producing organisms the number ​of chromosomes in the ​progeny remains the same as ​that of the ​parents.
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   Female $44+\text{XX}$, male $44+\text{XY}$ (46 each); meiosis halves the number in gametes and fertilisation restores $23+23=46$.

   A human female has $46$ chromosomes ($23$ pairs): $22$ pairs of autosomes and one pair of sex chromosomes, XX, that is $44+\text{XX}$. A human male also has $46$: $22$ pairs of autosomes and the sex chromosomes XY, that is $44+\text{XY}$. In sexually reproducing organisms the germ cells (gametes) are formed by a special cell division, meiosis, in which each gamete receives only one chromosome of each pair, that is half the number: in humans an egg has $22+\text{X}=23$ chromosomes and a sperm has $22+\text{X}$ or $22+\text{Y}=23$. At fertilisation a male gamete fuses with a female gamete, and the zygote gets one set from each parent, so the original number is restored ($23+23=46$). The progeny therefore have the same number of chromosomes as the parents.

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8. Q248 CBSE 2017
   How ​did ​Mendel’s ​experiments ​show that ​different traits ​are inherited ​independently ? Explain.
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   Self-pollinated ${\text{F}}_1$ gave round yellow, round green, wrinkled yellow and wrinkled green in the ratio $9:3:3:1$, showing the traits are inherited independently.

   Mendel crossed pure-breeding pea plants with round, yellow seeds ($\text{RRYY}$) with plants having wrinkled, green seeds ($\text{rryy}$). The ${\text{F}}_1$ plants all had round, yellow seeds ($\text{RrYy}$), showing that round and yellow are dominant. When the ${\text{F}}_1$ plants were self-pollinated, the ${\text{F}}_2$ generation had four kinds of seed: round-yellow, round-green, wrinkled-yellow and wrinkled-green, in the ratio $9:3:3:1$. Round-green and wrinkled-yellow are new combinations, found in neither parent. They appeared, about 3 in every 16 seeds each, as independent separation predicts, because the factors for seed shape and for seed colour separated from each other when the gametes formed, each gamete receiving one factor for shape and one for colour at random (RY, Ry, rY and ry in equal numbers). This showed that the two traits are inherited independently of each other. Each trait on its own still gave a $3:1$ ratio (round : wrinkled $=12:4$, yellow : green $=12:4$).

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9. Q249 CBSE 2017
   With ​the ​help of two suitable examples, explain why certain experiences ​and traits earned by ​people during their ​lifetime ​are not ​passed on ​to their next generations. When can such traits be passed on ?
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   Acquired traits change only body tissues, not germ-cell DNA, so they are not inherited; a change in germ-cell DNA can be passed on.

   Examples: (1) A person who learns to swim, dance or speak a new language acquires the skill by practice; their children are not born with it and have to learn it themselves. (2) A person who builds strong muscles by regular exercise does not have children born with strong muscles; similarly, a person who loses a finger in an accident has children with normal fingers. Reason: these experiences and traits change only the non-reproductive (body) tissues — the brain, the muscles, the skin — and not the DNA of the germ cells. The next generation develops from a zygote formed by the fusion of gametes, so it receives only the DNA of the germ cells, and changes in body tissues cannot be passed on to it. Such traits are acquired traits and are not inherited. Such traits can be passed on only when the change occurs in the DNA (the genes) of the germ cells, that is, in the reproductive tissue.

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10. Q250 CBSE 2017
    List ​in tabular ​form ​any three ​distinguishing features between ​the acquired traits and the inherited ​traits.
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    Acquired traits arise in a lifetime, change only body tissues and are not passed on; inherited traits come from parents through germ-cell DNA.

    Acquired traits compared with inherited traits: (1) Origin: acquired traits develop during the lifetime of the individual, through its experiences; inherited traits are received from the parents. (2) Basis: acquired traits are changes in the body (non-reproductive) tissues only and do not change the DNA of the germ cells; inherited traits are coded in the DNA (genes) of the germ cells. (3) Transmission: acquired traits are not passed on to the offspring; inherited traits are passed on from one generation to the next. Examples: the low weight of a starved beetle is an acquired trait; the green colour of a beetle, which arose from a change in its DNA, is an inherited trait.

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11. Q251 CBSE 2017
    Explain with an example for each, how the ​following provide evidences in ​favour ​of evolution in ​organisms ​:

    1. (a)Homologous organs
    2. (b)Analogous organs
    3. (c)​Fossils
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    Homologous organs show a common ancestor, analogous organs show adaptation to one function, and fossils like Archaeopteryx suggest birds evolved from reptiles.

