Sex determinationSpec 3.15
Humans have 23 pairs of chromosomes. One pair is the sex chromosomes. Females have two X chromosomes (XX). Males have one X and one Y chromosome (XY).
All eggs carry an X chromosome. Half of the sperm carry an X and half carry a Y. The sex of the offspring is decided at fertilisation by which sperm fertilises the egg.
| X (egg) | X (egg) | |
|---|---|---|
| X (sperm) | XX female | XX female |
| Y (sperm) | XY male | XY male |
Two outcomes are female and two are male, so the ratio is 1 : 1 and there is a 50% probability that a baby will be a boy or a girl.