Students should be able to carry out a genetic cross to show sex…

The idea

A genetic cross is a diagram that predicts the possible genotypes of offspring from two parents. For sex inheritance, the mother is XX and the father is XY. Their gametes are haploid: every egg carries X, while half the sperm carry X and half carry Y. Draw a grid with the mother's gametes along one side and the father's gametes along the other. Each cell shows the genotype formed when those gametes fuse. The four outcomes are XX, XX, XY and XY, so two of the four possible offspring are female and two are male. This gives a 1:1 ratio of females to males, or a 50% chance of each sex. The cross shows that the father's sperm determines the sex of the offspring.

What earns the marks

• Write the parental genotypes as XX for the mother and XY for the father.
• Show the gametes as X from the mother and X or Y from the father.
• Complete the grid so that each cell combines one gamete from each parent.
• Identify the offspring genotypes as XX and XY in equal numbers.
• State the expected ratio of female to male offspring as 1:1, equivalent to a 50% chance of each sex.
• Conclude that the sperm, not the egg, determines the sex of the offspring.

Common mistakes and how to put them right

• Putting two gametes from the same parent into one offspring cell.

  Put it right: Each cell must contain one gamete from each parent.

• Writing the father's gametes as XX and XY instead of X and Y.

  Put it right: Gametes are haploid and carry only one sex chromosome.

• Forgetting to state the ratio or probability after completing the grid.

  Put it right: The cross should end with a conclusion about expected offspring.

Exam tips

• Label the parental genotypes, the gametes and the offspring clearly so the logic of the cross is easy to follow.
• Check that every offspring cell contains exactly two sex chromosomes, one from each parent.
• Finish by stating the expected ratio or percentage chance of female and male offspring.