Sex determination in humans

In males, the sex chromosomes are heterozygous – one X and one Y chromosome.

In females, the sex chromosomes are homozygous – two X chromosomes.

An image of a complete set of chromosomes is known as a karyotype.

Meiosis forms haploid gametes (sperm and egg cells).

All eggs contain one X chromosome.

50% of sperm contain one X chromosome and 50% contain one Y chromosome.

Human sex chromosomes

During fertilisation, there is a 50% chance that a sperm containing an X chromosome will fuse with the egg; this would result in a baby girl – XX.

There is also a 50% chance that a sperm containing a Y chromosome will fuse with the egg; this would result in a baby boy – XY.

Within a population there will be equal numbers of females and males.