With a labelled line diagram of gametes and zygotes, show how a child's sex is fixed at fertilisation, and use it to explain why neither parent chooses whether the baby is a boy or a girl.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the basis. Sex depends on the combination of sex chromosomes in the zygote.
2 · Body cells. Female: 44 + XX. Male: 44 + XY.
3 · Gametes. Eggs: all 22 + X. Sperms: 22 + X or 22 + Y.
4 · Draw the chart. [Line diagram: parents 44 + XX × 44 + XY → gametes 22 + X; 22 + X and 22 + Y → zygotes 44 + XX (girl) and 44 + XY (boy)]
5 · Explain fertilisation.
X sperm + egg → 44 + XX: girl.
Y sperm + egg → 44 + XY: boy.
6 · No one to blame. Which sperm meets the egg is pure chance, so neither parent can choose or be blamed; the law also bans sex tests of a foetus.
7 · Close in one line. So a girl and a boy are equally likely in every birth.
Scoring tip: Draw circles for the gametes and write 44 + XX and 44 + XY in full; label girl and boy.
