The sex of an individual is usually determined by the sex chromosomes that the organism has. Each diploid cell in the organism has one pair of sex chromosomes that differs from the other homologous pairs of chromosomes. For example, in humans, one of the 23 pairs of homologous chromosomes makes up the sex chromosomes. However, unlike all of the other homologous pairs, the sex chromosomes may not be the same as each other in size. There are two types of sex chromosomes, the X chromosome and the Y chromosome. The X chromosome is larger than the Y chromosome, and therefore has spaces for genes that are not present on the Y chromosome.

In humans and many other species, females have two X chromosomes (symbolized as XX). However, the chromosomes that make up the pair of sex chromosomes in males are different. Males have one X chromosome and...