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 14 of 15

1. Q261 CBSE 2017
   Explain with the help of an ​example each, ​how ​the following provide ​evidences in favour of evolution :

   1. (a)​Homologous ​organs
   2. (b)Analogous organs
   3. (c)Fossils
   Show answer

   Homologous organs show common ancestry, analogous organs show that similar function can evolve separately, and fossils such as Archaeopteryx show change over time.

   (a) Homologous organs have the same basic structure and origin, even though they may perform different functions. Example: the forelimbs of a frog, a lizard, a bird and a human being; the human arm (grasping) and the bird's wing (flying) both have one upper-arm bone, two forearm bones, and wrist and hand bones. This common basic design shows that these animals have descended from a common ancestor, whose limb has been modified for different uses: evidence of evolution. (b) Analogous organs have different basic structures and origins but perform the same function, so they look alike. Example: the wings of a bird and of an insect such as a butterfly are both used for flying, but a bird's wing is a feathered forelimb with bones, while an insect's wing is a membrane with no bones. They show that organs which do not come from a common ancestral structure, in organisms only distantly related, can evolve to look alike as adaptations to the same function, so similarity of shape alone does not show a common ancestry. (c) Fossils are the preserved remains or traces of organisms that lived long ago; their age can be judged from the depth of the rock layer or found from isotope ratios. Arranged by age, they show that living forms have changed over time, and some show features of two groups. Example: Archaeopteryx had feathers and wings like a bird, but teeth, clawed fingers and a long bony tail like a reptile, which suggests that birds have evolved from reptiles.

   Step-by-step solution →
2. Q262 CBSE 2017
   List ​any three factors and ​mention ​how ​they could lead to the ​rise of ​a ​new species.
   Show answer

   Geographical isolation, genetic drift and natural selection make separated groups differ until they cannot interbreed, forming a new species.

   (1) Geographical isolation: a barrier such as a large river or a mountain range splits a population into sub-populations that cannot meet, so the gene flow that kept them alike stops and each group changes on its own. (2) Genetic drift: in each isolated group, especially a small one, the frequencies of genes change by chance from generation to generation, so different changes accumulate in the two groups. (3) Natural selection: the places the groups live in may differ in food, climate or enemies, so different variations survive and spread in each group, and the groups become adapted differently. Over many generations these differences build up in the DNA until the groups can no longer interbreed even if they meet again (for example, a change in the number of chromosomes can stop their germ cells from fusing). The separated group has then become a new species. Severe variation in DNA, such as a change in the number of chromosomes, is also accepted as a factor.

   Step-by-step solution →
3. Q263 CBSE 2017
   What ​are ​fossils ? How ​do ​they ​act as an evidence for ​organic evolution ?
   Show answer

   Fossils are preserved remains of past organisms; ordered by age they show change over time, and Archaeopteryx links birds and reptiles.

   Fossils are the preserved remains or traces of organisms that lived in the remote past, such as bones, shells, or impressions of leaves or bodies left in rocks. They act as evidence for evolution in these ways: (1) Rock layers form one above another, so in undisturbed rock the fossils in deeper layers are older than those near the surface, and their age can also be found by fossil-dating (the ratios of isotopes of an element in the fossil). Arranged by age, fossils show that organisms very different from today's lived in the past, and that, on the whole, simpler forms appeared first and more complex forms later: life has changed gradually over time. (2) Some fossils have features of two groups and link them. Archaeopteryx had feathers and wings like a bird but teeth and a long bony tail like a reptile, and fossils show that some dinosaurs had feathers — evidence that birds evolved from reptiles.

   Step-by-step solution →
4. Q264 CBSE 2017
   “Evolution and ​classification of organisms ​are interlinked.” ​Give ​reasons to justify this ​statement.
   Show answer

   Classification groups organisms by inherited similarities, and more shared characteristics mean a more recent common ancestor, so classification reflects evolutionary relationships.

   (1) Organisms are classified on the basis of their similarities and differences in characteristics, as groups within groups. (2) Characteristics are inherited, so the more characteristics two species have in common, the more closely they are related and the more recently they had a common ancestor; for example, the lion and the tiger share more characteristics, and a more recent common ancestor, than either shares with the dog. (3) Hence each group in a classification is a set of species descended from a common ancestor: the classification of organisms reflects their evolutionary relationships. Characteristics inherited from common ancestors, such as the homologous forelimbs of vertebrates, are used both to classify organisms and to trace their evolution.

