Class 10 Science Chapter – 8 Heredity Questions and Answers
Questions (Page 129)
Question 1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer: Trait B is likely to have arisen earlier. In an asexually reproducing species, variations accumulate over generations. A trait found in a larger percentage of the population (60%) has had more time to spread through the population, so it must have arisen earlier than the rarer trait A (10%).
Question 2. How does the creation of variations in a species promote survival?
Answer: Variations give different individuals different kinds of advantages. For example, bacteria that can withstand heat will survive better in a heat wave. The selection of useful variants by environmental factors helps some individuals of the species survive changing conditions, and this forms the basis for evolutionary processes.
Questions (Page 133)
Question 1. How do Mendel’s experiments show that traits may be dominant or recessive?
Answer: When Mendel crossed tall pea plants with short pea plants, the F1 progeny had no medium-height plants — all were tall. This showed that only one parental trait (tallness) was expressed. But when the F1 tall plants self-pollinated, the F2 progeny included short plants (about one quarter). This proves that both tallness and shortness were inherited in the F1 plants, but only tallness was expressed. Hence, tallness is a dominant trait and shortness is a recessive trait.
Question 2. How do Mendel’s experiments show that traits are inherited independently?
Answer: When Mendel crossed a tall plant with round seeds and a short plant with wrinkled seeds, all F1 progeny were tall with round seeds. But in the F2 generation, besides tall–round and short–wrinkled plants, there were also new combinations: tall plants with wrinkled seeds and short plants with round seeds. This shows that the tall/short trait and the round/wrinkled seed trait are inherited independently of each other.
Question 3. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?
Answer: No, this information is not enough. If O is recessive, the daughter with group O could have received an O allele from each parent — meaning the father (group A) could be heterozygous (AO). But if A were recessive and O dominant, the daughter could not be O with an A father unless he carried O. Since the father’s exact genotype (AA or AO) is unknown, we cannot conclude from this cross alone which trait is dominant.
Question 4. How is the sex of the child determined in human beings?
Answer: In human beings, sex is largely genetically determined. Humans have 22 pairs of matching chromosomes plus one pair of sex chromosomes. Women have a perfect pair (XX), while men have a mismatched pair (XY). Every child inherits an X chromosome from the mother. From the father, the child inherits either an X or a Y chromosome. A child who inherits an X chromosome from the father will be a girl (XX), and one who inherits a Y chromosome will be a boy (XY). Thus, the sex of the child is determined by what it inherits from the father.
Exercise Questions (Page 133)
Question 1. 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:
(a) TTWW (b) TTww (c) TtWW (d) TtWw
Answer: (c) TtWW.
Explanation: Since almost half the progeny are short, the tall parent must carry the recessive allele ‘t’, so it must be heterozygous for height (Tt). Since all progeny bear violet flowers, the tall parent must be homozygous dominant for flower colour (WW). Hence the genotype is TtWW.
Question 2. 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?
Answer: No, we cannot conclude whether light eye colour is dominant or recessive from this information alone. This is only a correlation showing that the trait runs in families — it does not tell us the genotypes of the parents or children. To determine dominance, we would need controlled crosses (like Mendel’s) showing how the trait appears or disappears across generations.
Question 3. Outline a project which aims to find the dominant coat colour in dogs.
Answer: Select dogs with two clearly contrasting coat colours (for example, black and brown). Cross a pure black dog with a pure brown dog and observe the F1 progeny — the coat colour that appears in all F1 dogs is the dominant trait. Then self/cross the F1 dogs to get F2 progeny and record the ratio of coat colours. If the F2 shows roughly a 3:1 ratio, the colour appearing in three-fourths of the progeny is confirmed as dominant. Keeping careful counts of each generation, as Mendel did, is essential.
Question 4. How is the equal genetic contribution of male and female parents ensured in the progeny?
Answer: Each cell has two copies of each chromosome, one from each parent. During gamete (germ cell) formation, each germ cell takes only one chromosome from each pair — which may be of maternal or paternal origin. Thus each germ cell carries one complete set of genes. When two germ cells (one from each parent) combine during fertilisation, the normal number of chromosomes is restored in the progeny, ensuring equal genetic contribution from the male and female parents and stability of the species’ DNA.