Reproduction and Inheritance · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 17% of your exam marks.
Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.
The 23rd pair of human chromosomes is the sex chromosomes. The other 22 pairs are called autosomes and are the same in males and females.
The X chromosome is much larger than the Y and carries many more genes. The Y chromosome is short and carries relatively few genes, but it does contain the SRY gene that triggers the development of male reproductive organs in an embryo.
When gametes are made:
So the father determines the sex of the child:
Punnett square for inheritance of sex:
| X (from mother) | X (from mother) | |
|---|---|---|
| X (from father) | XX (female) | XX (female) |
| Y (from father) | XY (male) | XY (male) |
The ratio is 1 male : 1 female, so each new child has a 50% chance of being male and a 50% chance of being female.