Chapter 9 · Heredity and Evolution

NCERT Exemplar Class 10 Science Chapter 9 Long Answer Questions

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

1. Q37
   Does ​geographical ​isolation of ​individuals of a species ​lead ​to formation of a new species? Provide ​a ​suitable ​explanation.
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   Yes: a barrier stops gene flow, each group accumulates different variations under selection, and after many generations they become reproductively isolated species.

   Yes, in sexually reproducing organisms geographical isolation can lead to the formation of a new species. A physical barrier such as a river, a mountain range or a sea divides one interbreeding population into sub-populations that live apart. Gene flow between them stops, so a variation arising in one group can no longer spread to the other. Each group then accumulates its own genetic changes: new mutations arise independently in each, and genetic drift alters gene frequencies by chance, strongly so in a small group. Because the two habitats differ, natural selection favours different variations on the two sides, and the two gene pools move steadily apart. After a large number of generations the differences become so great that even if the barrier is removed the members of one group can no longer interbreed with the other, or their offspring are not fertile. At that point they are reproductively isolated and are two separate species. Two qualifications apply: isolation alone is not enough, since the genetic changes must have time to accumulate; and geographical isolation is not an important factor for self-pollinating plants or for asexually reproducing organisms, whose reproduction does not depend on reaching a partner.

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2. Q38
   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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   No: body complexity is not degree of evolution; bacteria have evolved as long as humans and succeed in a wider range of habitats.

   No. Human beings have a more complex body design than bacteria, but that does not make them more evolved. Complexity of body plan and degree of evolution are different things. Bacteria and human beings are both alive today and both descend from the earliest forms of life, so the bacterial lineage has been evolving for exactly as long as ours. Evolution has no goal and no direction; changes are kept when they help an organism survive and reproduce in its own surroundings, and a simple, small, fast-reproducing body is as good a solution as a complex one. Judged by success, bacteria are outstanding: they live in hot springs, deep sea vents, ice, acidic and saline waters, soil, air and inside the bodies of other organisms, including ours; they occupy a far wider range of habitats than human beings and far outnumber all other organisms. Many of their habitats are places where no complex organism could survive at all. So human beings are more complex, not more evolved; bacteria are simply adapted in a different and highly successful way.

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3. Q39
   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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   Fertile interbreeding, very small DNA differences, oldest fossils from Africa and shared anatomy all point to one common ancestor.

   Several independent lines of evidence support a single common ancestor. (i) All human races interbreed freely and their children are fully fertile, which is the definition of one species; all human beings therefore belong to Homo sapiens and share one line of descent. (ii) Studies of human DNA show that the genetic differences between the races are very small, and smaller than the differences between individuals within a single race; comparisons along the maternal and paternal lines trace all living people to one small ancestral population, and the greatest genetic diversity is found in Africa. (iii) The earliest fossils of modern human beings come from Africa and are older than modern human fossils found anywhere else, and human fossils and tools appear at progressively later dates through Asia, Europe, Australia and the Americas, recording a spread outwards from one region. (iv) All races share the same forty-six chromosomes, the same anatomy and physiology, the same blood group systems and the same biochemistry, while the features that differ, such as skin colour, hair form and body build, are superficial adaptations to climate controlled by a few genes. Taken together these show that the races are branches of one species that spread from a common ancestral population and diverged only in surface characters.

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4. Q40
   Differentiate between ​inherited ​and acquired ​characters. ​Give ​one ​example for ​each type.
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   Inherited characters, such as eye colour, pass to offspring through gametes; acquired ones, such as low weight from starvation, affect only body cells.

   Inherited character: it is received from the parents through their gametes and is present in the DNA from the moment of fertilisation; it is controlled by genes and is present in the germ cells as well as the body cells; it is transmitted to the offspring; it cannot be removed by a change in the surroundings and lasts for life; and because it accumulates over generations it provides the raw material for evolution. Acquired character: it develops during the lifetime of the individual as a result of use or disuse, food, training, injury, disease or climate; it changes only the body or somatic cells and leaves the DNA of the germ cells unaltered; it is therefore not transmitted to the offspring; it may appear and disappear within one lifetime and dies with the individual; and it causes no evolution. Examples: colour of the eyes is an inherited character, and low body weight caused by starvation is an acquired character.

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5. Q41
   Give ​reasons ​why ​acquired characters ​are ​not ​inherited.
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   Acquired changes affect only body cells, not germ-cell DNA, so the gametes carry no record of them to the offspring.

