The two chromosomes in each body cell of a living thing that determine what sex it is.
- As with other pairs of chromosomes, one of the sex chromosomes is contributed by each parent; they are of two types, X and Y. The mother supplies only an X-chromosome, but the father can transmit both the X- and the Y-chromosome. At fertilization, if the father's sperm is also carrying an X-chromosome, the child will be female. If the father's sperm is carrying a Y-chromosome, the child will be male.