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.
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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.