6.9 Sex determination
5 exam questions worth 9 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Give the sex chromosomes of (a) a female and (b) a male.
Mark scheme
- (a) XX (1)
- (b) XY (1)
Model answer
A female has XX, and a male has XY.
Draw a genetic diagram to show that there is an equal chance of a child being a boy or a girl.
Mark scheme
- parents XX and XY (1)
- gametes X, X and X, Y (1)
- offspring XX, XX, XY, XY / 1:1 (1)
Model answer
Mother XX, father XY. Eggs X and X; sperm X and Y. Offspring XX, XY, XX, XY: two girls and two boys, a 1:1 ratio, so a 50% chance of each.
Explain why it is the father, not the mother, who decides the sex of a child.
Mark scheme
- all eggs carry an X chromosome (1)
- sperm carry either X or Y (1)
Model answer
All eggs carry an X chromosome, but sperm carry either an X or a Y, so the sperm that fertilises the egg decides the sex.
A couple have two daughters. What is the chance that their next child is a boy?
Mark scheme
50% / 1 in 2 / 0.5
Model answer
50%: each child is independent.
How many pairs of chromosomes in a human body cell carry the genes that decide sex?
Choose one answer.
Correct Not quite: the answer is One
Mark scheme
A: one
Model answer
A: one pair.
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