Each and every one of us has inherited half our genes from each parent. So generally, we have two copies of every gene, one copy from our mother and the other copy from our father. The genetic information is held as a code in a substance called deoxyribonucleic acid (DNA) and genes are just pieces or fragments of DNA.
Our cells store DNA very efficiently by packaging it first into genes which are themselves packaged into chromosomes. There are as many as 20 – 25,000 genes found in human genome and are all packaged into 46 chromosomes. The chromosomes are arranged in 23 pairs which split into two so that each parent contributes one chromosome from each pair, hence half his or her genes to each child. It may be easier to think of genes as “recipes” which are then packaged into chromosomes or “recipe books”.
The 23 pairs of chromosomes consist of 22 pairs called autosomes and one pair which is called sex chromosomes. Each pair in the 22 pairs of autosomes is identical and the autosomes are of different sizes. They can be arranged according to their size and numbered, with chromosome 1 being the largest and chromosome 22 the smallest.
The sex chromosomes carry genes responsible for the sex of an individual (as well as other genetic information). Women have an identical pair of sex chromosomes both called X chromosomes, while men have one X and one Y chromosome. This means that the mother of a child will contribute one chromosome from each pair, and therefore the sex chromosome will always be an X chromosome. The father, on the other hand, will contribute either an X or a Y chromosome along with one chromosome from each pair of autosomes. In this way, the sex of the child depends on the father’s contribution of his sex chromosome.