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NCERT Exemplar · Class 10 Science Heredity and Evolution

47 questions · 47 still being checked

Short Answer Questions 26–36 (part 4 of 5)

  1. Exercise 26

    How is the sex of a newborn determined in humans?

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    NCERT’s answer
    The sex of the individual is genetically determined i.e., genes inherited from parents decide whether the new born will be a boy or a girl. A new born who inherits an 'X' chromosome from father will be a girl and one who inherits a 'Y' chromosome will be a boy.
    The 23rd chromosome pair carries the sex information: \(\displaystyle XX\) in a female, \(\displaystyle XY\) in a male. \[\text{Mother (XX)} \xrightarrow{\text{meiosis}} \text{every ovum} = X \] \[\text{Father (XY)} \xrightarrow{\text{meiosis}} \text{sperm} = X \ \text{or} \ Y \] \[X_{\text{egg}} + X_{\text{sperm}} \rightarrow XX \ (\text{girl}), \qquad X_{\text{egg}} + Y_{\text{sperm}} \rightarrow XY \ (\text{boy}) \]The father's sperm decides the outcome. Answer: Chromosomally -- by whether an X-bearing or Y-bearing sperm fertilises the ovum.

  2. Exercise 27

    Do genetic combination of mothers play a significant role in determining the sex of a new born?

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    NCERT’s answer
    No, because mothers have a pair of X-chromosomes. All children will inherit an 'X' chromosome from their mother regardless of whether they are boys or girls.
    \[\text{All ova}: X \quad (\text{mother is homogametic}) \] \[\text{Sperm}: X \ \text{or} \ Y \quad (\text{father is heterogametic}) \] No. The mother contributes an X chromosome to every child; her genetic combination cannot vary the outcome. Answer: No -- sex is fixed by the father's heterogametic sperm (X or Y); the mother is homogametic and always contributes X.

  3. Exercise 28

    Mention three important features of fossils which help in the study of evolution.

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    NCERT’s answer
    (a)
    Fossils represent modes of preservation of ancient species.
    (b)
    Fossils help in establishing evolutionary traits among organisms and their ancestors.
    (c)
    Fossils help in establishing the time period in which organisms lived.
    (i)
    Preserved remains or impressions of ancient, often extinct, organisms -- direct evidence of past life.
    (ii)
    Transitional forms, e.g. Archaeopteryx (reptile-like teeth and tail, bird feathers), link one group to its ancestor.
    (iii)
    Depth of rock strata or isotope dating gives their age -- when the organisms lived.
    Answer: Fossils are direct remains of extinct species, show transitional links between groups, and date evolutionary events.

  4. Exercise 29

    Why do all the gametes formed in human females have an X chromosome?

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    NCERT’s answer
    Human females have two X chromosomes called sex chromosomes. During meiosis at the time of gamete formation, one X chromosome enters each gamete. Hence all the gametes possess an X chromosome.
    \[\text{Female germ cell (XX)} \xrightarrow{\text{meiosis}} \text{each ovum} = X \quad (\text{no Y available}) \] Females are homogametic: both sex chromosomes are X, so meiosis has only X to distribute to a gamete. Answer: Because a female's sex-chromosome pair is XX, not XY -- there is no Y chromosome for meiosis to send to an egg.

  5. Exercise 30

    In human beings, the statistical probability of getting either a male or female child is 50\displaystyle 50 : 50. Give a suitable explanation.

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    NCERT’s answer
    The sex of an infant is determined by the type of sex chromosome contributed by the male gamete. Since the ratio of male gametes containing X chromosome and those containing Y chromosome is $\displaystyle 50$ : $\displaystyle 50$, the statistical probability of male or a female infant is also $\displaystyle 50$ : 50.
    \[P(X\text{-sperm}) = \tfrac12, \qquad P(Y\text{-sperm}) = \tfrac12 \] \[P(XX,\ \text{girl}) = 1 \times \tfrac12 = \tfrac12, \qquad P(XY,\ \text{boy}) = 1 \times \tfrac12 = \tfrac12 \] A man makes equal numbers of X- and Y-bearing sperm; a woman's eggs are always X. Whichever type fertilises the egg is a fair chance event. Answer: Equal numbers of X- and Y-sperm make fertilisation with either type equally likely, giving XX and XY each probability \(\displaystyle \tfrac12\).

