The chromosome that carries the fewest number of genes is the Y chromosome. The Y chromosome carries only about 344 genes. Genes on the Y chromosome determine sex and code for male sexual characteristics.
The absence or presence of the Y chromosome determines the sex of a human being. The Y chromosome pairs up with the X chromosome; this pair is called the sex chromosomes. Men have an X and a Y chromosome. When a man makes sperm cells, the Y chromosome, because it has fewer genes than the X chromosome, cannot exchange genetic material with its partner. The Y chromosome, therefore, passes from one male generation to the next male generation in a relatively unchanged form. The X chromosome is much larger than the Y chromosome. A woman possesses two X chromosomes. When her body forms egg cells, pieces of each X chromosome break off and recombine to create new genetic combinations.