    (a) Homologous organs have the same basic structure and origin, even though they may perform different functions. Example: the forelimbs of a human (grasping), a bat (flying) and a cat (walking) are built from the same set of bones (upper-arm bone, forearm bones, wrist bones and hand bones). The same design modified for different uses shows that these animals descended from a common ancestor. (b) Analogous organs have different basic structures and origins but perform the same function, so they look alike. Example: the wings of a bird (a feathered forelimb with bones) and of an insect (a thin membrane with no bones) are both used for flying; so are the wings of a bird and a bat (a feathery covering along the arm against a skin fold between elongated fingers). They show that organisms that are not closely related, adapting to the same function, evolve similar-looking organs from different structures, so a similar appearance alone does not show a common ancestry. (c) Fossils are the preserved remains or impressions of organisms that lived in the past. Example: Archaeopteryx, with feathers and wings like a bird but teeth, clawed fingers and a long bony tail like a reptile, suggests that birds evolved from reptiles. Fossils, dated from the depth of their rock layer or from isotope ratios, show that organisms have changed over time.

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12. Q252 CBSE 2017
    Differentiate ​between ​inherited and ​acquired traits ​by giving one example of each. Give reason why the traits ​acquired during the life time of an ​individual are not inherited ?
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    Inherited traits are coded in germ-cell DNA and passed on; acquired traits change only body tissues, so they are not inherited.

    Inherited traits are passed on from the parents to the offspring through the genes (DNA) of the germ cells; acquired traits are developed by an individual during its own lifetime, through use, training, injury or the environment, and are not passed on. Inherited traits are coded in the DNA of the germ cells; acquired traits are changes in the non-reproductive (body) tissues that leave the DNA of the germ cells unchanged. Examples: inherited - eye colour (or free versus attached earlobes); acquired - the low weight of a beetle caused by starvation (or a skill such as swimming). Reason: an acquired trait changes only the non-reproductive tissues of the body, not the DNA of the germ cells. Only the DNA of the germ cells is passed on to the next generation in sexual reproduction, so the traits acquired during the lifetime of an individual are not inherited.

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13. Q253 CBSE 2017
    Describe ​any three ​methods ​of ​tracing evolutionary relationship ​amongst ​organisms.
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    Evolutionary relationships can be traced by homologous organs, by fossils arranged by age, and by comparing DNA (molecular phylogeny).

    Three methods: (1) Homologous organs: organs with the same basic structure and origin but different functions, such as the forelimbs of frogs, lizards, birds and humans, show that the organisms share a common ancestor; the more such features two organisms share, the more closely related they are. Analogous organs (same function, different structure and origin, such as the wings of a bat and of a bird) must be told apart, since they do not show close relationship. (2) Fossils: preserved remains or impressions of past organisms. Fossils in deeper layers of rock are usually older, and their age can also be found from the ratios of isotopes in them; arranged by age, fossils show which forms came earlier and how features changed. (3) Molecular phylogeny: comparing the DNA of organisms. Changes in DNA build up over generations, so the fewer the differences between the DNA of two organisms, the more closely related they are.

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14. Q254 CBSE 2017
    What is an organic evolution ? It cannot ​be equated ​with ​progress. Explain with the help of ​a ​suitable ​example.
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    Evolution is gradual change forming new species, but it only creates diversity, not 'better' forms; humans and chimpanzees share a common ancestor.

    Organic evolution is the sequence of gradual changes that take place in organisms over very long periods of time (many generations), by which new species arise from pre-existing, often simpler, forms of life. It cannot be equated with progress because evolution only produces diversity, which is then shaped by environmental selection; it does not move towards a goal or towards 'better' forms. A newly formed species is not necessarily better than the older one - it is just different - and older, simpler designs continue to survive. Example: human beings did not evolve from chimpanzees. Humans and chimpanzees had a common ancestor long ago, which was neither a chimpanzee nor a human, and the two lines evolved separately in their own ways; neither is more 'advanced' than the other. Likewise bacteria, among the simplest organisms, still thrive in hot springs, deep-sea thermal vents and Antarctic ice - the simple design has not been replaced by a 'better' one.

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15. Q255 CBSE 2017
    What are ​fossils ? ​How ​are ​they formed ? Explain two ​roles of fossils in ​tracing ​evolutionary relationships.
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    Fossils are preserved remains of past organisms, like impressions in hardened mud; their age and features shared by two groups trace evolution.

    Fossils are the preserved remains or traces of organisms that lived in the remote past - bones, teeth, shells, footprints, or the impressions of leaves and bodies - usually found in rocks. Formation: usually a dead organism decomposes and is lost. But if the body, or part of it, is buried where it cannot decompose completely - for example, a dead insect caught in hot mud - the mud hardens and keeps the impression of the body. Over long periods more layers settle on top and harden into rock; in some fossils the hard parts are slowly replaced by minerals. Two roles in tracing evolutionary relationships: (1) Fossils can be dated - fossils in layers nearer the surface are more recent than those in deeper layers, and the ratios of isotopes in the fossil material give their age - so arranging them by age shows the order in which different forms of life appeared and how they changed over time. (2) Some fossils have features of two groups and so show how the groups are related: Archaeopteryx has feathers like a bird but teeth, clawed fingers and a long bony tail like a reptile, and fossil dinosaurs with feathers show that birds are closely related to reptiles.

