The sex chromosomes

1. A human body cell holds $23$ pairs of chromosomes, and $22$ of those pairs are autosomes, which look the same in both sexes.
2. The twenty third pair is the pair of sex chromosomes, and this pair decides whether an individual develops as female or male.
3. A female has two X chromosomes, written $XX$, so this pair is fully matched like any other pair.
4. A male has one X chromosome and one Y chromosome, written $XY$, and the Y is much shorter than the X.
5. The Y chromosome carries a gene that switches on the development of testes in the early embryo.
6. An embryo with no Y chromosome has no copy of that gene, so it develops ovaries instead.
7. The X chromosome also carries many genes with nothing to do with sex, so every individual needs at least one copy of it.

Gametes and a child's sex

1. Meiosis separates the two sex chromosomes, so every gamete receives exactly one of them.
2. A female is $XX$, so every egg cell she makes carries an X chromosome.
3. A male is $XY$, so half the sperm cells he makes carry an X chromosome and half carry a Y chromosome.
4. The egg cell always supplies an X, so the sex of the child depends entirely on which kind of sperm reaches it first.
5. An X sperm fusing with the egg gives a zygote of genotype $XX$, which develops as a female.
6. A Y sperm fusing with the egg gives a zygote of genotype $XY$, which develops as a male.
7. Equal numbers of X sperm and Y sperm are made, which is why the two outcomes are equally likely.

The genetic cross for sex

1. Sex determination is set out as a genetic cross in exactly the same way as any other single gene cross.
   1. Parent genotypes: the mother is $XX$ and the father is $XY$.
   2. Gametes: the mother produces $X$ and $X$, while the father produces $X$ and $Y$.
   3. Grid: the mother's gametes go along the top of a $2\times 2$ Punnett square and the father's gametes go down the side.
   4. Offspring genotypes: the four boxes give $XX$, $XX$, $XY$ and $XY$.
   5. Offspring phenotypes: two female and two male.
2. The phenotype ratio is therefore $1:1$ female to male.
3. The probability that any one child is male is $\frac{2}{4}=\frac{1}{2}$, which is $50\%$.
4. That prediction applies afresh to every pregnancy, because each fertilisation is an independent event.
5. Birth records show slightly more boys than girls born each year, which is close to but not exactly the predicted $1:1$.