Chromosome is a threadlike structure in the nucleus of a cell. Chromosomes are made up of many hundreds of genes, the messengers that carry the “instructions” that determine a person’s hereditary makeup.
There are 46 chromosomes (arranged as 23 pairs) in each human cell except the ova (eggs) and sperm, which have only 23 chromosomes.

What happens to the chromosomes when a cell divides?

The chromosomes divide at the same time as the cell, so that the 2 new cells, each with 46 chromosomes, are identical to the parent cell. Exceptions are the cells that form sperm and ova, which divide to produce sex cells (gametes) with only 23 chromosomes each.
This means that when a sperm joins an ovum at fertilization to form a new cell of 46 chromosomes, it does so with half the genes from the mother and half from the father.

How is gender determined?

The male chromosome is called Y. It is smaller and contains fewer genes than the female chromosome. Each sperm contains either an X or a Y chromosome; each ovum contains a single X chromosome.
When a sperm and an ovum combine to form a new individual, the fertilized ovum contains either two X chromosomes (XX) and is female, or it contains an X and a Y (XY) and is male.