Chromosomes are a thread-like structure holding genetic information and is present in the cell nuclei. A normal cell contains 23 pairs (46n) of chromosomes. Out of these, the last pair (2n) is sex chromosomes. Sex chromosomes are denoted as X and Y. Females typically have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males typically have two different kinds of sex chromosomes (XY) and are called the heterogametic sex. The X chromosome resembling a large autosomal chromosome has a long and a short arm. The Y chromosome meanwhile has one long arm and a very short second arm. Other differences include a number of genes, the number of working genes on the chromosome and even abnormal chromosome pairings. The X chromosome contains more than 1,000 working genes, and the Y chromosome has less than 100 working genes. X chromosome is a sex chromosome that occurs paired in the female and single in the male. Meanwhile, Y chromosome is a sex chromosome that is normally present only in male cells. X chromosomes contain genes for female sex determination and Y chromosome contain genes for male sex determination. The X chromosome is bigger (about 155 million base pairs). Y chromosome is smaller (about 59 million base pairs). X chromosome does not contain the SYR gene, even as Y chromosome contains the SYR gene, which is involved in the development of the testes.