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 11 of 15

1. Q201 CBSE 2020
   Explain ​giving ​an ​example how the following ​provide evidences in favour ​of evolution in ​organisms.

   1. (i)​Homologous organs
   2. (ii)Fossils
   Show answer

   Homologous organs (same design, different use) show a common ancestor; fossils like Archaeopteryx show that birds evolved from reptiles.

   (i) Homologous organs have the same basic structure (and origin) but perform different functions. Example: the forelimbs of a lizard, a bird, a whale and a human are built on the same plan of bones (one upper-arm bone, two forearm bones, wrist, palm and finger bones) but are used for different jobs: walking, flying, swimming and grasping. The common design shows that these animals inherited it from a common ancestor and that it was modified during evolution. (ii) Fossils are the preserved remains or impressions of organisms of the remote past; fossils in deeper rock layers are older, so they show how organisms changed with time. Example: fossils of some dinosaurs show feathers, although these dinosaurs could not fly; feathers could have first given insulation in cold weather and later become useful for flight. The fossil Archaeopteryx has a bird's feathers and wings with a reptile's teeth and long bony tail. So birds have evolved from reptiles.

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2. Q202 CBSE 2020
   1. (a)​What are ​fossils ​?
   2. (b)Describe ​two ​methods of determining ​the ​age of fossils.
   Show answer

   Fossils are preserved remains of past organisms; their age is found from the depth of the rock layer or the carbon-14 to carbon-12 ratio.

   (a) Fossils are the preserved remains or traces of organisms that lived in the remote past, such as bones and shells, or impressions of leaves and footprints preserved in rock. (b) (i) Relative method: when we dig into the earth, the fossils found closer to the surface are more recent than those found in deeper layers, because the layers of rock were laid down one above the other over time. (ii) Dating fossils by isotopes: by detecting the ratios of different isotopes of the same element in the fossil material, for example carbon-14 to carbon-12. Carbon-14 decays at a fixed rate after the organism dies, so the less of it that remains, the older the fossil.

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3. Q203 CBSE 2020
   (a) Classify the ​following as homologous or ​analogous pairs :

   1. (i)Broccoli and Cabbage
   2. (ii)​Ginger and Raddish
   3. (iii)Fore ​limbs of birds and lizard
   4. (iv)Wings of a bat and Wings of a ​bird (b) State the main feature that categorises a given ​pair of organs as homologous or analogous.
   Show answer

   Broccoli-cabbage, ginger-radish and bat-bird wings are analogous; bird-lizard forelimbs are homologous; the basic structure and origin decide.

   (a) (i) Broccoli and cabbage: analogous. (ii) Ginger and radish: analogous. (iii) Forelimbs of birds and lizards: homologous. (iv) Wings of a bat and wings of a bird: analogous. (b) The origin, that is, the basic structure of the organs. Homologous organs have the same basic structure and origin but perform different functions; analogous organs have a different basic structure and origin but perform similar functions.

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4. Q204 CBSE 2020
   List ​three factors that could lead to ​speciation. ​Which of ​these cannot be a major factor ​in the ​speciation ​of a ​self-pollinating plant species and why ?
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   Geographical isolation, genetic drift and natural selection can cause speciation; isolation matters little for self-pollinating plants, which exchange no genes between plants anyway.

   Three factors that could lead to speciation: (1) geographical isolation of a population, which stops gene flow between the separated groups; (2) genetic drift; (3) natural selection. (Mutations, severe changes in DNA, are also accepted; other correct factors earn credit.) Geographical isolation cannot be a major factor in the speciation of a self-pollinating plant species. In such a plant, pollination takes place within the same plant, so genes are not exchanged between plants even without a barrier; isolating the plants therefore cuts off no gene flow, and pollination within the same plant brings little new variation.

