Question 49 - Biological and Biochemical Foundations of Living Systems Practice Test for the MCAT

Tracie is a 29-year-old female with Rett syndrome, a rare X-linked dominant neurological disorder. Tracie is heterozygous for the trait. She marries Jax and they have one daughter. If Jax does not carry any copies of the Rett syndrome gene, what are the chances that their daughter will inherit the gene for Rett syndrome? (You may consult the attached passage.)

Adrenoleukodystrophy (ALD) is a genetic disorder in which lipids with very high molecular weights are not metabolized and accumulate within cells. Accumulation of these fats in the brain damages the myelin that surrounds nerves. This progressive disease has two causes: an autosomal recessive allele, which causes neonatal ALD, and a mutation in the ABCD1 gene located on the X chromosome. A controversial treatment is the use of Lorenzo’s oil, which is expensive; despite this treatment, neurological degradation persists in many patients. Gene therapy as a potential treatment is currently in trials but is also very costly. An infant patient exhibits symptoms of neonatal ALD, which are difficult to distinguish from the X-linked form of the disease. The infant’s physician consults electronic health records to construct a pedigree showing family members who also presented symptoms similar to ALD. The pedigree is shown in this diagram. The infant patient is circled. Symbols are filled when symptoms are present.


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