Y chromosome

Y chromosome: The sex chromosome found in normal males, together with an X chromosome. Once thought to be a genetic wasteland, the Y now is known to contain at least 20 genes, some of them unique to the Y including the male-determining gene and male fitness genes that are active only in the testis and are thought responsible for the formation of sperm. Other genes on the Y have counterparts on the X chromosome, are active in many body tissues and play crucial "housekeeping" roles with the cell.