Ans) Mendel explained that it is possible that a trait is inherited but not expressed in an organism with the help of a monohybrid cross.

1. He crossed pea plants with different characteristics a tall plant (TT) and a short plant (tt).

Inheritance of traits over two generations

2. The progeny he received in the first generation (F) was tall. The dwarfness did not show up in the F1 generation.
3. He then crossed the tall plants of the F1 generation and found that one quarter of dwarf plants are obtained in second generation (F2). Out of four, one pea plant was dwarf.

This indicates that both the tallness and shortness traits were inherited in F, plants, but only tallness trait was expressed.

Ans) Ans. In human beings, the females have two X chromosomes and the males have one X and one Y chromosome. Therefore, the females are XX and the males are XY. The gametes, as we know, receive half of the chromosomes.

Most human chromosomes have a maternal and paternal copy and we have 22 such pairs. But one pair, (23rd pair) called the sex chromosome, is odd in not always being a perfect pair. The male gametes have a mismatched pair in which one is normal sized X while the other is a short one called Y. Therefore, type of male gametes: 22+ X or 22+ Y.

Sex determination in human beings

However, since the females have XX sex chromosomes, their gametes can only have X sex chromosome. Type of female gamete : 22 + X.

Thus, all children will inherit an X chromosome from their mother, regardless of whether they are boys or girls. Thus, the sex of the children will be determined by what they inherit from their father (See Fig.). A child who inherits an X chromosome from her father will be a girl and one who inherits a Y chromosome from him, will be a boy.

Ans) In human beings, equal genetic contribution of male and female parents is ensured in the progeny through inheritance of equal number of chromsomes from both parents. There are 23 pairs of chromosomes. All human chromosomes are not paired. Out of these 23 pairs, the first 22 pairs are known as autosomes and the remaining one pair is known as sex chromosome represented as X and Y. Females have a perfect pair of two X sex chromosomes and males have a mismatched pair of one X and one Y sex chromosome.

During the course of reproduction, as fertilisation takes place, the male gamete (haploid) fuses with the female gamete (haploid) resulting in the formation of the diploid zygote. The zygote in the progeny receives an equal contribution of genetic material from the parents. Out of 23 pairs of chromosomes in progeny, male parent contributes 22 autosomes and one X or Y chromosome and female parent contributes 22 autosomes and one X chromosome.