   The only physical link between a parent and its offspring is the gamete, so a character can be inherited only if the information for it is written in the DNA carried by the sperm or the egg. An acquired character develops during the life of the individual through use or disuse, diet, training, injury, disease or climate, and such an influence changes only the body or somatic cells and the tissues concerned. It does not alter the DNA of the germ cells in the reproductive organs. Since the germ-cell DNA is unchanged, the gametes carry no record of the acquired change and the offspring is born without it. The altered body cells themselves cannot help, because they are never handed on and die with the individual. Weismann tested this directly by removing the tails of mice for many successive generations, and every generation was still born with a normal tail. Because acquired characters are not inherited they cannot accumulate over generations and cannot cause evolution, which is why Lamarck's theory of the inheritance of acquired characters is not accepted.

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6. Q42
   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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   Correct: all life shares DNA, RNA and the genetic code, but body forms diversified, because almost any change in core molecules is harmful.

   The statement is correct. Every organism, from a bacterium to a human being, uses DNA and RNA of the same chemistry, essentially the same genetic code so that the same triplet specifies the same amino acid throughout life, the same twenty amino acids, ATP as the energy currency, ribosomes for protein synthesis and core pathways such as glycolysis; even individual proteins such as cytochrome c are recognisably the same molecule in a yeast cell and in a human being. Over the very same period the external form has changed beyond recognition, so that a bacterium, a fern, a fish, a bird and a mammal share almost nothing in body plan, organs, size, nutrition or locomotion. The reason is that the core molecules are indispensable and every part of the cell depends on them, so almost any change in them is lethal or seriously harmful and is removed by natural selection as soon as it appears, whereas morphological characters can vary without killing the organism and different environments actively favour different forms. The same selection therefore conserves the molecules and diversifies the bodies. Greater stability does not mean no change: molecules do change, but slowly and steadily, and it is precisely this that allows the number of differences in DNA or protein between two species to measure how long ago they separated, which makes molecular phylogeny the most reliable modern evidence of evolutionary relationship.

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7. Q43
   In the following ​crosses write ​the characteristics of ​the progeny

   1. (a)$\text{RR YY}$ (Round, ​yellow) $\times$ $\text{RR YY}$ (Round, yellow)
   2. (b)$\text{Rr Yy}$ (Round, yellow) $\times$ $\text{Rr Yy}$ (Round, yellow)
   3. (c)$\text{rr yy}$ (wrinkled, green) $\times$ $\text{rr yy}$ (wrinkled, ​green)
   4. (d)$\text{RR YY}$ (Round, yellow) $\times$ $\text{rr yy}$ (wrinkled ​green)
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   (a) all $\text{RRYY}$; (b) round yellow, round green, wrinkled yellow, wrinkled green in the ratio $9:3:3:1$; (c) all $\text{rryy}$; (d) all $\text{RrYy}$.

   (a) $\text{RR YY}\times \text{RR YY}$: each parent forms only $\text{RY}$ gametes, so all the progeny are $\text{RRYY}$, that is $100\%$ round and yellow and true-breeding; only one genotype occurs. (b) $\text{Rr Yy}\times \text{Rr Yy}$: each parent forms four kinds of gamete, $\text{RY}$, $\text{Ry}$, $\text{rY}$ and $\text{ry}$, and the sixteen combinations give four kinds of progeny in the ratio $9$ round yellow $:3$ round green $:3$ wrinkled yellow $:1$ wrinkled green, with the genotypic ratio $1\text{RRYY}:2\text{RRYy}:2\text{RrYY}:4\text{RrYy}:1\text{RRyy}:2\text{Rryy}:1\text{rrYY}:2\text{rrYy}:1\text{rryy}$; round green and wrinkled yellow are new combinations. (c) $\text{rr yy}\times \text{rr yy}$: each parent forms only $\text{ry}$ gametes, so all the progeny are $\text{rryy}$, that is $100\%$ wrinkled and green, identical to the parents; only one genotype occurs. (d) $\text{RR YY}\times \text{rr yy}$: one parent gives only $\text{RY}$ and the other only $\text{ry}$, so every offspring is $\text{RrYy}$, that is $100\%$ round and yellow but heterozygous for both characters; only one genotype occurs.

