Question

Marfan syndrome is caused by a loss of function mutation in the human fibrillin-1 (FBN1) gene. Most loss of function mutations cause recessive phenotypes, but Marfan syndrome is a dominant condition. Why is this the case?

A) Marfan syndrome is X-linked and only appears in males
B) individuals with Marfan syndrome are homozygous for the FBN1 mutant allele
C) the FBN1 gene is haploinsufficient
D) gain of function alleles are dominant over loss of function alleles E) both parents must be homozygous for a child to have Marfan syndrome

Answer 1

Marfan syndrome is caused by a loss of function mutation in the FBN1 gene(C), which is haploinsufficient. This means that having only one functional copy of the gene is not enough to produce the normal amount of fibrillin protein needed for proper connective tissue function, resulting in a dominant phenotype.

Step-by-step explanation:

Marfan syndrome is caused by a loss of function mutation in the human fibrillin-1 (FBN1) gene. Unlike most loss-of-function mutations that cause recessive phenotypes, Marfan syndrome is a dominant condition. The reason for this is that the FBN1 gene is haploinsufficient, meaning that having only one functional copy of the gene is not enough to produce the normal amount of fibrillin protein needed for proper connective tissue function.

When a person has Marfan syndrome, they have one mutated copy and one normal copy of the FBN1 gene. The normal copy cannot fully compensate for the loss of function caused by the mutated copy, resulting in a dominant phenotype. Even though the normal copy is present, it is not enough to produce the proper amount of functional fibrillin protein.

The correct answer to the question is C) the FBN1 gene is haploinsufficient.