Sex determination

Inheritance of biological sex

Human body cells have 23 pairs of chromosomes in the nucleus. Twenty two pairs are known as autosomes, and control characteristics, but one pair carries genes that determine sex - whether offspring are male or female:

  • males have two different sex chromosomes, X Y
  • females have two X chromosomes, XX

Chromosomes from a male

Chromosomes from a male, since it contains a Y chromosome

These diagrams are known as human karyotypes, and show all the chromosomes aligned in pairs.

The blue box shows the two sex chromosomes – these are different sizes. The X chromosome is the larger chromosome and the Y chromosome is the smaller one.

Chromosomes from a female

Females have two X chromosomes

The red box shows the two sex chromosomes which are both the same size. They are two larger X chromosomes.

Genetic crosses

A genetic cross, like a Punnett square, shows how the alleles inherited from the parents may combine in a zygote. The diagram below shows how biological sex is inherited.

Mother has 2 x chromosomes, Father has an x and y chromosome.  If the male x and one of the female x chromosomes combine they have a girl, if the x and y chromosomes combine they have a boy.Mothers/female alleles - XX and the fathers/male alleles - XY

The two possible combinations are:

  • an X chromosome from the mother and an X chromosome from the father - producing a girl (female phenotype from the XX genotype)
  • an X chromosome from the mother and a Y chromosome from the father - producing a boy (male phenotype from the XY phenotype)

The ratio of female to male offspring is 1:1 – on average, half of the offspring will be girls and half will be boys. This can also be converted into a probability of 50% (XX) and 50% (XY).

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