The smallest human chromosome carries a decisive developmental switch. The Y chromosome is normally present only in males and contains the SRY gene that initiates testis development. Without SRY the bipotential gonad becomes an ovary.

Most of the Y does not recombine with the X and has therefore accumulated repetitive DNA and lost many ancestral genes. A few genes in the male-specific region are essential for sperm production. The pseudoautosomal regions at the ends still pair with the X during meiosis.

Y-chromosome deletions can cause infertility. Because the Y is passed strictly from father to son, it is used in studies of male lineages and population history. Despite its small size and limited gene content, the chromosome remains central to sex determination and male reproductive biology.

• Sex chromosome normally found only in males
• Carries the SRY sex-determining gene
• Largely non-recombining and gene-poor
• Critical for male fertility and sex determination