In some families, either rural or urban, females are tortured for giving birth to a female child. They do not seem to understand the scientific reason behind the birth of a boy or a girl. In fact the mother is not responsible for the sex of the child and it has been genetically proved that the sex of a newborn is determined by what the child inherits from the father. (a) State the basis on which the sex of a newborn baby is determined in humans. (b) Why is the pair of sex chromosomes called a mismatched pair in males? (c) How is the original number of chromosomes present in the parents restored in the progeny? OR (c) Explain by giving two examples of the organisms in which the sex is not genetically determined.

(a) Basis of sex determination in humans:

In human beings, the sex of a newborn is genetically determined. It depends entirely on the type of sex chromosome the child inherits from the father:

• All children inherit an $X$ chromosome from their mother.
• If a child inherits an $X$ chromosome from the father, the child will be a girl ($XX$).
• If a child inherits a $Y$ chromosome from the father, the child will be a boy ($XY$).

(b) Mismatched pair of sex chromosomes in males:

In humans, there are $22$ perfect pairs of chromosomes. The $23^{\text{rd}}$ pair, called the sex chromosomes, is a perfect pair of $XX$ in females. However, in males, this pair is mismatched because it consists of one normal-sized $X$ chromosome and one shorter $Y$ chromosome ($XY$).

(c) Restoration of original chromosome number in progeny:

During sexual reproduction, germ cells (gametes) are formed such that they contain only one chromosome from each pair (a single set of genes). When the male germ cell (sperm) and female germ cell (egg) combine during fertilization, they form a zygote. This fusion restores the normal double set of chromosomes ($22$ pairs of autosomes and $1$ pair of sex chromosomes) in the progeny, ensuring the stability of the DNA of the species.

OR

(c) Organisms where sex is not genetically determined:

In some species, sex is determined by non-genetic factors:

1. Reptiles (e.g., some turtles/lizards): The temperature at which fertilized eggs are kept determines whether the developing embryo will become male or female.
2. Snails: Individuals can change their sex during their lifetime, showing that their sex is not genetically fixed.