Question 79

A Punnett square is drawn for the sex chromosomes of two human parents. Which statements about the completed square are true? Select all that apply.

Only one box in four holds an X with a Y, a chance of 25 per cent
Two of the four boxes hold two X chromosomes and two hold an X with a YCorrect answer
It predicts a 50 per cent chance of each sex for every childCorrect answer
Three of the boxes hold two X chromosomes, because the mother supplies two of them

Explanation

The mother contributes an X to every box, while the father contributes an X to half of them and a Y to the other half, so the four boxes divide evenly between the two arrangements. A 25 per cent figure comes from treating the mismatched arrangement as though it needed a rare double helping of one allele, which is how a recessive genotype behaves and not how this pair works.

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