Which contribution decides
Some babies are girls. Some are boys. One pair of sex chromosomes decides which. A chromosome is a thread of genes inside a cell.
- A mother's cells carry two X chromosomes.
- A father's cells carry one X and one Y.
- Every egg carries an X.
- A sperm carries an X, or a Y.
- X plus X makes XX: a girl.
- X plus Y makes XY: a boy.
- Each outcome is about equally likely.
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Show children. Watch the egg. Watch the sperm.
The rule
A chromosome is a thread of genes inside a cell. People have 23 pairs. One pair is the sex chromosomes, X and Y.
A pair is mixed (XY) or matching (XX). A parent passes on one chromosome of the pair. The chromosome passed is X or Y.
A parent with a mixed pair can pass either kind. A parent with a matching pair always passes the same kind. In people, the father has the mixed pair, XY. So the father's sperm decides the sex.
A mother's cells carry XX. So every egg gets an X. A father's cells carry XY. A sperm gets X or Y, about half each. An X sperm makes a girl. A Y sperm makes a boy.
Nobody chooses which sperm meets the egg. Chance decides.
In some animals, temperature decides the sex. People are not like that.