Sex determination
Inheritance of biological sex
Human body cells have 23 pairs of The structure made of DNA that codes for all the characteristics of an organism. in the nucleus. Twenty two pairs are known as autosomes, and control characteristics, but one pair carries genes that determine sex - whether offspring are born male or female:
- males have two different sex chromosomes, X Y
- females have two X chromosomes, XX
Chromosomes from a male
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
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 A fertilised egg cell.. The diagram below shows how biological sex is inherited.
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).
