CoachPair 23 comes in two versions: two matching X chromosomes — the karyotype of a girl; or one X and a small Y — a boy. The halving then runs the flip: every egg cell carries an X; a sperm cell carries X or Y, half and half. Fertilization by an X-sperm makes XX, by a Y-sperm XY — one chance in two (Chapter 63’s language of chance, finally with its mechanism), decided by which swimmer arrives, at the instant of fertilization.
Examples
Example 64.7 (Old questions, short answers)
The deck settles historic scores. Whether a child is a boy or a girl owes nothing to either parent’s wishes, diets or seasons — it is the flip, run in Definition 64.6’s mechanism (and, for the record of old injustices: the deciding difference rides in the sperm). Equal halves also answer the family album: mother and father contribute one full half-deck each — 23 and 23 — and heredity’s arithmetic honors both lines alike.