Science (086) — AI-generated practice question

(a) Human females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). A human cell has 23 pairs of chromosomes in total — 22 pairs are autosomes (perfectly matched pairs) and 1 pair is the sex chromosomes. Unlike the 22 autosomes, the sex chromosomes in males are mismatched: one normal-sized X and one shorter Y chromosome.

(b)

| Mother (XX) | × | Father (XY) |
|---|---|---|
| Eggs: all X | | Sperms: X or Y |

Offspring: XX (girl) or XY (boy) — 50% girls, 50% boys.

The father's contribution determines sex because the mother can only pass X chromosomes, while the father passes either X (→ girl) or Y (→ boy).

(c) In reptiles like crocodiles, sex is determined by incubation temperature, not genes. In snails, individuals can change sex, so sex is not genetically fixed. This shows that sex determination varies widely across species — it can be genetic (humans), environmental (reptiles), or flexible (snails).

Source: Chapter 8, Section 8.2.4 — Sex Determination

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• (a): Examiners expect the numbers 22 + 1 = 23 pairs, and the key distinction that autosomes are matched while the male sex chromosomes are mismatched (X and Y).
• (b): A simple cross table earns marks. The key conclusion — father's X or Y decides the child's sex — must be explicitly stated.
• (c): This 1-mark part only needs a brief, accurate contrast of the three examples and a one-line conclusion about diversity of sex-determination mechanisms. Don't over-write here.