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 control most of the characteristics. Pair 23 are the sex chromosomes. They carry genes that determine an embryo's sex - whether offspring are biologically male or female:
- typically males have two different sex chromosomes, X Y
- typically females have two X chromosomes, XX
- In humans, some genetic conditions occur when there is a variation in the number of chromosomes. For example, people with Down's Syndrome generally have 47 chromosomes in their body cells rather than 46.
Chromosomes from biological males
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 biological females
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).
