Sex Determination
- Sex is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a number of genes)
- Females have the sex chromosomes XX
- Males have the sex chromosomes XY
- As only a father can pass on a Y chromosome, he is responsible for determining the sex of the child
- He does this because:
- He produces (ejaculates) around 250 million sperm cells during sexual intercourse
- Of those, half (125 million sperm) will be carrying his X chromosome
- If one of these sperm fertilises the egg, the fetus will be female
- The other 125 million of his sperm will be carrying his Y chromosome
- Which will result in a male foetus if one of these fertilises the egg
Inheritance of sex diagram
Sperm cells determine the sex of offspring
- The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes
Punnet square sex determination diagram
Punnett square showing the inheritance of sex
