Solution of Heredity and Evolution (Page No - 193 - Biology by Lakhmir Singh, Class 10 | Extra Documents, Videos & Tests for Class 10 PDF Download

Question 34:
(a) Why did Mendel choose pea plants for conducting his experiments on inheritance ?
(b) State Mendel’s second law of inheritance.
Solution :
(a) Mendel choose pea plants for studying inheritance because pea plants had a number of clear cut differences which were easy to tell apart. Another reason for choosing pea plants are they were self pollinating and many generations can be produced in a short time span.
(b) According to Mendel’s second law of inheritance: In the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.
(c) How does the creation of variation in a species ensure its survival ?

Question 35:
(a) What do you understand by the term ‘variation’ ?
(b) Name two human traits which show variation.
Solution :
(a) The differences in the characters among the individual of a species is called variation.
(b) Human height and free ear lobe are the traits which show variation in humans.
(c) Due to the creation of variations, a species can adjust to the changing environment around it. And this promotes the survival of the species in the changing environment. Example: The accumulation of ‘heat resistant’ variation (or trait) in some bacteria will ensure its survival even when the temperature in its environment rises too much due to a heat wave or some other reasons. On the other hand, the bacteria which did not have this variation to withstand heat would not survive under these circumstances and die.

Question 36:
(a) What are genes ? Where are they located in our body ?(a) What are genes ? Where are they located in our body ?
(b) What is meant by dominant genes and recessive genes ? Give one example of each.
(c) Explain how, characteristics (or traits) are inherited through genes.
Solution :
(a) Genes are the units of heredity which transfer characteristic from parents to their offspring’s during reproduction. Genes are located on the chromosomes.
(b) The gene which decides the appearance of an organism even in the presence of an alternative gene is known as dominant gene. The gene which can decide the appearance of an organism only in the presence of another identical gene is called a recessive gene. The dominant gene is represented by a capital letter and the corresponding recessive gene is represented by the corresponding small letter. For example, in pea plants, the dominant gene for tallness is T and the recessive gene for dwarfness is t.
(c) There is a pair of genes for each characteristic of an organism, one is dominant gene and the other is recessive gene. Each parent passes only one of the two genes of the pair for each characteristic to its progeny through gametes. Thus, the male gamete and the female gamete carry one gene for each characteristic from the gene pairs of the parents. When a male gamete fuses with a female gamete during fertilisation they make a new cell called zygote which grows and develops to form a new organism having characteristics from both the parents which it has inherited through genes.

Question 37:
(a) How do Mendel’s experiments show that traits may be dominant or recessive ?
(b) How do Mendel’s experiments show that traits are inherited independently ?
Solution :
(a) Mendel first crossed pure bred tall pea plants with pure bred dwarf pea plants and found that only tall pea plants were produced in the first generation. No dwarf pea plants were obtained in F1 generation. Mendel concluded that the first generation showed the traits of only one of the parent plants; tallness. The trait of other parent plant, dwarfness, did not show up in the progeny of first generation. Mendel then crossed the tall pea plants of the first generation (F1 Generation) and found that tall plants and dwarf plants were obtained in the second generation (or F2 generation) in the ratio of 3:1. Mendel noted that the dwarf trait of the parent pea plant which had seemingly disappeared in the first generation progeny reappeared in the second generation. In this way, Mendel’s experiments with tall and dwarf pea plants showed that the traits may be dominant and recessive.

A cross of tall and dwarf pea plant

A cross of tall plants of F₁ Generation

(b) When Mendel crossed pure-bred tall pea plants with pure-bred dwarf pea plants, he found that only tall pea plants were produced in the F1 generation. When Mendel further crossed the tall pea plants of the F1generation, he found that the tall plants and dwarf plants were obtained in the ratio 3:1 in the F2 generation. Mendel noted that all the pea plants produced in the F2 generation were either tall or dwarf. There were no plants with intermediate height (or medium height) in-between the tall and dwarf plants. In this way, Mendel’s experiment showed that the traits (like tallness and dwarfness) are inherited independently. This is because if the traits of tallness and dwarfness had blended (or mixed up), then medium sized pea plants would have been produced.