Question

A hypothesis states that all mutations leading to hemophilia occur within the Factor VIII gene. Which of the following pairs of inheritance patterns WOULD FAIL TO SUPPORT this hypothesis? (Factor VIII gene lies on the X chromosome, so the inheritance of the recessive hemophilia allele follows the typical pattern of sex linkage).....(11)
I. Normal sons born to hemophiliac fathers
II. Normal sons born to hemophiliac mothers
III. Hemophiliac sons born to normal fathers
IV. Hemophiliac daughters born to normal fathers

A. I and II only
B. I and III only
C. II and IV only
D. III and IV only

Answer 1

Answer:

C. II and IV only

Explanation:

X linked recessive inheritance follows the criss-cross pattern in which affected mothers always have affected sons as sons get their X chromosome from mothers only. Condition "II" can not be true for X linked inheritance as hemophiliac mothers (XhXh) should have hemophiliac sons (XhY) only.

Daughters get one copy of their X chromosome from the father and the other from mother. To have affected daughters, mother should be carrier or hemophiliac and the father should be hemophiliac (XhY). This would make it possible for both the parent to transmit one copy of X chromosome with the gene to daughters (XhXh).