Exam questions · Biology · Inheritance
Sex-Linked Genetic Disorders (Higher tier)
- 6 exam questions
- 21 marks
- 9 quick checks
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1 State [2 marks]
State what is meant by a sex-linked disorder.
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Model answer
A disorder caused by an allele on a sex chromosome, usually the X chromosome, so that it is inherited differently in males and females.
Mark scheme
- Allele on a sex chromosome / X chromosome — 1 mark
- Different pattern of inheritance in males and females — 1 mark
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2 Explain [3 marks]
Explain why a male is more likely than a female to have a sex-linked recessive disorder.
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Model answer
A male has only one X chromosome, so he has only one allele for the gene. If it is the recessive allele, it will show its effect. A female has two X chromosomes, so a dominant allele on the other X would hide the recessive one, and she needs two recessive alleles to be affected.
Mark scheme
- Males have only one X — 1 mark
- A single recessive allele shows — 1 mark
- Females need two recessive alleles — 1 mark
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3 Complete [3 marks]
Haemophilia is caused by a recessive allele, \(h\), on the X chromosome. State the genotype of (a) a man with haemophilia, (b) a female carrier, and (c) a man without haemophilia.
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Model answer
(a) \(X^h Y\). (b) \(X^H X^h\). (c) \(X^H Y\).
Mark scheme
- \(X^h Y\) — 1 mark
- \(X^H X^h\) — 1 mark
- \(X^H Y\) — 1 mark
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4 Draw [4 marks]
A woman who is a carrier of colour blindness has a child with a man who has normal vision. Draw a genetic diagram to show the possible children.
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Model answer
The mother is \(X^B X^b\) and the father is \(X^B Y\). The gametes are \(X^B\) and \(X^b\) from the mother, and \(X^B\) and \(Y\) from the father. The children are \(X^B X^B\) (normal girl), \(X^B X^b\) (carrier girl), \(X^B Y\) (normal boy) and \(X^b Y\) (colour-blind boy).
Mark scheme
- Parents' genotypes — 1 mark
- Gametes — 1 mark
- All four offspring genotypes — 1 mark
- Phenotypes identified — 1 mark
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5 Calculate [3 marks]
A woman is a carrier of haemophilia and her partner is unaffected. Calculate the percentage of their sons expected to have haemophilia, and the percentage of their daughters expected to have haemophilia.
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Model answer
Half of the sons, 50%, are expected to have haemophilia, because the sons are \(X^H Y\) or \(X^h Y\). None of the daughters, 0%, will have haemophilia, as they all receive the father's normal \(X^H\).
Mark scheme
- 50% of sons — 1 mark
- 0% of daughters — 1 mark
- Reasoning about the father's X — 1 mark
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6 Explain [6 marks]
A man with haemophilia has a daughter and a son. Explain, using a genetic diagram, whether the children are likely to have haemophilia, and why haemophilia cannot be passed from a father to his son.
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Model answer
The father is \(X^h Y\) and the mother is \(X^H X^H\). The father makes \(X^h\) and \(Y\) sperm, and the mother makes only \(X^H\) eggs. The daughters are all \(X^H X^h\), so they are carriers and do not have haemophilia. The sons are all \(X^H Y\), so they are normal. A father passes the Y chromosome to his sons, and the allele is on the X chromosome, so he cannot pass it to them. His sons get their X from their mother, who has normal alleles. A carrier daughter could, however, pass the allele to her own children.
Mark scheme
- Parents' genotypes — 1 mark
- Correct gametes — 1 mark
- Daughters are carriers — 1 mark
- Sons are unaffected — 1 mark
- Sons get the Y from the father — 1 mark
- Allele is on the X chromosome — 1 mark
Quick check
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1
On which chromosome is the haemophilia allele found?
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B: The X chromosome
Haemophilia is caused by a recessive allele on the X chromosome.
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2
Why are sex-linked recessive disorders more common in males?
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C: Males only have one X chromosome
A male has only one X chromosome, so one recessive allele is enough to cause the disorder.
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3
What is the genotype of a female carrier of haemophilia?
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A: \(X^H X^h\)
A carrier has one normal and one haemophilia allele: \(X^H X^h\).
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4
What is the genotype of a man with haemophilia?
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D: \(X^h Y\)
He has the haemophilia allele on his only X chromosome, and a Y: \(X^h Y\).
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5
A carrier woman and an unaffected man have a son. What is the chance that he has haemophilia?
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B: 50%
He gets his X from his mother, which is \(X^H\) or \(X^h\) with equal chance, so 50%.
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6
What are the daughters of a man with haemophilia and a woman with two normal alleles?
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C: All carriers
Each daughter receives his \(X^h\) and her mother's \(X^H\), so all are carriers.
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7
Why can a father not pass haemophilia on to his sons?
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D: He passes the Y chromosome to his sons, and the allele is on his X
A father passes his Y chromosome to his sons, and the haemophilia allele is on the X chromosome.
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8
Which of these is a sex-linked recessive disorder?
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A: Red-green colour blindness
Red-green colour blindness is caused by recessive alleles on the X chromosome.
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9
A woman who has colour blindness has a son. What is the chance that he is colour blind?
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B: 100%
She has the genotype \(X^b X^b\), so every son receives an \(X^b\) from her, and he is colour blind (\(X^b Y\)).