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NCERT Exemplar · Class 10 Science Heredity and Evolution

47 questions · 47 still being checked

Long Answer Questions 37–47 (part 5 of 5)

  1. Exercise 37

    Does geographical isolation of individuals of a species lead to formation of a new species? Provide a suitable explanation.

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    NCERT’s answer
    Yes, geographical isolation gradually leads to genetic drift. This may impose limitations to sexual reproduction of the separated population. Slowly the separated individuals will reproduce among themselves and generate new variations. Continuous accumulation of those variations through a few generations may ultimately lead to the formation of a new species.
    Yes — geographical isolation is a common route to speciation.A mountain range, river, or sea arm splits one interbreeding population into two sub-populations. Gene flow between the two stops. Independent mutations and genetic drift then accumulate differently in each group, and natural selection favours different traits under each group's local conditions.Given enough generations, the DNA of the two groups diverges so far that, even if the barrier disappears and the groups meet again, they can no longer interbreed to produce fertile offspring.Answer: Reproductive isolation is the end result — the two diverged populations are now separate species.

  2. Exercise 38

    Bacteria have a simpler body plan when compared with human beings. Does it mean that human beings are more evolved than bacteria? Provide a suitable explanation.

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    NCERT’s answer
    This is a debatable issue. If appearance of complexity is concurrent with evolution then, human beings are certainly more evolved than bacteria. But if we take the totality of life characteristics into account, then it is hard to label either organism as evolved.
    No — "more evolved" is a misleading way to compare them.Evolution has no target of increasing complexity; it only keeps whatever variant survives and reproduces best in its own environment. Bacteria and humans have both been evolving by natural selection for the same length of time since life began, along separate lineages.A simple body plan is not an unfinished one — it is what natural selection has kept because it works: fast reproduction and a wide metabolic range, with no more complexity than the bacterium's niche needs.Answer: Both are equally evolved; a simple body plan reflects the environment a lineage adapted to, not a lesser degree of evolution.

  3. Exercise 39

    All the human races like Africans, Asians, Europeans, Americans and others might have evolved from a common ancestor. Provide a few evidences in support of this view.

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    NCERT’s answer
    Hints— Common body plan, structure, physiology and metabolism. Constant chromosome number Common genetic blue print Freely inter-breeding
    Interbreeding: members of any two races mate and have fertile children, so all belong to one species.Same genetic make-up: the same $\displaystyle 46$ chromosomes, the same blood-group types and the same genetic code in every population.Same body plan: the same skeleton and organ systems; skin colour, hair form and build differ only as adaptations to climate and sunlight.Fossils and DNA: the oldest Homo sapiens fossils are African, and the DNA of living populations traces back to that one group:\[\text{Africa} \rightarrow \text{spread over continents} \rightarrow \text{geographical isolation} \rightarrow \text{regional traits} \]Answer: Shared fertility, chromosomes, body plan and African fossil-DNA lineage point to one common ancestor.

  4. Exercise 40

    Differentiate between inherited and acquired characters. Give one example for each type.

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    NCERT’s answer
    Characters that are passed on from parents to offspring are inherited characters e.g., colour of seeds, colour of eyes. Characters appearing in an individual's life time but cannot be transmitted to next generation are acquired characters e.g., obese body, loss of a finger in an accident.
    Inherited: a trait coded in the DNA of germ cells (sperm, egg), so it passes to offspring and can spread through a population. Example: blood group.Acquired: a change made in body (somatic) cells by use, disuse, injury or environment; it ends with that individual. Example: enlarged muscles of a weightlifter.\[\text{germ-cell DNA} \rightarrow \text{offspring} \;\Rightarrow\; \text{inherited} \qquad \text{somatic change} \nrightarrow \text{offspring} \;\Rightarrow\; \text{acquired} \]Answer: The difference is where the change sits: germ-cell DNA (inherited) or somatic cells (acquired).

  5. Exercise 41

    Give reasons why acquired characters are not inherited.

