Free X-linked Recessive Inheritance MCQs with Answers

1. Red green colour blindness is far commoner in men than in women because

• A. a man has only one X chromosome, so a single recessive allele is expressed
• B. the gene lies on the Y chromosome
• C. the allele is dominant in men and recessive in women
• D. women lose the allele during meiosis

Explanation: The gene is on the X chromosome and a man is hemizygous, so there is no second copy that could mask a defective allele. A woman needs the recessive allele on both of her X chromosomes to be colour blind, which is a far less likely combination, and with only one she is a carrier with normal vision. The Y chromosome carries no corresponding gene.

2. A woman who is a carrier for colour blindness marries a man with normal colour vision. The probability that any son of theirs is colour blind is

• A. 0
• B. one in four
• C. one in two
• D. certain

Explanation: A son takes his single X from his mother, and half of her X chromosomes carry the defective allele, so half the sons are colour blind. The answer of one in four applies to the different question of the chance that a child of either sex is colour blind, which is the usual mistake here. None of the daughters is colour blind, since each receives a normal X from the father, but half are carriers.

3. Haemophilia A is caused by the deficiency of

• A. clotting factor VIII
• B. vitamin K
• C. platelets
• D. haemoglobin

Explanation: The X linked recessive gene codes for antihaemophilic factor VIII, and without it the clotting cascade stalls, so even a small injury bleeds for a very long time and bleeding into joints is common. Vitamin K deficiency also impairs clotting but does so by affecting several liver made factors and is dietary rather than inherited. The platelet count in haemophilia is normal.

4. A colour blind girl can be born only if

• A. her mother alone is colour blind
• B. her father alone is colour blind
• C. her father is colour blind and her mother is at least a carrier
• D. neither parent carries the allele

Explanation: A girl has two X chromosomes and needs the recessive allele on both, one from each parent. The father can only supply it if he is colour blind himself, and the mother must be either a carrier or colour blind. This is why affected women are rare, and it explains the pattern in which the condition seems to skip generations on the mother's side.

5. The sex of a human child is determined by

• A. the ovum, which may carry either an X or a Y chromosome
• B. the hormones of the mother during pregnancy
• C. the total number of chromosomes in the zygote
• D. the sperm, which carries either an X or a Y chromosome

Explanation: The mother is XX and so every ovum carries an X, while the father is XY and produces X bearing and Y bearing sperm in equal numbers, so it is the sperm that decides. A Y bearing sperm gives an XY male and an X bearing sperm an XX female, which also explains the roughly equal sex ratio at birth. Blaming the mother for the sex of a child has no biological basis whatsoever.

6. In X linked recessive inheritance, the criss cross pattern means that a trait typically passes from

• A. father to son directly
• B. mother to daughter only
• C. father to every one of his sons
• D. an affected man to his grandsons through his carrier daughters

Explanation: An affected father gives his X, and therefore the allele, to all of his daughters, who become carriers, and half of their sons then inherit it, so the trait reappears in the grandsons. A father can never pass an X linked allele to a son, because a son receives his Y from the father and his X from the mother. That impossibility is the quickest way to rule out X linkage in a pedigree.

7. Haemophilia is

• A. Autosomal dominant
• B. Autosomal recessive
• C. X-linked recessive
• D. Y-linked

Explanation: Haemophilia is an X-linked recessive disorder affecting blood clotting, more common in males.

8. A carrier of an X-linked recessive disorder is

• A. Male with the disorder
• B. Female with one affected X
• C. Female with both X affected
• D. Male without the disorder

Explanation: Carrier females are heterozygous (XAXa) and phenotypically normal but can pass the allele to offspring.

9. A baby girl is born with hemophilia, which is an X-linked recessive disorder. What are the most likely genotypes of her parents?

• A. The mother is a carrier and the father is normal
• B. The mother is hemophiliac and the father is normal
• C. The mother is carrier and the father is hemophiliac
• D. Both are normal

Explanation: A girl has two X chromosomes and needs the recessive allele on both, one from each parent, so the father must be haemophiliac himself and the mother must carry at least one copy. A carrier mother with a normal father could only produce affected sons, which is why the first option fails. This is why affected females are so rare.

10. If a normal person marries with colour blind female what will be the possibility of normal male child:

• A. 0%
• B. 25%
• C. 50%
• D. 75%

Explanation: A colour blind mother carries the defective allele on both her X chromosomes, and a son receives his only X from her, so every son must be colour blind. The daughters each receive a normal X from the father and are carriers with normal vision. This is the clearest illustration of why an X linked recessive trait passes from mother to son.