   Step-by-step solution →
5. Q265 CBSE 2017
   What ​are homologous ​and analogous organs ? ​How ​do they provide evidences ​in favour ​of evolution ?
   Show answer

   Homologous organs share structure and origin but differ in function, showing common ancestry; analogous organs share only function and show no close relationship.

   Homologous organs are organs that have the same basic structure and origin but perform different functions, for example the forelimbs of a frog, a lizard, a bird, a whale and a human (upper-arm bone, forearm bones, wrist bones and digits in all). Analogous organs are organs that have a different basic structure and origin but look alike and perform similar functions, for example the wings of a bird and of a butterfly (or, in NCERT's example, the wings of a bat and a bird). Evidence of evolution: homologous organs show that different organisms have descended from a common ancestor, the same basic plan having been modified for different uses (divergent evolution). Analogous organs show that unrelated organisms adapting to a similar function evolve similar-looking organs (convergent evolution); they also show that similar function alone does not mean a close relationship, so evolutionary relationships are traced through homologous organs.

   Step-by-step solution →
6. Q266 CBSE 2017
   Distinguish ​between inherited traits ​and acquired traits giving ​one ​example of each. Give ​reason why ​the traits acquired by an ​individual during the life time are ​not inherited.
   Show answer

   Inherited traits pass on through germ-cell DNA; acquired traits change only body tissues, so they are not inherited.

   Inherited traits are passed from parents to offspring through the genes (DNA) in the germ cells; they come from the genetic make-up of the individual and can appear in the next generation. Example: eye colour (or free and attached earlobes). Acquired traits are developed by an individual during its own lifetime, through experience, surroundings, use or training; they change only the body tissues and are not passed on. Example: the low weight of a beetle that has been starved (or a skill such as swimming). Reason: an acquired trait is a change in the non-reproductive (body) tissues. It does not change the DNA of the germ cells, and in sexual reproduction only the DNA of the germ cells is passed on to the next generation, so the trait is not inherited.

   Step-by-step solution →
7. Q267 CBSE 2017
   What are ​homologous organs ? Give one example. Can the wings of a butterfly and ​the wings of ​a bat be ​regarded as ​homologous ​? Give reason in support of your ​answer.
   Show answer

   Homologous organs have the same basic structure and origin; bat and butterfly wings are not homologous because their structure and origin differ.

   Homologous organs are organs of different organisms that have the same basic structure and the same origin but may perform different functions. Example: the forelimbs of a frog, a lizard, a bird, a bat and a human, which share the same basic arrangement of bones but are used for different purposes, such as supporting the body, flying and grasping. No, the wings of a butterfly and the wings of a bat cannot be regarded as homologous. Both are used for flying, but their basic structure and origin are different: a bat's wing is a modified forelimb, a fold of skin stretched mainly between long finger bones, whereas a butterfly's wing is a thin membrane of the body wall, stiffened by veins, with no bones. Organs with the same function but a different structure and origin are analogous, so these wings are analogous organs.

   Step-by-step solution →
8. Q268 CBSE 2017
   What ​are ​Chromosomes ? ​Explain how in sexually reproducing ​organisms the number of chromosomes in the progeny ​is ​maintained.
   Show answer

   Chromosomes are DNA threads carrying genes; germ cells get half the number and fertilisation restores it, so progeny keep the same number.

   Chromosomes are thread-like structures in the nucleus of a cell, made of DNA (with proteins), that carry the genes, the information for the inheritance of features from parents to offspring. In the body cells they occur in pairs, one member of each pair from each parent (46, or 23 pairs, in human beings). How the number is maintained: the germ cells (gametes) are formed by a special cell division in which each germ cell receives only one chromosome of each pair, so it has half the number of chromosomes (23 in human beings). At fertilisation a male and a female germ cell fuse, and the zygote gets one set from each parent, which restores the normal number ($23+23=46$). So in every generation the progeny have the same number of chromosomes as their parents, and the DNA of the species stays stable.

   Step-by-step solution →
9. Q269 CBSE 2017
   If we cross-bred tall (dominant) pea ​plant with pure-bred dwarf ​(recessive) pea plant, we will get plants of $F_1$ generation. If we now self-cross the pea plant of $F_1$ generation, we obtain pea plants of $F_2$ generation.

   1. (i)What do ​the ​plants of $F_1$ generation look like ?
   2. (ii)State ​the ​ratio of tall plants to dwarf plants in $F_2$ ​generation.
   3. (iii)State the type of ​plants not found in $F_1$ generation but appeared in $F_2$ generation. Write the reason for the same.
   Show answer

   ${\text{F}}_1$ plants are all tall; ${\text{F}}_2$ tall : dwarf $=3:1$; dwarf plants reappear when two gametes carrying $t$ fuse.

