The genetic phenotype of a missense mutation resulting in Marfan syndrome is:
a. Dominant.
b. Dominant with delay onset.
c. Recessive.
d. Co-Dominant.
e. None of the above
The correct answer and explanation is:
Correct Answer: a. Dominant
Explanation (Approx. 300 words):
Marfan syndrome is a genetic disorder that affects the body’s connective tissue. It is caused by mutations in the FBN1 gene, which encodes fibrillin-1, an essential protein that contributes to the strength and elasticity of connective tissue.
The genetic phenotype of Marfan syndrome is dominant, meaning that a single copy of the mutated gene (from either parent) is enough to cause the disorder. This pattern is referred to as autosomal dominant inheritance. Therefore, if one parent has Marfan syndrome, there is a 50% chance with each pregnancy that the child will inherit the mutation and develop the condition.
A missense mutation, which leads to the substitution of one amino acid for another in the protein, is a common type of mutation observed in Marfan syndrome. Many such mutations result in an abnormal fibrillin-1 protein that disrupts the structural integrity of connective tissue.
Unlike recessive conditions (which require two defective copies of the gene to manifest the disorder), Marfan syndrome can arise from just one mutated allele. This dominant expression of the disease makes it easier to track in families and contributes to its relatively consistent appearance across generations.
Although the severity and age of onset can vary greatly—even among individuals within the same family—delayed onset is not a defining feature of the syndrome in the way it is for some other genetic conditions like Huntington’s disease. Symptoms of Marfan syndrome can often be recognized in childhood or adolescence, although some individuals may not be diagnosed until adulthood.
Co-dominance, in which both alleles are fully expressed (as in blood types AB), does not apply to Marfan syndrome.
In conclusion, Marfan syndrome resulting from a missense mutation in the FBN1 gene demonstrates a dominant inheritance pattern, making option a the correct choice.