  6. Exercise 31

    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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    NCERT’s answer
    Fewer individuals in a species impose extensive inbreeding among them. This limits the appearance of variations and puts the species at a disadvantage if there are changes in the environment. Since the individuals fail to cope up with the environmental changes, they may become extinct.
    \[N \downarrow \ \Rightarrow\ \text{allele diversity} \downarrow \ \Rightarrow\ \text{adaptability} \downarrow \ \Rightarrow\ \text{extinction risk} \uparrow \] A small population has few individuals and so few allele combinations -- a narrow gene pool. Inbreeding and drift shrink variation further, leaving little raw material for selection when conditions change. Answer: Low genetic variation in a small population limits its ability to adapt, so environmental change is more likely to wipe it out.

  7. Exercise 32

    What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor?

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    NCERT’s answer
    Structures which have a common basic structure but perform different functions are called homologous structures. e.g. fore limbs of reptiles, amphibians and mammals. Yes, they have common ancestor but variously modified to carry out different activities.
    Homologous structures share the same basic structure and origin but perform different functions, having evolved by divergent evolution from a common ancestor. Example: forelimbs of humans, bats and whales share the same bone arrangement — humerus, radius-ulna, carpals, metacarpals, phalanges — modified for grasping, flying and swimming respectively. Answer: Yes; common descent from one ancestral structure is what defines homology.

  8. Exercise 33

    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. Body-form diversity does not need diverse ancestry: species arise when heritable variation accumulates in a population over generations by natural selection and drift, causing divergence from its parent stock. Present-day diversity is thus the branching, tree-like outcome of gradual divergence from shared, often common ancestors — not of many independent origins. Answer: No — diversity reflects branching descent from common ancestors, not diverse origins.

  9. Exercise 34

    Give the pair of contrasting traits of the following characters in pea plant and mention which is dominant and recessive
    (i)
    yellow seed
    (ii)
    round seed

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    NCERT’s answer
    (a)
    yellow — dominant
    green — recessive
    (b)
    round — dominant
    wrinkled — recessive
    (i)
    Seed colour: yellow (dominant) and green (recessive). (ii) Seed shape: round (dominant) and wrinkled (recessive). Answer: Yellow and round are dominant; green and wrinkled are recessive.

  10. Exercise 35

    Why did Mendel choose pea plant for his experiments?

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    NCERT’s answer
    (a)
    Easy to grow
    (b)
    Short life span
    (c)
    Easily distinguishable characters
    (d)
    Larger size of flower
    (e)
    Self pollinated
    The pea plant suited controlled, countable crosses. Answer: Easy to grow; short life span; many easily distinguishable, true-breeding contrasting traits; large flowers, easy to emasculate and cross-pollinate by hand; naturally self-pollinating; many seeds per cross.

  11. Exercise 36

    A woman has only daughters. Analyse the situation genetically and provide a suitable explanation.

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    NCERT’s answer
    ((a) The woman produces ova with 'X' chromosome
    (b)
    The man produces sperms with X and Y chromosome which actually determines the sex of the baby.
    Females are XX (eggs always carry X); males are XY, giving X- and Y-bearing sperm in equal numbers. Sex is fixed by the sperm's chromosome, i.e. by the father, not the mother. \[P(\text{daughter at any birth}) = P(\text{sperm carries X}) = \tfrac12 \] \[P(\text{all } n \text{ births daughters}) = \left(\tfrac12\right)^n \] Answer: By chance alone — an X-bearing sperm fertilised the egg at each independent conception; the mother is not responsible.