   (i) All the plants of the $F_1$ generation are tall (genotype Tt); none of them is dwarf. (ii) The ratio of tall plants to dwarf plants is $3:1$ (the genotypes TT, Tt and tt are in the ratio $1:2:1$). (iii) Dwarf plants. The $F_1$ plants are Tt: they carry the recessive factor t for dwarfness, but it does not show because the dominant factor T masks it. When the $F_1$ plants are self-crossed, the two factors separate into different gametes, and when two gametes carrying t fuse, the offspring is tt, which is dwarf. So the recessive trait, hidden in $F_1$, reappears in about one-fourth of the $F_2$ plants.

   Step-by-step solution →
10. Q270 CBSE 2017
    How ​are ​fossils formed ​? State any ​one role of ​fossils in ​the study of ​organic evolution.
    Show answer

    Quick burial in sediment that hardens into rock preserves remains as fossils, which show how past organisms differed from present ones.

    Formation: usually, when an organism dies, its body is decomposed by microorganisms and lost. Sometimes, however, the body or some of its parts (especially hard parts such as bones, teeth and shells) is buried quickly in mud, silt or sand - for example at the bottom of a lake or a sea - where it cannot decompose completely. Layers of sediment settle over it and, over millions of years, harden into rock; minerals slowly replace the original material, so the remains are preserved in the rock as a fossil. An organism may also leave its impression, for example an insect caught in hot mud that later hardens. All such preserved traces of organisms are fossils. Role in the study of evolution (any one): fossils provide evidence of evolution by showing organisms that lived in the past and how they differed from present-day ones. Dated by the depth of their rock layer (deeper layers are older) or by isotope ratios, they show the order in which different groups appeared - for example, fossils of feathered dinosaurs show that birds are very closely related to reptiles.

    Step-by-step solution →
11. Q271 CBSE 2017
    How do ​Mendel’s experiments show ​that

    1. (a)traits may be dominant or ​recessive ?
    2. (b)inheritance ​of two traits is independent ​of ​each other ?
    Show answer

    All-tall $F_1$ and a $3:1$ $F_2$ show dominance; the $9:3:3:1$ ratio shows seed shape and colour are inherited independently.

    (a) Mendel crossed pure tall pea plants ($TT$) with pure short plants ($tt$). All the $F_1$ plants were tall ($Tt$); none was short. When the $F_1$ plants were self-pollinated, the $F_2$ generation had tall and short plants in the ratio $3:1$ ($TT:Tt:tt=1:2:1$). The short trait, which did not appear in $F_1$, reappeared in $F_2$, so it had been carried by the $F_1$ plants without being expressed. The trait expressed in $F_1$ (tallness) is dominant, and the trait that remained hidden in $F_1$ (shortness) is recessive. (b) Mendel crossed plants with round, yellow seeds ($RRYY$) with plants with wrinkled, green seeds ($rryy$). All the $F_1$ plants had round, yellow seeds ($RrYy$). On self-pollination of $F_1$, the $F_2$ plants had four kinds of seeds: round-yellow, round-green, wrinkled-yellow and wrinkled-green, in the ratio $9:3:3:1$. Round-green and wrinkled-yellow are new combinations not present in either parent. This shows that the factors for seed shape and seed colour are inherited independently of each other.

    Step-by-step solution →
12. Q272 CBSE 2017
    What are fossils ​? How are ​fossils formed and how ​is their age determined ? ​State the importance of fossils in the ​study of evolution.
    Show answer

    Fossils are preserved remains of ancient organisms, formed by quick burial; dated by layer depth and isotope ratios, they show that life has evolved.

    Fossils are the preserved traces of organisms that lived long ago: the remains of hard parts such as bones, shells and teeth, or impressions of the body, usually found in rocks. Formation: when an organism dies its body usually decomposes, but sometimes the body or some part of it is buried quickly, for example in mud or sand, where it cannot decompose completely. Soft parts usually decay while hard parts or an impression remain. More sediment settles on top, and over long periods, often millions of years, the layers harden into rock with the remains or their impression preserved inside; erosion or digging may later expose them. Age: (i) by the depth of the layer: in undisturbed rock, fossils found nearer the surface are more recent than those found in deeper layers; (ii) by fossil dating: detecting the ratios of different isotopes of the same element in the fossil material (for young remains, the ratio of carbon-14 to carbon-12), since such isotopes change at a steady rate. Importance: fossils are direct evidence that the organisms of the past were different from those of today, so living things have evolved; arranged by age, they show the order in which groups appeared and help trace evolutionary relationships; and some show links between groups, for example fossil dinosaurs with feathers show that birds are closely related to reptiles (Archaeopteryx, with features of both, is often cited as such a link).