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5. Q205 CBSE 2020
   1. (a)Why is the ​F1 progeny always of tall plants when a tall ​pea plant is crossed ​with ​a short pea plant ?
   2. (b)How is F2 ​progeny obtained by ​self-pollination of F1 progeny different from F1 progeny ? ​Give reason for this observation.
   3. (c)State a conclusion that ​can be drawn on the basis of this observation.
   Show answer

   All ${\text{F}}_1$ are tall as tallness is dominant; ${\text{F}}_2$ gives tall and short in $3:1$, showing the recessive trait was hidden, not lost.

   (a) Tallness is the dominant trait. Every F1 plant receives a factor for tallness (T) from the tall parent and a factor for shortness (t) from the short parent; being Tt, it shows only the dominant trait, so all F1 plants are tall. (b) The F1 plants are all tall, but the F2 contains both tall and short plants, in the ratio $3:1$. The reason: the recessive character, shortness, is expressed in the F2 generation when two copies of the recessive factor come together in a plant (tt). (c) Conclusion: in the F2 progeny the dominant character is expressed along with the recessive character in the ratio $3:1$; the recessive trait was not lost in the F1 but only hidden, because each trait is controlled by a pair of factors, one inherited from each parent.

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6. Q206 CBSE 2020
   Define the ​term evolution. ​“Evolution cannot ​be ​equated ​with ​progress.” Justify ​this statement.
   Show answer

   Evolution is the generation and selection of diversity, not progress: simple bacteria still inhabit inhospitable habitats and older designs are not inefficient.

   Evolution: the gradual change in living organisms over very long periods of time, since the beginning of life, which results in the formation of new species; it is the generation of diversity and the shaping of that diversity by environmental selection. Justification: evolution produces diversity, not progress. More and more complex body designs have emerged over time, but this does not mean that the older, simpler designs are inefficient. For example, bacteria, among the simplest organisms, still inhabit the most inhospitable habitats, such as hot springs, deep-sea thermal vents and the ice in Antarctica. A new species is not necessarily better than an older one, and human beings are not the pinnacle of evolution but one more species in the spectrum of evolving life.

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7. Q207 CBSE 2020
   “During the course ​of evolution, organs or features ​may ​be ​adapted for ​new functions.” ​Explain ​this fact by choosing ​an appropriate example.
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   Feathers first gave insulation in feathered dinosaurs and were later adapted by birds for flight, a new function.

   Example: feathers. (1) Feathers first provided insulation, keeping the body warm in cold weather. Some dinosaurs had feathers, although they could not fly using them. (2) Birds later adapted the feathers for flight, a new function. (3) So a feature that evolved for one function became useful later for a quite different function. This also shows that birds are very closely related to reptiles, since dinosaurs were reptiles.

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8. Q208 CBSE 2020
   List two distinguishing ​features between inherited and acquired ​characters. Also ​give one example for ​each ​type.
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   Inherited characters, such as eye colour, are passed on through germ-cell DNA; acquired ones, such as a dancing skill, are not.

   1. Inherited characters are passed on from parents to offspring, from one generation to the next; acquired characters are developed by an individual during its own lifetime and are not passed on to its offspring. 2. Inherited characters are coded in the DNA of the germ (reproductive) cells; acquired characters are changes in the body (non-reproductive) cells that leave the DNA of the germ cells unchanged. (So inherited characters can bring about evolution, while acquired characters play no role in it.) Examples: inherited character, eye colour; acquired character, a skill such as dancing. Other correct examples earn the same credit.

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9. Q209 CBSE 2020
   What are chromosomes ? Explain how ​in ​sexually reproducing organisms the ​number ​of chromosomes in ​the ​progeny is ​maintained.
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   Chromosomes are DNA-protein threads carrying genes; the number is halved in gametes and restored at fertilisation, so it stays constant ($23+23=46$).

   Chromosomes are thread-like structures present in the nucleus of a cell. They are made of DNA and proteins and carry the genes, the hereditary information passed from parents to offspring. In sexually reproducing organisms, body cells have two sets of chromosomes (a pair of each kind; $46$ in human beings). During the formation of germ cells (gametes), the number of chromosomes is reduced to half, so each gamete receives only one chromosome of each pair ($23$ in human beings). At fertilisation a male and a female gamete fuse, and the zygote receives one set from each parent, restoring the original number ($23+23=46$). In this way the number of chromosomes in the progeny is maintained, generation after generation.