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    NCERT’s answer
    Acquired characters do not produce change in the DNA of germ cells, so they cannot be inherited. Only those characters which have a gene for them can be inherited.
    Reproduction hands on only the DNA in the gametes:\[\text{acquired change} \Rightarrow \text{somatic cells only} \Rightarrow \text{germ-cell DNA unchanged} \Rightarrow \text{gametes carry the old code} \Rightarrow \text{offspring unchanged} \]Weismann cut off the tails of parent mice over several generations before breeding them; the young were still born with full tails, because the tail-length code in the germ cells was never touched.Answer: Acquired changes stay in somatic cells and never alter germ-cell DNA, so they are not inherited.

  6. Exercise 42

    Evolution has exhibited a greater stability of molecular structure when compared with morphological structures. Comment on the statement and justify your opinion.

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    NCERT’s answer
    We see immense diversity in size, form, structure and morphological features in the living world. But at the molecular level these, diverse types of organisms exhibit unbelievable similarity. For instance, the basic biomolecules like DNA, RNA, carbohydrates, proteins etc. exhibit remarkable similarity in all organisms.
    True — molecules central to metabolism are far more conserved than outward body form.Cytochrome c, DNA-repair enzymes and other essential proteins do the same core biochemical job in almost every organism, so a mutation that alters their structure is usually harmful and gets removed by selection — their sequence stays nearly unchanged across hundreds of millions of years.Morphology (limbs, wings, fins, body covering) is what directly meets the environment — diet, habitat, predators — so it is under strong, varying selection pressure and diverges quickly even between close relatives.Answer: Molecular structure is shielded by strong selection against change, while morphology is reshaped by fast-changing external pressures — so molecules stay stable while body form diversifies.

  7. Exercise 43

    In the following crosses write the characteristics of the progeny
    CrossProgeny
    (a) RR YY × RR YY Round, yellow Round, yellow____\displaystyle \_\_\_\_
    (b) Rr Yy × Rr Yy Round, yellow Round, yellow____\displaystyle \_\_\_\_
    (c) rr yy × rr yy wrinkled, green wrinkled, green____\displaystyle \_\_\_\_
    (d) RR YY × rr yy Round, yellow wrinkled green____\displaystyle \_\_\_\_

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    (a)
    Round, yellow
    (b)
    Round, yellow
    Round, green
    Wrinkled, yellow
    Wrinkled, green
    (c)
    Wrinkled, green
    (d)
    Round, yellow
    (a)
    Pure-breeding parents form one gamete type each.
    \[RRYY \times RRYY \rightarrow RY \times RY \rightarrow RRYY\ \text{(round, yellow)} \]
    (b)
    Each parent forms four gamete types by independent assortment.
    \[\begin{array}{c|cccc} \times & RY & Ry & rY & ry \\ \hline RY & RRYY & RRYy & RrYY & RrYy \\ Ry & RRYy & RRyy & RrYy & Rryy \\ rY & RrYY & RrYy & rrYY & rrYy \\ ry & RrYy & Rryy & rrYy & rryy \end{array} \]
    \[9\ \text{round, yellow} : 3\ \text{round, green} : 3\ \text{wrinkled, yellow} : 1\ \text{wrinkled, green} \]
    (c)
    \[rryy \times rryy \rightarrow ry \times ry \rightarrow rryy\ \text{(wrinkled, green)} \]
    (d)
    \(\displaystyle R\) and \(\displaystyle Y\) are dominant.
    \[RRYY \times rryy \rightarrow RY \times ry \rightarrow RrYy\ \text{(round, yellow)} \]
    Answer: (a) all round, yellow; (b) $\displaystyle 9$ round, yellow : $\displaystyle 3$ round, green : $\displaystyle 3$ wrinkled, yellow : $\displaystyle 1$ wrinkled, green; (c) all wrinkled, green; (d) all round, yellow.

  8. Exercise 44

    Study the following cross and showing self pollination in F1\displaystyle \mathrm{F}_1, fill in the blank and answer the question that follows
    ParentsRRYY×rryy
    Round, yellowwrinkled, green
    F1\displaystyle \mathrm{F}_1 —Rr Yy×?
    Round, yellow

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    NCERT’s answer
    Rr Yy Round, yellow
    Self-pollination means the \(\displaystyle F_1\) plant is crossed with itself, so the missing parent carries the same genotype as the round, yellow \(\displaystyle F_1\). \[F_1:\ RrYy \times RrYy \] Both plants form the four gamete types \(\displaystyle RY,\ Ry,\ rY,\ ry\) in equal proportion by independent assortment of the two gene pairs. Answer: the blank is Rr Yy — the round, yellow \(\displaystyle F_1\) self-pollinates with itself.