    Step-by-step solution →
13. Q273 CBSE 2017
    With ​the help of one example for ​each, distinguish between the acquired traits and the inherited traits. Why ​are the traits/experiences ​acquired during the entire lifetime of an individual not ​inherited in the next generation ​? Explain the reason of this fact with an example.
    Show answer

    Acquired traits change body tissue, not germ-cell DNA, so they are not inherited; inherited traits pass through germ-cell DNA, as green colour does in beetles.

    Acquired traits: developed during an individual's lifetime because of its surroundings, use or experience; they change body (non-reproductive) tissue, not the DNA of the germ cells, and are not passed on to the offspring. Example: the low weight of a beetle that has been starved (or a skill such as swimming). Inherited traits: received from the parents through the DNA (genes) of their germ cells, and passed on to the offspring. Example: a beetle that is green because of a change in its DNA has green progeny (or the colour of the eyes, which a child inherits). Reason: only the DNA of the germ cells enters the zygote, and changes in non-reproductive tissues cannot be passed on to the DNA of the germ cells, so an acquired trait does not reach the next generation. Example: if the tails of mice are cut off by surgery in every generation, their progeny are still born with tails, because removing the tail does not change the genes of the germ cells.

    Step-by-step solution →
14. Q274 CBSE 2017
    1. (a)What ​is the total number of ​chromosomes in a ​human male and a human female ? Write ​the type and number of sex chromosomes in each case.
    2. (b)Distinguish between ​acquired traits and ​inherited traits. Why cannot we pass on our acquired experiences to our progeny ? Explain.
    Show answer

    Both sexes have 46 chromosomes, XX in females and XY in males; acquired traits change body cells, not germ-cell DNA, so they are not inherited.

    (a) A human male and a human female each have $46$ chromosomes ($23$ pairs) in their body cells: $22$ pairs of autosomes and $1$ pair of sex chromosomes. The female has two sex chromosomes of the same type, XX; the male has two sex chromosomes of different types, one X and one Y (XY), the Y being short. (b) Acquired traits versus inherited traits: 1. Acquired: developed during the individual's own lifetime, by experience, practice, use or injury. Inherited: received from the parents through their genes. 2. Acquired: changes in body (non-reproductive) cells, with no change in the DNA of the germ cells. Inherited: controlled by the DNA of the germ cells. 3. Acquired: not passed on to the progeny, e.g. weight lost through starvation, a skill such as swimming. Inherited: passed on from generation to generation, e.g. eye colour, free or attached earlobes. We cannot pass on our acquired experiences because they change only our non-reproductive tissues (such as the brain and muscles), not the DNA of our germ cells. The progeny develops from a zygote that receives DNA only from the egg and the sperm, so a change that has not reached the germ cells cannot be inherited.

    Step-by-step solution →
15. Q275 CBSE 2017
    How ​many pairs of chromosomes are present in human ​beings ? Out of these, ​how many are sex-chromosomes ? How many types of sex-chromosomes are found in human beings ? “The sex of a ​newborn child is a matter of chance in human beings and none of the parents (mother-father) may be considered responsible for it.” ​Justify ​this ​statement by drawing a flow chart showing the process of sex-determination.
    Show answer

    Humans have 23 chromosome pairs, one pair sex chromosomes (X, Y); chance decides which sperm fertilises the egg, so neither parent decides the sex.

    Human beings have 23 pairs of chromosomes (46 chromosomes) in each body cell. Out of these, one pair is the pair of sex chromosomes; the other 22 pairs are autosomes. There are two types of sex chromosomes, X and Y: a woman has XX and a man has XY. Justification (flow chart): Mother ($44+\text{XX}$) forms eggs that all carry $22+\text{X}$. Father ($44+\text{XY}$) forms two kinds of sperm in equal numbers, $22+\text{X}$ and $22+\text{Y}$. Egg ($22+\text{X}$) + sperm ($22+\text{X}$) gives a zygote $44+\text{XX}$, a girl; egg ($22+\text{X}$) + sperm ($22+\text{Y}$) gives a zygote $44+\text{XY}$, a boy. Which kind of sperm fertilises the egg is a matter of chance, so a girl and a boy are equally likely (50% each). The mother contributes the same X chromosome every time, and the father cannot control which of his two kinds of sperm fertilises the egg, so neither parent can be held responsible for the sex of the child.