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10. Q210 CBSE 2020
    1. (a)Why did Mendel ​carry out an experiment to ​study inheritance of two traits in garden-pea ?
    2. (b)What were his findings with respect to ​inheritance of traits in ${\text{F}}_1$ and ${\text{F}}_2$ ​generation ?
    3. (c)State the ratio obtained ​in the ${\text{F}}_2$ generation ​in the above mentioned ​experiment.
    Show answer

    Two traits are inherited independently; $F_1$ shows both dominant forms, and $F_2$ gives new combinations in the ratio $9:3:3:1$.

    (a) To find out whether two different traits, such as seed shape and seed colour, are inherited together or independently of each other; the cross showed that they are inherited independently (independent inheritance of traits). (b) ${\text{F}}_1$: all the plants showed the dominant form of both traits, round and yellow seeds. ${\text{F}}_2$: the parental combinations (round yellow, wrinkled green) reappeared along with new combinations (round green, wrinkled yellow). (c) $9:3:3:1$ (round yellow : round green : wrinkled yellow : wrinkled green).

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11. Q211 CBSE 2020
    A green stemmed rose plant denoted by GG ​and a brown stemmed rose plant ​denoted by gg are ​allowed to undergo a cross ​with each other. (a) List your observations regarding

    1. (i)Colour of stem in their ${\text{F}}_1$ progeny
    2. (ii)Percentage of brown stemmed ​plants in ${\text{F}}_2$ progeny if ${\text{F}}_1$ plants are self pollinated.
    3. (iii)Ratio of GG and Gg in the ${\text{F}}_2$ progeny. (b) Based on ​the findings of this cross, ​what conclusion can ​be drawn ?
    Show answer

    ${\text{F}}_1$ is all green ($Gg$); $25\%$ of ${\text{F}}_2$ are brown and $GG:Gg=1:2$; green is dominant, brown recessive.

    (a) (i) All the ${\text{F}}_1$ plants have green stems (genotype Gg). (ii) $25\%$ of the ${\text{F}}_2$ plants have brown stems (gg is $1$ box in $4$). (iii) GG : Gg $=1:2$ in the ${\text{F}}_2$ progeny. (b) Green stem colour is the dominant trait and brown the recessive trait: the trait expressed in the ${\text{F}}_1$ progeny is dominant, while the trait that is not expressed in ${\text{F}}_1$ but reappears in the ${\text{F}}_2$ progeny is recessive.

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12. Q212 CBSE 2020
    1. (a)​What provides nutrition ​to human sperms ? State the genetic ​constitution ​of a ​sperm.
    2. (b)Mention the ​chromosome pair present in zygote which determines the sex ​of (i) a female child, and ​(ii) a male child.
    Show answer

    Seminal vesicle and prostate secretions nourish sperms; a sperm is $22+\text{X}$ or $22+\text{Y}$; XX gives a female zygote, XY a male.

    (a) The secretions of the seminal vesicles (together with those of the prostate gland), added as the sperms pass along the vas deferens, provide nutrition to the sperms and make their transport easier. A sperm has $23$ chromosomes, $22$ autosomes and one sex chromosome, so its genetic constitution is either $22+\text{X}$ or $22+\text{Y}$. (b) (i) Female child: the zygote has the sex-chromosome pair XX. (ii) Male child: the zygote has the pair XY.

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13. Q213 CBSE 2020
    Define ​the ​following ​:

    1. (a)​Speciation
    2. (b)​Natural ​Selection
    3. (c)Genetic ​Drift
    Show answer

    Speciation is the origin of a new species, natural selection favours the best-suited variations, and genetic drift is chance change in gene frequency.

    (a) Speciation: the origin of a new species from an existing (pre-existing) species, whose members can no longer interbreed with the original population. (b) Natural selection: nature selects the individuals with variations best suited to the environment; they survive and reproduce more (survival of the fittest), and the species evolves. (c) Genetic drift: accidental (random) changes in the frequency of genes in a population, especially a small one, without any survival advantage.

