Question

Sol. Key concept: Classical haemophilia A and B show X-linked recessive inheritance.

Haemophilia A involves deficient factor VIII, and haemophilia B deficient factor IX. Their genes lie on the X chromosome. A male has only one X, so a disease-causing allele on that X can be expressed without a second affected allele.

a) Correct in the standard pedigree sense: an affected father passes his X-linked allele to daughters, who may transmit it to their sons. This is the familiar criss-cross pattern; there is no direct father-to-son transmission of the affected X.

b) Incorrect: the inheritance is X-linked recessive, not Y-linked dominant. c Incorrect: classical haemophilia is much more common in males; affected females are less common, although females can have clinically significant bleeding.

d) Correct: it is historically called the royal disease because it appeared in several European royal families. The question asks for incorrect statements, so choose b and c.

Correction / clarification: The former solution simply repeated statements (b) and (c). They are the false statements being selected, not correct descriptions of haemophilia. [R7]