    Step-by-step solution →
16. Q276 CBSE 2017
    How do Mendel’s experiment ​show that

    1. (i)the traits may ​be ​dominant or recessive, ​and
    2. (ii)​different ​traits are inherited independently ?
    Show answer

    Tall $F_1$ and a $3:1$ $F_2$ show dominance; new combinations in a $9:3:3:1$ $F_2$ show seed shape and colour are inherited independently.

    (i) Mendel crossed pure-breeding tall pea plants ($TT$) with pure-breeding short plants ($tt$). All the $F_1$ plants were tall ($Tt$); none was short or of medium height, so only tallness was expressed although each $F_1$ plant had also inherited shortness. When the $F_1$ plants were self-pollinated, the $F_2$ had tall and short plants in the ratio $3:1$ ($TT:Tt:tt=1:2:1$): shortness reappeared. So a single copy of $T$ makes a plant tall, while a plant is short only when both copies are $t$. The trait expressed in $F_1$ (tallness) is dominant, and the trait hidden in $F_1$ that reappears in $F_2$ (shortness) is recessive. (ii) Mendel crossed plants that differed in two traits, for example round yellow seeds ($RRYY$) with wrinkled green seeds ($rryy$). The $F_1$ were all round and yellow ($RrYy$). On self-pollination the $F_2$ had round yellow, round green, wrinkled yellow and wrinkled green seeds in the ratio $9:3:3:1$. Round green and wrinkled yellow are new combinations found in neither parent, and each trait by itself still shows $3:1$ (round : wrinkled $=12:4$, yellow : green $=12:4$). So seed shape and seed colour are inherited independently of each other.

    Step-by-step solution →
17. Q277 CBSE 2017
    Define ​evolution. How ​does ​it occur ? ​Explain how fossils provide evidences ​in support of evolution.
    Show answer

    Evolution is gradual change in populations producing new species through variation and selection; fossils show past forms, their ages and links such as Archaeopteryx.

    Evolution is the gradual change in the inherited characteristics of populations of organisms over many generations (over millions of years), through which new species arise from pre-existing ones. How it occurs: (1) Variation: during reproduction, DNA copying is not perfectly accurate and sexual reproduction combines the DNA of two parents, so offspring differ slightly; many of these variations are inherited. (2) Natural selection: variations that help organisms survive and reproduce in their environment are passed on to more offspring and become common over generations, as green beetles hidden from crows became common in NCERT's example. (3) Genetic drift: in small populations, some variations become common or are lost purely by chance. (4) Over many generations these changes accumulate; when a population is split, for example by geographical isolation, the separated groups accumulate different variations until they can no longer interbreed, and a new species forms. Changes acquired during an individual's life are not inherited and do not contribute. Fossils as evidence: fossils are the preserved remains or traces of organisms that lived long ago. (1) They show that many organisms of the past were different from those living now, and that many have become extinct. (2) Their age can be found, relatively because in undisturbed rock layers a deeper fossil is older, and absolutely by radioactive dating of the fossil material; arranged by age, the oldest layers contain only simple organisms and more complex ones appear in later layers, showing that life changed gradually. (3) Some fossils show features of two groups: Archaeopteryx had feathers and wings like a bird but teeth, clawed fingers and a long bony tail like a reptile, which suggests that birds evolved from reptiles.

    Step-by-step solution →
18. Q278 CBSE 2017
    Select ​a ​set of homologous ​organs ​from the ​following :

    1. (a)Forelimb of frog, bird and lizard
    2. (b)Wings of birds and wings of bats
    3. (c)Wings of bats and wings of butterflies
    4. (d)Spines of cactus and thorns of Bougainvillea
    Show answer

    (a) Forelimb of frog, bird and lizard

    Step-by-step solution →
19. Q279 CBSE 2017
    Select the ​set ​of homologous ​organs ​from ​the following ​:

    1. (a)Wings of pigeon and a butterfly
    2. (b)Wings of bat and a pigeon
    3. (c)Forelimbs of cow, a duck and a lizard
    4. (d)Wings of butterfly and a bat
    Show answer

    (c) Forelimbs of cow, a duck and a lizard

    Step-by-step solution →
20. Q280 CBSE 2017
    Select a ​set of homologous organs ​from ​the ​following :

    1. (a)Wings of a bat and wings of a butterfly
    2. (b)Wings of a pigeon and wings of a bat
    3. (c)Wings of a butterfly and wings of a pigeon
    4. (d)Forelimbs of a duck, forelimbs of a cow and forelimbs of a lizard
    Show answer

    (d) Forelimbs of a duck, forelimbs of a cow and forelimbs of a lizard

    Step-by-step solution →