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14. Q214 CBSE 2020
    (a) We ​see eyes in Planaria, insects, octopus and vertebrates.

    1. (i)Can eyes be grouped together in ​case of the ​above mentioned organisms to establish a common evolutionary origin ? Why ?
    2. (ii)​Can ​these organisms be ​grouped together on the basis of the eyes they possess ? Give justification to ​your answer. (b) State evidence to prove that birds have evolved from reptiles.
    Show answer

    Eyes of Planaria, insects, octopus and vertebrates are analogous, so show no common origin or grouping; feathered dinosaurs and Archaeopteryx link birds to reptiles.

    (a) (i) No. The eyes of Planaria, insects, octopus and vertebrates do the same job, detecting light, but their structure is very different (simple eye-spots in Planaria, compound eyes in insects, camera-like eyes built differently in the octopus and in vertebrates). They are analogous organs that arose separately, not homologous organs inherited from a common ancestor, so they cannot establish a common evolutionary origin. (ii) No. Organisms are grouped by characteristics inherited from a common ancestor. The eyes evolved separately, so grouping by them would bring together unrelated animals: a flatworm (Planaria), arthropods (insects), a mollusc (octopus) and chordates (vertebrates). (b) Fossils show that some dinosaurs, which were reptiles, had feathers although they could not fly with them; birds later adapted feathers for flight, so birds are very closely related to reptiles. Fossils of Archaeopteryx show feathers and wings like a bird together with teeth, clawed fingers and a long bony tail like a reptile, a connecting link between the two groups. Birds also still have scales on their legs and lay shelled eggs, as reptiles do.

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15. Q215 CBSE 2020
    What ​are fossils ? How are ​they formed ? ​How ​do we determine ​the age ​of ​fossils ?
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    Fossils are preserved traces of ancient organisms, formed when remains are buried and hardened into rock; age comes from layer depth and isotope ratios.

    Fossils are the preserved traces of living organisms that lived long ago, such as bones, teeth, shells, or impressions of their bodies in rock. Formation: normally a dead organism decomposes, but if its body or a part of it is buried quickly in mud, sand or silt, it cannot decompose completely. The sediment hardens around it and keeps the remains or the impression of its body parts (an insect caught in hot mud, for example); as more layers pile up over millions of years, their pressure turns the sediment into rock with the fossil inside. Age: (1) from the depth of the rock layer: fossils found closer to the surface are more recent than those in deeper layers (relative age); (2) by fossil dating: detecting the ratios of different isotopes of the same element in the fossil material, since a radioactive isotope such as carbon-14 decays at a known rate (carbon-14 halves about every $5730$ years).

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16. Q216 CBSE 2020
    1. (a)​What is the law of dominance of traits ​? ​Explain with an ​example.
    2. (b)​Why ​are the ​traits acquired during ​the life time of an individual not inherited? Explain.
    Show answer

    Dominant traits show in $F_1$ (tall over dwarf in peas); acquired traits change only body cells, not germ-cell DNA, so they are not inherited.

    (a) Law of dominance: when a pair of contrasting forms of a trait is crossed, only one of the two parental traits is expressed in the ${\text{F}}_1$ generation; it is called the dominant trait, and the one that is not expressed is called the recessive trait. Example: Mendel crossed pure tall pea plants (TT) with pure dwarf pea plants (tt). The gametes were T and t, and every ${\text{F}}_1$ plant was Tt and tall. So the factor for tallness (T) is dominant over the factor for dwarfness (t), which is present in the ${\text{F}}_1$ but cannot express itself in the presence of T; the dwarf trait reappears in one-fourth of the ${\text{F}}_2$ plants. (b) Traits acquired during the lifetime of an individual (for example, a tail cut off or muscles built up by exercise) are changes in the somatic (body) cells only; they do not change the DNA of the germ cells (sperm and egg). Only the DNA of the germ cells is passed on to the next generation, so acquired traits are not inherited.

