How sex is decided

The cross is the same kind you have already drawn twice. Put the mother's gametes down one side and the father's along the top.

XX × XY	X	Y
X	XX	XY
X	XX	XY

Every ovum carries X, so both rows are the same. The father's two kinds of sperm fill the two columns.

Reading the four boxes

Two boxes are XX — a girl.
Two boxes are XY — a boy.

Figure to be added

Figure 20.23 — the sex-determination cross drawn as cells: the mother producing ova all carrying X, the father producing sperm of which half carry X and half carry Y, and the four possible fertilisations.

Fifty-fifty at each conception, not across a family

The equal chance applies afresh to every pregnancy. A family of four girls has not used up its chances; the fifth child still has an even chance of being a boy. This is the same point as the pea seeds, and it is worth more here because people believe otherwise.

Half and half

XX × XY gives two XX boxes and two XY boxes — an equal chance of a girl or a boy at every conception.