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Haemophilia A and haemophilia B are inherited in an X-linked recessive pattern. The genes associated with these conditions are located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, it is very rare for females to have haemophilia. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

1. Autosomal dominant
2. Autosomal recessive
3. X-linked recessive
4. Y-linked inheritance

Correct answer: X-linked recessive

Solution

The condition is caused by genes on the X chromosome, and males need only one altered X to express it, which fits X-linked recessive inheritance. The fact that fathers do not pass the trait to sons also supports an X-linked pattern rather than autosomal or Y-linked inheritance.

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