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17. Q217 CBSE 2020
    1. (a)What is ​genetics ?
    2. (b)​What are genes? ​Where are the genes ​located?
    3. (c)State ​and define ​three factors responsible ​for the rise of a new species.
    Show answer

    Genetics studies heredity and variation; genes are DNA segments on chromosomes; natural selection, genetic drift and reproductive isolation can give rise to new species.

    (a) Genetics is the study of heredity and variation. (b) Genes are segments of DNA that carry the information for traits (for making particular proteins) from one generation to the next. They are located on the chromosomes, in the nucleus of the cell. (c) (i) Natural selection: individuals whose variations suit the environment survive and reproduce more, so over generations these variations accumulate and can give rise to a new species. (ii) Genetic drift: random change in the frequency of genes in a small population over successive generations. (iii) Reproductive isolation: members of a group separated by a barrier (for example geographical) can no longer mate with the rest of the species, so their differences accumulate into a new species. Other correct factors, such as a large change in DNA, also earn credit.

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18. Q218 CBSE 2020
    (a) Giving one example for each, explain the ​following terms :

    1. (i)​Homologous organs
    2. (ii)Analogous ​organs
    3. (iii)​Fossils (b) Explain two ​methods of ​determining the age ​of ​fossils.
    Show answer

    Homologous organs have the same structure but different functions, analogous the reverse; fossil age comes from layer depth and isotope ratios.

    (a) (i) Homologous organs have the same basic structure (a common origin) but perform different functions. Example: the forelimbs of amphibians, reptiles, birds and mammals, such as a lizard's forelimb (walking), a bird's wing (flying) and a human arm (grasping). (ii) Analogous organs have different basic structures (different origins) but perform similar functions. Example: the wings of a bat (skin stretched between the fingers) and the wings of a bird (feathers along the arm), both used for flying; the wings of an insect and a bird are another example. (iii) Fossils are the preserved traces (remains or impressions) of organisms that lived in the past. Example: fossils of an ammonite, a trilobite or the dinosaur Rajasaurus. (b) Method 1, relative dating: on digging into the earth, fossils found closer to the surface are more recent than those found in deeper layers. Method 2, isotope (carbon) dating: the ratio of different isotopes of the same element in the fossil, such as carbon-14 to carbon-12, falls at a known rate after death, so measuring it gives the age of the fossil. Any other correct example earns credit.

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19. Q219 CBSE 2019
    What ​are ​fossils ​? ​Describe ​briefly two methods of determining ​the age of ​fossils.
    Show answer

    Fossils are preserved remains of past organisms; their age is found by the depth of the rock layer or carbon (isotope) dating.

    Fossils are the preserved remains or traces (such as bones, shells or impressions) of plants and animals that lived long ago, found buried in the rocks of the earth's crust. Two methods of determining their age: (i) Relative dating: when we dig into the earth, the fossils found in layers closer to the surface are more recent than those found in deeper layers, because the deeper layers were laid down earlier. (ii) Carbon dating (isotope dating): the ratio of different isotopes of the same element, such as carbon-14 to carbon-12, is measured in the fossil material. Carbon-14 decays at a known rate after death, so this ratio tells how old the fossil is.

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20. Q220 CBSE 2019
    Define the term ​evolution. ​“Evolution cannot ​be equated with progress.” ​Give examples ​to ​justify ​this statement.
    Show answer

    Evolution is gradual change over millions of years giving new species; it is not progress, as simple forms like bacteria still thrive.

    Evolution: The sequence of gradual changes that take place in living organisms over millions of years, by which new species arise from existing ones. Why evolution cannot be equated with progress: Evolution only generates diversity, and the environment selects what suits it. More complex body designs have appeared over time, but the simpler, older designs have not been replaced; they continue to survive and flourish and are equally efficient in their own habitats. Examples: (i) Bacteria, among the simplest and earliest forms of life, still thrive, even in the most hostile habitats such as hot springs, deep-sea thermal vents and the ice of Antarctica. (ii) Humans have not evolved from chimpanzees; both evolved from a common ancestor along separate lines, so humans are not the pinnacle of evolution but one more species among the evolving forms of life.

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