We know that the process of reproduction gives rise to new individuals. But you must have always wondered what determines the sex of a newly born individual or sex determination? Isn�t it? In different species, there are different factors that affect the sex of the newborn. In some animals, the temperature at which the fertilized eggs are kept is the deciding factor.

Let us take some examples to understand this fact.

In members of the order Testudines (for example- in turtle and tortoise), a temperature above 33 degrees celsius leads to a female progeny, while temperature below 28 degrees celsius leads to the birth of a male progeny. Similarly, in lizards, a high temperature favors the birth of a male while a temperature ranging below 28�C favors the birth of a female progeny.

There are also certain animals in which sex is not genetically determined. Snail is an example of such a category of animals as they can change sex as and when required. However, in human beings, the sex of an individual is largely genetically determined and depends entirely on the genes that we inherit from our parents.

In diploid organisms having separate sexes, there are 22 pairs of chromosomes that are perfect pairs. They are called autosomes. Whereas, a specific pair that is not perfectly paired determines the sex of an individual. This pair is called the sex chromosomes.

A female individual possesses a perfect pair of sex chromosomes having 2 similar X chromosomes, that is, XX. These are homomorphic as they both are of the same size and shape. A female, therefore, produces the same type of gametes called the ova.

On the other hand, a male individual possesses one �X� and one �Y� chromosome, that is, XY. These are heteromorphic since they differ in size. This is the reason why a male produces two different kinds of gametes or sperms, with half of them possessing the �X� chromosome and the other half carrying the Y chromosome.

The sex determination happens at the time of fertilization when the male and female gametes fuse to form a zygote. If a sperm carrying an X chromosome fertilizes an egg carrying an X chromosome, then the offspring will be a girl. And the resultant offspring will have a XX combination of chromosomes.

Fertilization of an egg by a sperm carrying Y-chromosome produces a male offspring possessing XY combination of sex chromosomes. Thus, the sex of a child is largely determined by what they inherit from their father. In other words, it is the type of sperm fusing with the ovum at the time of fertilization that determines the sex of a child.

There is, therefore, a 50 percent chance of the child being a male and a 50 percent chance of the child being a female. The same mechanism of sex determination is observed in Drosophila melanogaster, commonly known as fruitfly.

Read More: Rules for Inheritance of Traits: Mendel�s Contributions- Science Class 10