Chapter 9 · Heredity and Evolution

NCERT Exemplar Class 10 Science Chapter 9 Short Answer Questions

All 11 questions of NCERT Exemplar Class 10 Science Chapter 9 Short Answer Questions (Heredity and Evolution), in the book's order, each with its answer and a step-by-step solution.

1. Q26
   How ​is the sex ​of ​a ​newborn ​determined in humans?
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   The sperm decides: an $\text{X}$ sperm gives a girl ($\text{XX}$), a $\text{Y}$ sperm a boy ($\text{XY}$), each with a $50:50$ chance.

   The sex is fixed at fertilisation by the sex chromosome carried in the sperm. A woman is $\text{XX}$ and makes only $\text{X}$ eggs; a man is $\text{XY}$ and makes $\text{X}$ and $\text{Y}$ sperms in equal numbers. An $\text{X}$ sperm gives an $\text{XX}$ zygote, which develops into a girl, and a $\text{Y}$ sperm gives an $\text{XY}$ zygote, which develops into a boy. The father's gamete therefore determines the sex of the newborn, with a $50:50$ chance either way.

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2. Q27
   Do ​genetic combination ​of mothers ​play a significant role in ​determining the sex of ​a ​new ​born?
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   No: every egg carries an $\text{X}$ whatever the mother's genotype, so the father's sperm ($\text{X}$ or $\text{Y}$) decides the sex.

   No. The genetic combination of the mother plays no significant role. A human female is $\text{XX}$, so both members of her sex-chromosome pair are alike and every egg she makes carries an $\text{X}$ chromosome, whatever the rest of her genotype. The father is $\text{XY}$ and makes $\text{X}$ and $\text{Y}$ sperms in equal numbers. The zygote becomes $\text{XX}$ (girl) or $\text{XY}$ (boy) according to which sperm fertilises the egg, so the sex of the newborn is decided by the father's gamete and by chance, not by the mother.

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3. Q28
   Mention ​three important features ​of fossils ​which ​help ​in ​the ​study of evolution.
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   Position in the earth's layers gives their order, radioactive dating gives their age, and preserved body design allows comparison with living forms.

   (i) Position in the earth's layers: fossils in the deeper layers are older than those nearer the surface, so the order in which different organisms lived can be read from the strata. (ii) Measurable age: the age of a fossil is found by dating the fossil or its surrounding rock from the decay of radioactive elements, for example carbon dating, so the timing of evolutionary change can be fixed. (iii) Preserved body design: a fossil keeps the structure of an organism that is now extinct, so it can be compared with living forms, and connecting links such as Archaeopteryx, which has feathers and wings of a bird together with the teeth and tail of a reptile, show that one group has arisen from another.

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4. Q29
   Why ​do all ​the ​gametes ​formed ​in ​human females have ​an X chromosome?
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   A human female is $\text{XX}$, and meiosis separates the pair, so every egg receives one $\text{X}$ chromosome.

   Because the sex-chromosome pair of a human female is $\text{XX}$, that is, both of its members are $\text{X}$ chromosomes. During meiosis the two members of every pair separate and each gamete receives exactly one of them, so an egg must receive an $\text{X}$; a female has no other kind of sex chromosome to pass on. The female is therefore the homogametic sex, producing only one kind of gamete, unlike the male, whose $\text{XY}$ pair yields $\text{X}$ and $\text{Y}$ sperms in equal numbers.

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5. Q30
   In human ​beings, the statistical probability of ​getting either ​a ​male or female child ​is 50 : 50. ​Give a suitable explanation.
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   Every egg carries $\text{X}$ while half the sperms carry $\text{X}$ and half $\text{Y}$, so girls and boys are equally likely, $50:50$.

   The mother is $\text{XX}$ and gives an $\text{X}$ chromosome to every egg, so her contribution is the same each time. The father is $\text{XY}$, and every meiotic division separates that pair, so he produces $\text{X}$-bearing and $\text{Y}$-bearing sperms in equal numbers. Fertilisation is random, so an $\text{X}$ sperm and a $\text{Y}$ sperm have the same chance of reaching the egg. Half the zygotes are therefore $\text{XX}$, which develop into girls, and half are $\text{XY}$, which develop into boys, giving $P(\text{girl})=P(\text{boy})=\frac{1}{2}$, that is $50:50$. Being a statistical probability it holds over a large number of births; an individual family may still have children of only one sex.

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6. Q31
   A very small population of a species faces ​a greater ​threat ​of extinction than ​a ​larger population. Provide ​a suitable genetic ​explanation.
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   A small gene pool means few variations to cope with change, strong genetic drift and inbreeding, so extinction is more likely.

