X and Y
The two sex chromosomes are named after their shapes. One is large; the other is much smaller and carries far fewer genes.
Female
44 + XX
Two X chromosomes — a matching pair, like all the autosomes.
Male
44 + XY
One X and one Y — the only pair in the body that does not match.
What that means for the gametes
Meiosis puts one chromosome of each pair into each gamete. Because a woman's pair is XX and a man's is XY, the two sexes do not produce the same kinds of gamete.
| Parent | Sex chromosomes in body cells | Sex chromosome in the gametes |
|---|---|---|
| Mother | XX | Every ovum carries X — one kind only |
| Father | XY | Half the sperm carry X, half carry Y — two kinds |
This is the heterozygous case again
A woman is like a homozygote for the sex chromosomes: one kind of gamete only. A man is like a heterozygote: two kinds, in equal numbers. The reasoning from the Punnett square carries straight across.
The pair that does not match is the informative one
Every other chromosome in the body has a partner of the same size carrying the same genes. The Y has no such partner in the male, and the consequences of that reach much further than sex — they are the subject of the whole skill after this one.