In females, the sex chromosomes are homozygous – two X chromosomes.
An image of a complete set of chromosomes is known as a karyotype.
All eggs contain one X chromosome.
50% of sperm contain one X chromosome and 50% contain one Y chromosome.
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.