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8. Q44
   Study the following ​cross and showing self ​pollination ​in $F_1$, fill in the ​blank and answer the ​question that follows Parents: $\text{RRYY}$ (Round, yellow) $\times$ $\text{rryy}$ (wrinkled, green) $F_1$: $\text{Rr Yy}$ (Round, yellow) $\times$ ​?
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   The blank is $\text{Rr Yy}$, because self pollination crosses the $F_1$ plant with itself.

   The blank is $\text{Rr Yy}$, round and yellow. Self pollination means that the pollen and the ovule come from the same plant, so the second partner in the $F_1$ cross is the $F_1$ plant itself, whose genotype is $\text{Rr Yy}$. The $F_1$ cross is therefore $\text{Rr Yy}\times \text{Rr Yy}$, and it is this cross that gives rise to the $F_2$ generation.

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9. Q45
   In question 44, ​what are the ​combinations of character in the $F_2$ ​progeny? ​What ​are their ratios?
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   Round yellow, round green, wrinkled yellow and wrinkled green in the ratio $9:3:3:1$.

   The $F_1$ plants of question $44$ are $\text{Rr Yy}$ and are self pollinated, so the cross is $\text{Rr Yy}\times \text{Rr Yy}$. Each plant forms four kinds of gamete, $\text{RY}$, $\text{Ry}$, $\text{rY}$ and $\text{ry}$, in equal numbers, and the sixteen combinations give four combinations of characters in the $F_2$: round and yellow, round and green, wrinkled and yellow, and wrinkled and green. Their phenotypic ratio is $9:3:3:1$. Two of these, round green and wrinkled yellow, are new combinations not present in either grandparent. The corresponding genotypic ratio is $1\text{RRYY}:2\text{RRYy}:2\text{RrYY}:4\text{RrYy}:1\text{RRyy}:2\text{Rryy}:1\text{rrYY}:2\text{rrYy}:1\text{rryy}$.

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10. Q46
    Give the basic ​features ​of ​the ​mechanism ​of inheritance.
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    Genes come in two copies, one from each parent; gametes get one copy, fertilisation restores two, and different genes are inherited independently.

    The basic features are these. Every character is controlled by a gene, a section of the DNA of a chromosome that carries the information for one protein. In a sexually reproducing organism the chromosomes are in pairs, so each gene is present in two copies, one inherited from the father and one from the mother, situated at the same place on the two chromosomes of a pair. The two copies may be identical or may be different versions of the gene, and when they differ the dominant copy is expressed while the recessive copy is carried without being expressed and can reappear in a later generation. During gamete formation the two chromosomes of every pair separate, so each gamete receives only one copy of each gene and the number is halved. At fertilisation the two gametes fuse, restoring two copies in the progeny, one from each parent, so the chromosome number of the species stays constant. Finally, genes for different characters lying on different chromosomes are inherited independently of one another, which is why the progeny can show combinations of characters that neither parent had.

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11. Q47
    Give reasons ​for the ​appearance of new combinations of ​characters ​in the $F_2$ progeny.
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    Independent assortment makes new gamete combinations and random fertilisation brings them together, giving round green and wrinkled yellow, 6 in 16 of $F_2$.

    New combinations appear for two reasons acting in turn. First, independent assortment during gamete formation. The $F_1$ plant is $\text{Rr Yy}$, heterozygous for both characters, and the two gene pairs lie on different pairs of chromosomes, so when the pairs separate in meiosis the separation of $\text{R}$ from $\text{r}$ does not decide whether $\text{Y}$ or $\text{y}$ goes with it. The $F_1$ therefore makes four kinds of gamete in equal numbers, $\text{RY}$, $\text{Ry}$, $\text{rY}$ and $\text{ry}$, and two of these, $\text{Ry}$ and $\text{rY}$, carry combinations that neither grandparent could produce. Second, random fusion at fertilisation. Any male gamete may fuse with any female gamete, giving sixteen equally likely combinations, so the recombined gametes are brought together in every possible way. Dominance then decides when the result is seen: in the $F_1$ every plant looks round and yellow and the new combinations are hidden, while in the $F_2$ the recessive copies can meet and the new appearances become visible. Sorting the sixteen by appearance gives $9$ round yellow $:3$ round green $:3$ wrinkled yellow $:1$ wrinkled green, in which round green and wrinkled yellow, six seeds in every sixteen, are the new combinations. No new gene has been created; the existing genes have simply been redistributed.

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