5.10 - Sex Determination

Sex chromosomes and the inheritance of sex

• The nucleus of every ordinary body cell contains chromosomes arranged in pairs - 23 pairs in humans (46 in total). The two chromosomes in a pair carry the same genes, with a gene for each characteristic in the same position on both, except for the sex chromosomes of a male (X and Y), which are different.
• 22 pairs are the ordinary chromosomes, called autosomes. These carry the genes for the body's characteristics, such as eye colour and blood group.
• The 23rd pair are the sex chromosomes, which decide whether a person is male or female.

The sex chromosomes

• There are two kinds of sex chromosome:
  • The X chromosome
  • The Y chromosome, which is smaller
• The combination of sex chromosomes determines whether a person is biologically female or male:
  • XX - Female
  • XY - Male

How sex is inherited

Sex inheritance can be shown with a genetic cross (a Punnett square), using the sex chromosomes in place of ordinary alleles.

1. The parents are a female (XX) and a male (XY).
2. The mother is XX, so every egg carries an X.
3. The father is XY, so half his sperm carry an X and half carry a Y.
4. The egg always provides an X, so it is the sperm that decides the sex of the child:
   • A sperm carrying an X gives an XX child, who develops as female.
   • A sperm carrying a Y gives an XY child, who develops as male.

Crossing XX with XY gives four equally likely combinations:

• Two are XX (female) and two are XY (male), so there's an equal chance of a female or a male - a 1:1 ratio, the same as a 50% chance of each at every birth.
• This is a probability, not a guarantee: over many births the numbers of girls and boys come out roughly equal, but one family could by chance have children who are all the same sex.