Describe how the sex of offspring is determined at fertilisation, using…

The idea

Human sex is determined at fertilisation by which sex chromosomes the sperm brings. A female has two X chromosomes, so every egg carries one X. A male has one X and one Y, so half his sperm carry X and half carry Y. If an X-bearing sperm fertilises the egg the zygote is XX and develops as female; if a Y-bearing sperm fertilises it the zygote is XY and develops as male. Because the sperm type is a 50:50 chance, each fertilisation gives roughly equal numbers of each sex. A genetic diagram shows this: write the parents' sex chromosomes, put the egg's X across the top and the sperm's X and Y down the side, then combine them in the four boxes to give XX, XY, XX, XY. The diagram, not the environment, shows the outcome.

What earns the marks

• A female is XX, so all eggs contain a single X chromosome.
• A male is XY, so sperm are of two types: half carry X and half carry Y.
• At fertilisation the sperm's chromosome combines with the egg's X: X + X gives XX (female) and X + Y gives XY (male).
• A genetic diagram (Punnett square) with egg X and sperm X/Y produces XX, XY, XX, XY, showing a 1:1 ratio of female to male.
• The sex of the offspring is decided by the sperm, not the egg, because the egg always contributes X.

Common mistakes and how to put them right

• Thinking the mother determines the sex.

  Put it right by noting all eggs carry X, so only the sperm's X or Y changes the outcome.

• Writing that a male is YY or a female is XY.

  Put it right: Correct to male XY and female XX.

• Claiming the diagram guarantees exactly two boys and two girls.

  Put it right: Correct to a probability of about half each, not a fixed count.

Exam tips

• Always label the egg row and sperm column with the actual sex chromosomes before combining them.
• State clearly that the sperm determines sex and explain why in one sentence.
• When asked for a ratio, read it from the diagram boxes (for example 1:1) rather than guessing.