IMAT 2014 Q30 [Sex-linked Conditions]

The diagram below shows the inheritance of a non­lethal recessive sex­-linked condition.


Which row in the table below does not correctly state the probability for each person possessing one copy of the allele for the condition?


A. Person 2
B. Person 5
C. Person 4
D. Person 1
E. Person 3

The answer to this question is Choice D, person 1 has the wrong probability.

Let’s establish a few reminders first that will be useful in interpreting this pedigree diagram: (to prevent redundancy, these concepts will not be explained when rationalizing the choices below)

  1. Males have the XY sex chromosome, while females have the XX chromosome
  2. If a condition is a sex-linked recessive condition, it will NOT present in females (since they have XX, and thus will be carriers), but it will present in males (because they only have a single X sex chromosome, hence XY).
  3. Fathers pass on their X sex chromosome to daughters. On the other hand, fathers will pass on their Y sex chromosome to sons. Mothers pass on either of the two X sex chromosomes they have, regardless if sons or daughters.

Before we discuss Person #1, let’s try to see why Persons 2-5 have correct probabilities, and this are not the correct choices for this question:

  • Person 2, 0%: This person is the father, since this condition is a sex-linked recessive condition, it makes sense that this person will NOT present with the condition.
  • Person 3, 50%: This person is one of the daughters. Since this person has no offspring yet, the maximum probability that we can assign this person is only 50%. The reason is that the person could have inherited either of the X sex chromosome of the mother, since the mother is a carrier, it is possible for this daughter to inherit either the X chromosome with the allele, or the one without the allele. (Keep in mind reminder #3)
  • Person 4, 0%: This is a son that did NOT present with the condition, hence, the reason why this son did not present with the disease is stated in reminder #2.
  • Person 5, 100%: This is a female that had a son that presented with the recessive condition. Because the son’s father (Person 5’s partner) did NOT present with the condition, the only possible explanation is that the allele came from the mother. Thus, the maximum probability that person #5 has at least one allele is 100%, given that she was able to pass it on to her offspring. You might be wondering, why is there a difference in probability between person 3 and 5 if they’re both females? This is because the question is asking for “probability” or our certainty of the person having at least 1 copy of the allele. Person 3 does not have an offspring yet, thus we cannot be 100% certain that she carries the allele, compared to person 5, wherein there is concrete evidence that she does carry it, given that her son presents with the condition.

Let’s now rationalize why person 1 has a wrong probability:

  • Person 1 was given a probability of 50% of carrying at least 1 copy of the allele, which is incorrect. This has to be 100% because of 1 main reason (with the basis that this is a sex-linked recessive condition): The husband (person 2) and their son (person 4) did not present with the condition (refer to reminder #2), thus the only source of the allele would only be the mother, or person 1. If this was a Y-chromosome linked disease, person 2 would have presented with the condition, and person 5 would not have been able to pass on the allele to her offspring.

Thus, the only answer would be choice D, where person 1 has the wrong probability.