Science (086) — AI-generated practice question

Probability of the fourth child being a boy = 1/2 (50%)

Chromosomal Diagram at Fertilisation:

```
Mother (XX) Father (XY)
| |
X X X Y
\ / \ /
egg sperm
X X or Y (equal chance)
```

| Sperm from Father | Egg from Mother | Child | Sex |
|---|---|---|---|
| X | X | XX | Girl |
| Y | X | XY | Boy |

Explanation:

Women are XX and men are XY. During gamete formation, the mother always produces eggs carrying one X chromosome. The father produces two types of sperm — half carrying X and half carrying Y — in equal numbers.

The sex of the child depends entirely on which sperm fertilises the egg. An X-bearing sperm produces a girl (XX); a Y-bearing sperm produces a boy (XY).

Each pregnancy is an independent event. The previous three births being girls do not affect the sperm available in the next fertilisation. Therefore, the probability of a boy remains 1/2 every time.

Source: Sex Determination, Chapter 8, Section 8.2.4

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• Examiners expect you to state the probability clearly at the start.
• The diagram (Punnett square or cross diagram) is expected — it earns dedicated marks.
• Key point to stress: each pregnancy is independent; past outcomes don't change probabilities.
• Quote the textbook idea directly: "sex of the children is determined by what they inherit from their father."
• Avoid writing more than needed — the table + two short paragraphs is the ideal length for 5 marks.