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NCERT Class 10 Science Heredity: Exercise Solutions

Step-by-step working for every question of End-of-chapter exercises in NCERT's Class 10 Science book, chapter “Heredity”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. Exercises Q1

   Tall violet-flowered pea plants were crossed with short white-flowered ones; all progeny were violet but about half were short. The tall parent's make-up is (a) TTWW (b) TTww (c) TtWW (d) TtWw.

   1. Let T represent the dominant tall allele and t represent the recessive short allele.
   2. Let W represent the dominant violet allele and w represent the recessive white allele.
   3. The cross is between a tall violet parent and a short white parent with genotype ttww.
   4. All progeny have violet flowers, meaning every offspring received a dominant W allele from the tall parent.
   5. Since the short parent can only provide w alleles, the tall parent must be homozygous dominant WW to ensure all offspring are violet.
   6. About half of the progeny are short, indicating a 1:1 ratio for the height trait in the offspring.
   7. A 1:1 ratio for height occurs when a heterozygous Tt parent is crossed with a homozygous recessive tt parent.
   8. Combining the height genotype Tt and the flower color genotype WW, the tall parent's make-up is TtWW.
   9. This corresponds to option (c).

   Answer: (c) TtWW

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2. Exercises Q2

   Children with light-coloured eyes tend to have parents with light-coloured eyes. Can we say whether the trait is dominant or recessive?

   1. The observation states that children with light-coloured eyes tend to have parents with light-coloured eyes.
   2. This pattern suggests that the light-coloured eye trait is passed from parents to offspring.
   3. However, this observation alone does not reveal the dominance relationship between the alleles.
   4. A recessive trait can also appear in children if both parents carry the recessive allele, even if one parent shows the dominant trait.
   5. To determine if a trait is dominant or recessive, we need to know the phenotypes of the parents and the ratio of traits in the offspring from a specific cross.
   6. Since the question only gives a general tendency and not specific cross data, we cannot conclude dominance or recessiveness.
   7. Therefore, the given information is insufficient to classify the trait as dominant or recessive.

   Answer: No, we cannot say whether the trait is dominant or recessive based on this observation alone.

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3. Exercises Q3

   Outline a project to find the dominant coat colour in dogs.

   1. Select two pure-breeding dog breeds with different coat colours, such as one with black fur and one with white fur.
   2. Mate the black dog with the white dog to produce the first filial generation, known as the F1 generation.
   3. Observe the coat colour of all the F1 puppies: the colour they all show is the dominant one (the other colour is hidden, i.e. recessive).
   4. Mate two F1 puppies with each other to produce the second filial generation, known as the F2 generation.
   5. Observe and record the coat colour of all the puppies in the F2 generation.
   6. Count the number of puppies with each coat colour in the F2 generation and calculate the ratio.
   7. In the F2 generation the dominant colour appears in about three-fourths of the puppies and the recessive colour reappears in about one-fourth (roughly 3 : 1), which confirms the result.

   Answer: Cross pure-breeding dogs of two coat colours: the colour shown by all the F1 puppies is dominant, and in the F2 generation it appears in about 3 out of 4 puppies (3 : 1), confirming it.

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4. Exercises Q4

   How is equal genetic contribution of the male and female parents ensured in the progeny?

   1. In humans, the male parent produces sperm cells that carry either an X or a Y sex chromosome.
   2. The female parent produces egg cells that always carry an X sex chromosome.
   3. During fertilization, one sperm cell fuses with one egg cell to form a zygote.
   4. The zygote receives exactly one set of 23 chromosomes from the father and one set of 23 chromosomes from the mother.
   5. This fusion ensures that the offspring has a total of 46 chromosomes, with half contributed by each parent.
   6. Thus, the genetic material is equally shared between the male and female parents in the progeny.

   Answer: Equal genetic contribution is ensured because the zygote receives one set of chromosomes from the father (via sperm) and one set from the mother (via egg), resulting in a 50-50 share of genetic material.

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