Why do only males have Y-chromosomes?
• The Y-chromosome contains a gene that directs the development of a human embryo along the pathway to becoming male. As long as this gene is working properly, any human embryo which has a Y-chromosome will grow into a baby boy. Men produce two types of sperm in equal amounts. Half contain a Y-chromosome and half contain an X-chromosome. The eggs produced by women have only X-chromosomes. Fertilised eggs have either two X-chromosomes, and become girls, or one X and one Y and become boys.
