IMAT 2018 Q31 [X-linked Condition]

A condition caused by the dominant allele of a gene on the X chromosome is passed down through a family. Individual Z inherits the condition.

For this family, which of the following statements must be correct, assuming no spontaneous mutations have occurred?

  1. If Z is a boy, then he must have inherited the dominant allele from his father.
  2. If Z is a boy, then one of his mother’s parents must have also had the condition.
  3. If Z is a girl, then both of her parents must have had the allele.
  4. If Z is a boy, and has children, then all of his daughters will have the condition.

A. 2 and 4 only
B. 1 and 3 only
C. 1, 3 and 4 only
D. 1 and 4 only
E. 2 and 3 only

The answer to this question is choice A.

Let’s establish a few reminders first that will be useful in interpreting this pedigree diagram:

  1. Males have the XY sex chromosome, while females have the XX chromosome.
  2. If a condition is a sex-linked dominant condition, it will present REGARDLESS if the offspring is a male or female, since both males and females have an X chromosome. 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.

Let’s then rationalize the 4 different statements, to see if they are consistent with the reminders above.

  • Statement 1: This is already wrong immediately, since the condition is an X-linked dominant condition. If you refer to reminder #3,(reminder, Z in this case is a male) males inherit the Y chromosome from the father, thus, it is NOT possible for Z to inherited an X-LINKED condition from his father.
  • Statement 2: This is correct, again referring to reminder #3. The only source of the X-linked condition of Z is from his mother, which would have not been possible if none of the mother’s parents had the condition in the first place. Thus it makes sense that the X-linked dominant condition was passed on to the mother, which was subsequently passed on the son (who has an XY sex chromosome), making this a correct consideration.
  • Statement 3: In this case, Z is a girl, where she has XX sex chromosomes, 1 X chromosome from each parent. Since this is an X-linked DOMINANT condition, the offspring needs to inherit at least 1 dominant allele to present with the condition! This leads then to the reason why this statement is wrong. Read the statement again, and notice that it said “both of her parents must have had the allele”. This is false, because as mentioned earlier, the offspring needs to inherit at least 1 dominant allele, which means that it is not a requirement that BOTH parents have the allele. Do not be confused however, it IS possible for both parents to have the allele, BUT it is NOT a requirement for Z to present with the condition.
  • Statement 4: This is a correct statement, as it is consistent with reminder #3. Remember that daughters will inherit the ONLY x chromosome that their father has (since males have XY sex chromosomes). Thus if the father has an X-linked dominant condition on his ONLY X chromosome, then all of his daughters WILL present with the condition (refer to reminder #2 as well).

Thus, the only correct consideration would be choice A.