Chapter 8 · Heredity

NCERT Class 10 Science Chapter 8 Exercises

All 4 questions of NCERT Class 10 Science Chapter 8 Exercises (Heredity), in the book's order, each with its answer and a step-by-step solution.

1. Q1
   A ​Mendelian experiment ​consisted ​of breeding tall ​pea plants bearing violet flowers ​with short pea plants bearing white flowers. The progeny all bore violet flowers, ​but almost half of them were short. This suggests that the genetic make-up of ​the ​tall parent can be depicted as

   1. (a)TTWW
   2. (b)TTww
   3. (c)TtWW
   4. (d)TtWw
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   (c) TtWW

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2. Q2
   A study found that children with ​light-coloured eyes are likely to have parents with ​light-coloured eyes. On this basis, can we say anything about ​whether the light eye colour trait is dominant ​or recessive? Why or why ​not?
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   No: light-eyed children of light-eyed parents are expected whether light is recessive or dominant, so the observation cannot decide.

   No, nothing can be concluded about dominance from this observation. The finding shows only that eye colour is inherited — that a child's eye colour is related to the parents' — which was not in question. Dominance is decided by crossing parents with contrasting forms of a trait: the form that appears in all the first-generation progeny is dominant, and the form that disappears and then reappears in about a quarter of the second generation is recessive. The study reports no such cross. Worse, the observation is exactly what both possibilities predict. If light eye colour is recessive, a light-eyed person is $ll$, so two light-eyed parents can pass on only $l$ and every child of theirs is light-eyed. If light eye colour is dominant, a light-eyed parent carries at least one $L$, so two light-eyed parents would still have light-eyed children in most cases — all of them if either parent is $LL$, and about three in four if both are $Ll$. Since both hypotheses predict that light-eyed children usually have light-eyed parents, the observation cannot separate them. What would settle it is a family in which the parents have contrasting eye colours, and in particular a light-eyed child born to two dark-eyed parents: a trait that appears in a child but in neither parent must have been carried hidden by both, and only a recessive trait can be carried hidden.

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3. Q3
   Outline ​a ​project which ​aims to find ​the ​dominant coat ​colour ​in ​dogs.
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   Cross pure black and pure white dogs: the colour every first-generation puppy shows is dominant, and the second generation gives about $3:1$.

   The project follows Mendel's method, transferred from pea plants to dogs. (i) Choose one coat colour character with two clearly contrasting forms that cannot be confused — say black coat against white coat — and one known to be controlled by a single pair of genes, since several genes affect coat colour in dogs. (ii) Obtain dogs that breed true for each colour. A dog is taken as pure-breeding for black if, over several generations of matings within the black group, all its offspring have been black; breeders' and kennel-club records can establish this. (iii) Mate a pure-breeding black dog with a pure-breeding white one, and record the coat colour of every puppy of the first generation, over several pairs and several litters. (iv) Read the answer off the first generation: all the puppies will show one of the two colours and none the other, and the colour that every puppy shows is the **dominant** one, while the colour that has disappeared is recessive. (v) Confirm the result by mating the first-generation dogs among themselves. In the second generation the missing colour reappears, in about one puppy in four, giving a ratio of about $3:1$ of the dominant colour to the recessive one — which shows that the recessive allele was carried through the first generation unchanged and merely masked. Throughout, keep all the dogs in the same conditions, use healthy unrelated animals, record every puppy of every litter, and pool the counts from many litters before comparing them with the expected ratio, because a ratio only shows itself in large numbers.

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4. Q4
   How ​is ​the equal ​genetic ​contribution ​of male and female ​parents ​ensured in the ​progeny?
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   Sperm and egg each get one chromosome of every pair, and fertilisation restores the pairs, so each parent gives the same number.

   The equality is ensured by the way chromosomes are handled when germ cells are made and when they fuse. (i) In the body cells of a sexually reproducing organism the chromosomes occur in pairs, one member of each pair having come from the father and the other from the mother; in human beings there are $23$ such pairs, that is $46$ chromosomes. (ii) When germ cells are formed, a special kind of cell division halves this number: each germ cell receives only **one** chromosome of every pair, so a sperm carries a single complete set of $23$ and an egg carries a single complete set of $23$. Neither carries an arbitrary half of the DNA — each carries exactly one member of every pair. (iii) At fertilisation the sperm and the egg fuse, and the zygote again has the full $46$ arranged in $23$ pairs, with one member of every pair coming from the father and the other from the mother. So each parent contributes exactly the same number of chromosomes, and contributes to every pair, which is what makes the genetic contribution equal; and because the halving and the restoration happen in every generation, the chromosome number of the species stays constant. The one qualification is the sex chromosomes: the mother always contributes an X while the father contributes an X or a Y, and it is this that determines the sex of the child — but the number of chromosomes contributed by each parent is still the same.

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