Chapter 9: Heredity

Internal Questions and Answers

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, which exists in 60% of the population, is likely to have arisen earlier. In asexually reproducing species, variations are passed to the next generation through reproduction. A trait present in a larger proportion of the population has had more time to spread and accumulate over generations. Hence, the trait with higher frequency is generally considered to have originated earlier.

2. How does the creation of variations in a species promote survival?

Answer: Variations help a species survive by increasing the chances that some individuals can adapt to changes in the environment. During unfavourable conditions such as diseases, climate changes or shortage of food, individuals with useful variations are more likely to survive and reproduce. These favourable traits are passed to the next generation, helping the species continue and evolve over time.

Internal Questions and Answers:

1. How do Mendel’s experiments show that traits may be dominant or recessive?

Answer: Mendel’s experiments with pea plants showed that when two contrasting traits were crossed, only one appeared in the first generation (), while the other seemed to disappear. For example, when he crossed tall and dwarf plants, all  plants were tall. However, in the next generation (), both tall and dwarf plants reappeared in a 3:1 ratio. This showed that the trait expressed in the generation is dominant, while the hidden one is recessive but reappears in later generations.

2. How do Mendel’s experiments show that traits are inherited independently?

Answer: Mendel showed that traits are inherited independently through his dihybrid cross experiments. When he crossed pea plants differing in two traits (such as seed shape and seed colour), the F1 generation showed only the dominant traits. In the F2 generation, the traits reappeared in new combinations in a 9:3:3:1 ratio. This indicated that the inheritance of one trait does not affect the inheritance of another, proving that traits are passed independently of each other.

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 alone is not enough to tell which trait is dominant. The daughter has blood group O, which means she received one O allele from each parent. Therefore, the father with blood group A must have the genotype AO, while the mother has genotype OO. Since the father shows blood group A even with one A allele and one O allele, it shows that A is dominant over O.

4. How is the sex of the child determined in human beings?

Answer: In humans, the sex of a child is determined genetically. Women have two X chromosomes (XX), while men have one X and one Y chromosome (XY). Every child inherits an X chromosome from the mother, but the father can pass on either an X or a Y. If the child inherits the X from the father, it will be a girl; if it inherits the Y, it will be a boy. Thus, the father’s chromosome decides the child’s sex.

Exercise Questions and Answers :

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

[ Since all progeny have violet flowers, the flower colour allele in the tall parent must be homozygous dominant (WW). The appearance of almost half short plants indicates a 1:1 ratio of tall to short, which happens when a heterozygous tall plant (Tt) is crossed with a short plant (tt). Therefore, the tall parent must be TtWW. ]

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 say whether the light eye colour trait is dominant or recessive from this information alone. This is because both parents and children have light-coloured eyes, but no information is given about dark eye colour or how the trait appears in other generations. Therefore, the dominance or recessiveness of the trait cannot be determined.

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

Answer: A project to find the dominant coat colour in dogs can be carried out by observing the coat colours of parent dogs and their puppies over several generations. Different coloured dogs can be crossed, such as black and brown coated dogs, and the coat colour of the offspring can be recorded. If a particular coat colour appears more frequently in the offspring, even when only one parent has that colour, it is likely to be the dominant trait. The results can then be analysed to identify the dominant coat colour.

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

Answer: The equal genetic contribution of male and female parents is ensured in the progeny because both parents contribute an equal number of chromosomes to the offspring. Each parent produces gametes (sperm in males and egg in females) through the process of meiosis, where the chromosome number is reduced to half (23 chromosomes in humans). During fertilization, the sperm and egg fuse to form a zygote, restoring the full diploid number (46 chromosomes). Since the zygote receives 23 chromosomes from the father and 23 from the mother, the genetic contribution from both parents is equal.