Day 78 MCAT Practice Question

Passage 5: Genetics

A novel drug, Xenaplex, has shown remarkable efficacy in treating a rare genetic

disorder characterized by the accumulation of a toxic metabolite, “Xenic Acid”, in

the body. The disorder is caused by a recessive mutation in the XEN1 gene, which

encodes an enzyme responsible for the breakdown of the metabolite. Xenaplex

is an enantiomerically pure compound that acts as a potent activator of the

XEN1 enzyme, effectively reducing the levels of the toxic metabolite in affected

individuals.



However, clinical trials have revealed that Xenaplex is only effective in a subset of

patients with the disorder. Genetic analysis of responders and non responders

has uncovered a surprising finding: the presence of a specific single nucleotide

polymorphism (SNP) in the XEN1 gene strongly correlates with the drug’s efficacy.

The results of this study are shown below in figures 1 and 2.

Further investigation into the evolutionary history of the XEN1 gene has revealed

that the SNP is a relatively recent mutation, estimated to have arisen approximately

10,000 years ago. The frequency of the SNP varies significantly across different ethnic

groups, with the highest prevalence observed in populations of East Asian descent.

If two individuals heterozygous for the XEN1 mutation have a child, what is the

probability that the child will be affected by the disorder?

A) 0%

B) 25%

C) 50%

D) cannot be known