  9. Exercise 45

    In question 44\displaystyle 44, what are the combinations of character in the F2\displaystyle \mathrm{F}_2 progeny? What are their ratios?

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    NCERT’s answer
    (i)
    Round yellow — $\displaystyle 9$
    (ii)
    Round green — $\displaystyle 3$
    (iii)
    Wrinkled yellow — $\displaystyle 3$
    (iv)
    Wrinkled green — $\displaystyle 1$
    \(\displaystyle 9: 3: 3: 1\)
    The \(\displaystyle F_1\) self-cross \(\displaystyle RrYy \times RrYy\) gives $\displaystyle 16$ equally likely zygotes. \[\begin{array}{c|cccc} \times & RY & Ry & rY & ry \\ \hline RY & RRYY & RRYy & RrYY & RrYy \\ Ry & RRYy & RRyy & RrYy & Rryy \\ rY & RrYY & RrYy & rrYY & rrYy \\ ry & RrYy & Rryy & rrYy & rryy \end{array} \] Sorting these by phenotype: \[9\ \text{round, yellow} : 3\ \text{round, green} : 3\ \text{wrinkled, yellow} : 1\ \text{wrinkled, green} \] Answer: four combinations appear in ratio $\displaystyle 9$:$\displaystyle 3$:$\displaystyle 3$:$\displaystyle 1$; round, yellow and wrinkled, green match the parents, while round, green and wrinkled, yellow are new combinations not seen in either parent.

  10. Exercise 46

    Give the basic features of the mechanism of inheritance.

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    NCERT’s answer
    (i)
    Characters are controlled by genes.
    (ii)
    Each gene controls one character
    (iii)
    There may be two or more forms of the gene
    (iv)
    One form may be dominant over the other
    (v)
    Genes are present on chromosomes
    (vi)
    An individual has two forms of the gene whether similar or dissimilar
    (vii)
    The two forms separate at the time of gamete formation
    (viii)
    The two forms are brought together in the zygote
    Characters are controlled by genes, each gene controlling one character. Genes lie on chromosomes and occur in pairs (alleles), one inherited from each parent. The two alleles stay together in the organism's cells but separate at gamete formation, one allele per gamete. \[Aa \xrightarrow{\text{gamete formation}} A : a = 1:1 \] When the two alleles differ, only the dominant one is expressed; the recessive is masked. For two characters, each gene pair assorts into gametes independently of the other. \[AaBb \rightarrow AB : Ab : aB : ab = 1:1:1:1 \] Fertilisation restores the pair. \[A\ (\text{gamete}) + a\ (\text{gamete}) \xrightarrow{\text{fertilisation}} Aa\ (\text{zygote}) \] Answer: genes on chromosomes, each controlling one character, occur as pairs of alleles from both parents; dominant is expressed, recessive masked; pairs segregate into gametes and different pairs assort independently; fertilisation restores the pairs.

  11. Exercise 47

    Give reasons for the appearance of new combinations of characters in the F2\displaystyle \mathrm{F}_2 progeny.

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    NCERT’s answer
    The tall/short and round/wrinkled seed trait are independently inherited.
    Round, green and wrinkled, yellow peas are absent from the parents (round, yellow and wrinkled, green) yet appear in \(\displaystyle F_2\). In the \(\displaystyle F_1\) (\(\displaystyle RrYy\)) the seed-shape gene pair and the seed-colour gene pair separate independently of each other during gamete formation, so a gamete can carry either parental combination or a mixed one. \[RrYy \rightarrow RY,\ Ry,\ rY,\ ry\ \ (1:1:1:1) \] Random fertilisation between these four gamete types from each parent then produces genotypes for all four phenotype classes. Answer: independent assortment of the two gene pairs in the \(\displaystyle F_1\) lets alleles recombine, giving the new round-green and wrinkled-yellow classes ($\displaystyle 3$:$\displaystyle 3$ of the $\displaystyle 9$:$\displaystyle 3$:$\displaystyle 3$:$\displaystyle 1$ ratio in \(\displaystyle F_2\)).