Original practice · 5 questions · 15 marks · about 17 minutes

Sex Determination and Sex Linkage Practice Questions

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These questions test your understanding of how sex is determined by chromosomes and how some traits are linked to the X chromosome.

1. Question 1

   Paper 4 Both 3 marks Difficulty 2/5

   Explain how sex is determined in humans by the X and Y chromosomes.

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   1. Females have two X chromosomes (XX); males have one X and one Y chromosome (XY).
   2. The mother always contributes an X; the father contributes either X or Y.
   3. There is a 50% chance of a boy (XY) and 50% chance of a girl (XX) in each pregnancy.

2. Question 2

   Paper 4 Extended 4 marks Difficulty 4/5

   Red-green colour blindness is a sex-linked recessive condition carried on the X chromosome. A carrier mother (X^N X^n) and a normal father (X^N Y) have children. What proportion of their sons will be colour blind?

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   1. Mother's gametes: X^N or X^n. Father's gametes: X^N or Y.
   2. Sons receive Y from father and X from mother.
   3. Son genotypes: X^N Y (normal) or X^n Y (colour blind).
   4. 50% of sons will be colour blind.

3. Question 3

   Paper 4 Extended 3 marks Difficulty 4/5

   Explain why sex-linked conditions like colour blindness are more common in males than females.

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   1. Males only have one X chromosome: if it carries the recessive allele, the condition is expressed.
   2. Males have no second X to carry a dominant allele that could mask it.
   3. Females need two copies of the recessive allele (one on each X) to show the condition: much less likely.

4. Question 4

   Paper 2 Core 2 marks Difficulty 2/5

   State the sex chromosomes found in a human male.

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   1. XY.
   2. The Y chromosome determines male sex.

5. Question 5

   Paper 4 Extended 3 marks Difficulty 4/5

   Can a colour-blind father have a daughter who is colour blind? Explain your answer.

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   1. Yes: but only if the mother is at least a carrier (X^N X^n).
   2. The father contributes X^n; if the mother also contributes X^n, the daughter is X^n X^n (colour blind).
   3. If the mother is X^N X^N (not a carrier), all daughters will be carriers at most: never colour blind.