   A very small population carries a small gene pool, so it holds few variations of each gene. Three things follow. First, when the environment changes, survival depends on some individuals already carrying a variation that lets them cope; in a small population the chance that such a variant exists at all is low, so a single change can destroy the whole population. Second, genetic drift acts strongly: the genes passed on each generation are a small chance sample, so variations are lost by accident and the gene pool shrinks further. Third, with few partners available, mating between close relatives becomes unavoidable, and inbreeding brings harmful recessive genes into the homozygous state where they are expressed, reducing fertility and survival. A large population keeps a large store of variation, drift is weak and inbreeding is rare, so it survives the same challenge.

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7. Q32
   What ​are homologous structures? Give an example. ​Is it necessary that homologous structures ​always have ​a ​common ​ancestor?
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   Organs with the same basic design but different functions, as in vertebrate forelimbs; yes, they always trace back to a common ancestor.

   Homologous structures are organs of different organisms that have the same basic structural design and the same embryonic origin but may perform different functions, having been modified for different ways of life. Example: the forelimbs of vertebrates, where the arm of a human, the wing of a bat, the flipper of a whale and the wing of a bird all contain the same bones, humerus, radius and ulna, carpals, metacarpals and phalanges, in the same arrangement, although they are used for grasping, flying, swimming and flying respectively. Yes, it is necessary: homologous structures always trace back to a common ancestor, because the shared plan is inherited from an ancestor that already had it, and the differences arose later by divergent evolution. Structures that merely resemble each other but arose independently in unrelated lines are analogous, not homologous, as with the wing of a bird and the wing of an insect.

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8. Q33
   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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   No: diversity arises by divergence from common ancestors, shown by homologous organs, shared DNA and genetic code, connecting-link fossils and similar embryos.

   No. The diversity of animals does not mean that their ancestry is diverse. Evolution explains diversity as the result of divergence from common ancestors: a single ancestral population becomes divided, gene flow between the parts stops, each part accumulates its own variations, and natural selection in different environments drives them apart until they can no longer interbreed. Repeated over long periods, this produces many species from one. The evidence that this is what happened is that animals of very different appearance still share deep similarities: homologous organs such as the forelimbs of a human, a bat, a whale and a bird, which contain the same bones in the same order; the same DNA and RNA chemistry and essentially the same genetic code in all of them, with the closest species having the most similar DNA and proteins; connecting-link fossils such as Archaeopteryx; and closely similar early embryos in very different vertebrates. Diversity is therefore the outcome of common ancestry, not evidence against it.

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9. Q34
   Give the ​pair of ​contrasting traits of the ​following ​characters in pea ​plant ​and mention which is dominant ​and recessive

   1. (i)yellow seed ​
   2. (ii)round seed
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   Yellow (dominant) against green (recessive) seeds; round (dominant) against wrinkled (recessive) seeds.

   (i) Seed colour: the contrasting pair is yellow seed and green seed. Yellow is the dominant trait and green is the recessive trait. (ii) Seed shape: the contrasting pair is round seed and wrinkled seed. Round is the dominant trait and wrinkled is the recessive trait.

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10. Q35
    Why did Mendel ​choose pea ​plant ​for ​his ​experiments?
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    It has clearly contrasting characters, self-pollinating bisexual flowers that can also be cross-pollinated at will, a short life cycle and many seeds.

    Mendel chose the pea plant because it has several characters that occur in two sharply contrasting and easily distinguishable forms, such as tall and dwarf plants, round and wrinkled seeds, yellow and green seeds and violet and white flowers, so the offspring could be sorted into classes and counted. Its flowers are bisexual and normally self-pollinating, which produces pure-breeding lines and gave him parents of known constant type. At the same time the flower is large enough for the stamens to be removed and for pollen of his own choosing to be applied, so he could carry out controlled cross-pollination. The plant is an annual that is easy to grow and needs little care, so many generations could be raised in a short time, each cross yielded a large number of seeds, and the hybrids were fully fertile, so an $F_2$ generation could always be obtained and reliable ratios calculated.

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11. Q36
    A woman ​has only ​daughters. Analyse the situation genetically and ​provide a ​suitable ​explanation.
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    The mother gives only $\text{X}$, so the father's sperm decides each child's sex; having only daughters is a chance outcome.

    The woman is not responsible for having only daughters. Every human female is $\text{XX}$, so each of her eggs carries an $\text{X}$ chromosome and her contribution is the same at every conception. The father is $\text{XY}$ and produces $\text{X}$-bearing and $\text{Y}$-bearing sperms in equal numbers, so the sex of each child was decided by whether an $\text{X}$ sperm or a $\text{Y}$ sperm fertilised the egg: $\text{X}$ gives an $\text{XX}$ zygote, a girl, and $\text{Y}$ gives an $\text{XY}$ zygote, a boy. Each conception is an independent event with a probability of one half for each sex, so a run of daughters is simply a run of chance outcomes. For four children the probability of all being daughters is ${\left(\frac{1}{2}\right)}^4=\frac{1}{16}$, which is far from unusual. There is nothing in the woman's genetic make-up that could favour daughters, since she has no $\text{Y}$ chromosome to give.

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