6.1.8: Sex determination
Not started yet — this one needs some love.
Ordinary human body cells contain 23 pairs of chromosomes.
22 pairs control characteristics only.
One pair of chromosomes carries the genes that determine sex.
In females the sex chromosomes are the same: XX.
In males the sex chromosomes are different: XY.
Sex inheritance cross: eggs carry X; sperm carry X or Y, giving about a 1:1 ratio of XX female to XY male offspring.
Common exam mistakes
Females are XX and males are XY; do not swap these.
Eggs carry an X chromosome; sperm carry either X or Y.
A sex determination cross gives an expected 1:1 ratio, but this does not guarantee every small family has equal numbers of boys and girls.
If a question adds a sex condition to an inheritance probability, apply that extra probability rather than using only the Punnett square result.