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Inheritance and Genetics: Question 2

Syllabus 17.1, 17.4

Structured Core 7 marks

A woman and a man, who already have two daughters, are expecting another child. A genetic counsellor is explaining to them how the sex of a baby is determined.

(a) State the sex chromosome combination found in the body cells of a human female, and in the body cells of a human male. [2]

(b) Using a genetic diagram, show the possible combinations of sex chromosomes that this next child could inherit, and use it to state the expected ratio of daughters to sons. [3]

(c) The couple's next two children are both sons. Explain why this does not mean that the expected ratio from part (b) was wrong. [2]

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Worked solution

Part (a): Sex chromosomes in body cells

Every human body cell contains 23 pairs of chromosomes, one of which is the pair of sex chromosomes. In a female, both sex chromosomes are X chromosomes, so her genotype is written XX. In a male, one sex chromosome is an X and the other is a Y, so his genotype is written XY.

Part (b): Genetic diagram for sex determination

The mother is XX, so every egg cell she produces carries a single X chromosome. There is no other possibility. The father is XY, so his sperm cells are of two types, produced in equal numbers: half carry an X chromosome and half carry a Y chromosome.

Combining the mother’s gametes with the father’s gametes gives two equally likely outcomes for a child:

  • egg (X) + X-carrying sperm → XX → a daughter
  • egg (X) + Y-carrying sperm → XY → a son

Since the father produces X-carrying and Y-carrying sperm in equal numbers, and the mother’s eggs are always X, the two outcomes XX and XY are equally likely. The expected ratio of daughters to sons is therefore 1:1.

Part (c): Why two sons in a row does not disprove the ratio

The 1:1 ratio calculated in part (b) is a probability, describing what is expected on average over a large number of births. It is not a guarantee for any particular family. Each pregnancy is an independent event: the chance of a son or a daughter is the same each time, unaffected by the sexes of any earlier children. With only a small number of children in one family, the actual outcome can differ from the expected ratio purely by chance. Having two sons in a row is simply one of the possible outcomes of two independent 1:1 events, and does not mean the underlying 1:1 probability is incorrect.

Final answers

  • (a) Female: XX. Male: XY.
  • (b) Genetic diagram gives XX (daughter) and XY (son) in equal numbers, expected ratio 1:1.
  • (c) The 1:1 ratio applies over many births on average; each pregnancy is an independent event, so a small family can differ from the expected ratio by chance alone.