Part 05 · Genes and inheritance · Card 10 of 11
Sex chromosomes
The pair of chromosomes that decides whether a person is female (XX) or male (XY).
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Analogy
A coin toss at conception, because the father's sperm carries X or Y.
In practice
Every egg carries an X. A sperm carrying X gives a girl. A sperm carrying Y gives a boy.
Practice
Interview questions
- Describe how the sex of a baby is determined.
- Females are XX and males are XY
- Eggs always carry X
- Sperm carry X or Y with equal chance
- The sperm that fertilises the egg decides the sex
Questions people ask
Which parent decides the sex of a baby?
The father's sperm decides. Every egg carries an X chromosome. A sperm carries either an X or a Y. If an X sperm fertilises the egg, the baby is female, XX. If a Y sperm fertilises it, the baby is male, XY. The chance is about half each.
Why are some conditions more common in males?
Some genes, like those for red-green colour blindness, are on the X chromosome. Males have only one X, so a single faulty recessive allele will show. Females have two X chromosomes and the second may hide the faulty one, so they are more often just carriers.