IMAT 2011 Q50 [Haemophilia]

Haemophilia is caused by a recessive allele carried only on the X chromosome. A carrier female and a non-haemophiliac male decide to have a child. Which of the following four statements are correct?

  1. they have a 25% chance of producing a haemophiliac son
  2. they have a 25% chance of producing a haemophiliac daughter
  3. they have a 25% chance of producing a carrier son
  4. they have a 25% chance of producing a carrier daughter

A. 1 and 2 only

B. 1 and 4 only

C. 2 and 3 only

D. 2 and 4 only

E. 3 and 4 only

Classical genetic questions are common on the IMAT exam. There’s at least one question every year, if not more, on this subject.

Let’s analyze the statements;

Haemophilia is caused by recessive allele carried only on the X chromosome, a carrier female (Xx) and a non-hemophiliac male (XY) decide to have a child, which of the following four statements are correct?

First, we must know that the RECESSIVE X chromosome with this specific situation will only affect males because male (XY) has only one X chromosome from the mother. It doesn’t matter if the gene affected is dominant or recessive, the male will be affected by it. Females on the other hand, will only be affected if both of their X chromosomes are recessive for the disease, meaning the father MUST be sick himself, and the mother MUST be a carrier or either recessive and sick herself.

We know that both of our parents are HEALTHY, so the girls that will be born will be HEALTHY, because the other X chromosome is given by the father, which has only one X chromosome to “give,” and we are told the father is healthy!

We are given the genotype (genes combination) and the phenotype (the gender of the child) with each answer, the possibility to be a girl or a boy is 50/50.

“they have a 25% chance of producing a haemophiliac son” –

The mother will either inherit her ‘healthy X’ or ‘recessive X’ to the boy; therefore there is a 1/2 (the allele probability) times 1/2 (the gender probability), however, the father only donates Y chromosome and only affects the gender of the child, thus there’s no need to consider the gender probability this time.

The boy would be hemophilic, so this statement is true.

“they have a 25% chance of producing a hemophiliac daughter.”

Let’s analyze this statement. For someone to be ill with a recessive disease, both of the alleles should be recessive, the probability that a girl will inherit a recessive X chromosome from her healthy father is 0%. The father must have been sick as well; therefore, this statement is false.

“they have a 25% chance of producing a carrier son.”

Similar to the previous statement, the probability of inheriting a recessive X linked from the mother to produce a carrier son is 0%. Always remember that males have a single X chromosome which they inherited from their mothers. Assuming the mother is healthy but a carrier, it means 1 of her chromosome must be healthy, and the other is must be recessive.
So in conclusion, the boy can only be healthy or sick, but not a carrier.

they have a 25% chance of producing a carrier daughter .”

A carrier recessive X linked mother must have a single healthy chromosome while the other must be infected, therefore, considering we are talking specifically about the mother *and not the father like the previous statements), the probability of delivering to her daughter a recessive chromosome is 50%, and of course, producing a daughter is also 50%.
1/2 time 1/2 is 25% which means the statement is correct.

The answer must be B.