Sex chromosomes

Only one of the 23 chromosome pairs have genes that determine if a foster becomes a boy or a girl. The chromosomes in this pair are called sex chromosomes. A woman has to similar x-chromosomes. A man has one x-chromosome and one smaller chromosome called a y-chromosome. The x-chromosome contains lots of genes, whilst the y-chromosome contains a small amount of genes. The punnett square shows that there is an equal chance of receiving a boy or a girl.

After meiosis, the egg cells receive an additional x-chromosome, in addition to the 22 other chromosomes. The man's sperm-cells consists of two types. Half of the sperm-cells contain the y-chromosome, whilst the other half contains the x-chromosome.

Even if theoretically there is an equal chance of either sex being born, there is in practice a majority of boys born, comparative to girls(the margin is relatively small tho).

The relation is boys: girls = 106 : 100. But because the mortality rate for boy infants is greater, the difference quickly changes. Furthermore, the life expectancy is higher for males than females.