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16. Q256 CBSE 2017
    Distinguish between ​the ​acquired traits and the inherited traits in ​tabular form, ​giving one example ​for ​each.
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    Acquired traits develop in a lifetime and are not passed on; inherited traits are controlled by germ-cell DNA and are passed on.

    Acquired traits: developed by an individual during its own lifetime, through its experiences or its surroundings | Inherited traits: received from the parents. Acquired traits: bring changes only in the body (non-reproductive) tissues, not in the DNA of the germ cells | Inherited traits: controlled by genes present in the DNA of the germ cells. Acquired traits: are not passed on to the offspring | Inherited traits: are passed on from one generation to the next. Acquired traits: cannot lead to evolution | Inherited traits: can lead to evolution. Example of an acquired trait: the loss of weight of a beetle that is starved of food | Example of an inherited trait: the colour of the eyes in human beings.

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17. Q257 CBSE 2017
    “Natural ​selection and ​speciation leads to ​evolution.” ​Justify ​this ​statement.
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    Natural selection changes populations over generations and speciation forms new species; together they bring gradual change and diversity, which is evolution.

    Natural selection: The individuals of a population show variations, which arise during reproduction. Nature favours the variations that suit the environment: individuals having them survive better and reproduce more, so over generations these variations become common and the population changes. For example, green beetles hidden from crows on green bushes increase in number over red ones. Speciation: When a population is split into sub-populations that cannot interbreed, for example by geographical isolation, gene flow between them stops. Each sub-population changes in its own way by natural selection (and genetic drift). Over many generations the changes accumulate until the members of the two groups can no longer reproduce with each other, and a new species is formed. Thus natural selection changes populations over time and speciation gives rise to new species; together they produce the gradual change and the diversity of living organisms, which is evolution. Hence the statement is justified.

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18. Q258 CBSE 2017
    Give ​an example of ​the ​characteristics being used to determine ​how close two ​species are in ​evolutionary terms.
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    Homologous organs, like vertebrate forelimbs with the same bones but different uses, show a common ancestor; more shared characteristics mean closer relationship.

    Homologous organs, which have the same basic structure but different functions, are used. For example, the forelimbs of a frog, a lizard, a bird and a human all have one bone in the upper arm (humerus), two in the forearm (radius and ulna, fused into one in the frog), a group of wrist bones, and bones of the palm and fingers, although they do different jobs: propping up the body and cushioning landing (frog), walking (lizard), flying (bird) and grasping (human). A detailed design shared by organs doing such different jobs was inherited from a common ancestor, so it shows that these animals are related; the more such characteristics two species share, the more closely they are related. By contrast, the wings of a bird and a bat do the same job but are built differently; such analogous organs do not show a close relationship.

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19. Q259 CBSE 2017
    How ​did Mendel explain ​that ​it is ​possible ​that a trait is inherited but not ​expressed in an organism ?
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    Each plant carries two factors for a trait; a dominant factor masks the recessive one, which shows only when both copies are recessive.

    Mendel crossed pure tall pea plants with pure short (dwarf) ones. All the $F_1$ plants were tall, yet when they were self-pollinated, short plants reappeared in $F_2$, in the ratio of about $3$ tall : $1$ short. So the $F_1$ plants had inherited both traits but expressed only tallness. He explained this with factors (genes): each plant carries two copies of the factor for a trait, one from each parent. The $F_1$ plants are $\text{Tt}$; one copy of the dominant factor $\text{T}$ is enough to make the plant tall and masks the recessive factor $\text{t}$, which is expressed only when both copies are $\text{t}$ ($\text{tt}$, short). Thus a recessive trait can be inherited without being expressed.

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20. Q260 CBSE 2017
    How do ​Mendel’s ​experiments show ​that ​the ​traits are inherited independently ​?
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    Pure round yellow $\times$ wrinkled green gives $9:3:3:1$ in ${\text{F}}_2$, with new combinations, so the two traits are inherited independently.

    Mendel crossed pure-breeding pea plants differing in two traits: one with round, yellow seeds (RRYY) and one with wrinkled, green seeds (rryy). F1: all plants had round, yellow seeds (RrYy), so round and yellow are dominant. F2 (F1 self-pollinated): four kinds of seeds appeared, round yellow, round green, wrinkled yellow and wrinkled green, in the ratio $9:3:3:1$. Round green and wrinkled yellow are new combinations found in neither parent. They appear, $3$ and $3$ in every $16$, because the factor for seed shape and the factor for seed colour go into the gametes independently of each other (gametes RY, Ry, rY and ry in equal numbers); each trait on its own still shows $3:1$. Hence the two traits are inherited independently.

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