Question

The condition shown in this family has multiple possible modes of inheritance: X-linked recessive, autosomal dominant, and autosomal recessive. It is always one mode of inheritance for each family (ie it doesn't change between people within the same family). Assume full penetrance and onset from birth.

You are trying to determine the mode of inheritance for the condition in this family. Regardless of the gene it's on, finding a single pathogenic mutation in who in this family (other than the affected person) would allow you to determine the mode of inheritance?

Select one:

a. The mother

b. The father

c. The sister

d. Finding a pathogenic mutation in no one individual will give that information

Answer 1

Genetic diseases are inherited through generations by 2 methods: by sex chromosomes and autosomes(Chromosomes 1-22). Here in this family, the possible mode of inheritance are X-linked recessive (sex chromosomes) and autosomal (dominant and recessive).

1) X linked recessive: x chromosome causes infection. As male has only one x chromosome, they get infected and females only with 2 mutated x chromosome causes infection. This mode can be determined by examining sister.

2) Autosomal dominant: Infection occur if any one allele of either the parent is inherited to progeny.

3) Autosomal recessive: Infection occur only if infected chromosomes from both the parent is inherited. Others with one allele from either the parent makes them a carrier.

Autosomal disease can be determined with any one of the parent.

So, the determination of the mode of infection cannot be done by observing any one parent or sibling. It can be done by any two. So option d is the answer.