In normal cell division, i.e. when new cells are formed in muscle or skin tissue, chromosomes are duplicated and replicated. The resulting cell has the same number of chromosomes as the dividing 'parent cell'. The resulting new cells are exact copies of the previous cells.

During fertilization, an ovum and a sperm cell fuse together, forming a zygote. But how can the zygote have the correct number of chromosomes?

Germ cells have only half the number of chromosomes as a diploid cell one of each pair. They are considered haploid (n) cells. In a human egg or sperm cell, there are 23 chromosomes, one of which is a single X or Y chromosome.

The number of chromosomes is reduced from 46 to 23 during the process of meiosis. The fertilization of the egg by a sperm cell restores the diploid number of 46 chromosomes.

In a female, the primary oocyte is diploid, containing 44+XX chromosomes. However, when the primary oocyte undergoes its first meiotic division in order to form the secondary oocyte and a polar body, the secondary oocyte is haploid and contains 22+X chromosomes.

The primary spermatocyte contains 44 + XY chromosomes. As a result of meiotic division, two types of sperm can be produced: those with the chromosomes 22 + X and those with the chromosomes 22 + Y.

The sex of an individual is determined when a sperm cell fertilizes an egg.

If an egg is fertilized by a sperm cell with an X chromosome, the developing individual becomes a female.

Similarly, if the sex chromosome of the fertilizing sperm cell is Y, the developing individual becomes a male.