chromosome, X

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