Question

A male Haemophiliac, marries a normal woman, and they have four normal children, two boys (Mark and Mike) and two girls (Molly and Mary). Mark, Mike, Molly, and Mary all marry normal individuals and have children. The children (male and female) of Mark's or Mike's children were normal but the sons of Molly and Mary all display symptoms of Haemophilia while their daughters do not.
Which of the following statements best explains the reason that Mark, Mike, Molly, and Mary do not display symptoms of Haemophilia, even though their father, John, is a Haemophiliac?

A. Haemophilia is an X-linked disorder, and John can pass on only his Y chromosome.
B. Haemophilia is an X-linked disorder and even though Molly and Mary received a Haemophiliac X chromosome from father, mother gave them a normal X chromosome.
C. Haemophilia is a Y-linked disorder and therefore cannot be displayed in females.
D. Haemophilia is a Y-linked disorder and Mark and Mike must have received an X chromosome from John.

Answer 1

Hint: The gene that causes haemophilia is passed from parent to child. A mother that carries the gene is known as a carrier, and she has a half chance of having a son with Haemophilia and a half (50%) chance of having a daughter who is also a carrier.

Complete Answer:
Haemophilia is a bleeding disorder that slows the blood clotting process. Haemophilia A and haemophilia B are inherited in an X-linked recessive pattern. The genes involved with this phenomenon are present on the X chromosome that is one of the two sex chromosomes. In males which have only one X chromosome, one changed duplicate of the gene in every cell is enough to occur the phenomenon.

Now, let us find the solution from the option.
- Out of this family, the only members that express this condition are males. This is a tip-off for X-linked disorders, which are more usual in males, because they have only a single X chromosome.
- John’s genotype is XhY. He transferred his Y chromosome to Mark and Mike; they also got a normal X from Jane, so they do not have Haemophilia, nor can they transfer it on to their child. Molly and Mary got Xh from John but also got a normal X from Jane, so they are bearers of Haemophilia but do not show its symptoms.

Thus, the correct option is B. i.e. Haemophilia is an X-linked disorder and even though Molly and Mary received a Haemophiliac X chromosome from father, mother gave them a normal X chromosome.

Note: Haemophilia is usually an inherited bleeding disorder in which the blood does not clot properly. Blood consists of different types of proteins known as clotting factors that function as to prevent bleeding. Persons with haemophilia have low numbers of factor VIII or factor IX. The intensity of hemophilia that an individual has is recognized by the quantity of factors in the blood. The less the quantity of the factor, the more bleeding will occur which can cause severe health issues.