Question

A 20-year-old man has had no retinoblastomas but has produced two offspring with multiple retinoblastomas. In addition, his father had two retinoblastomas as a young child, and one of his siblings has had 3 retinoblastomas. What is the most likely explanation for the absence of retinoblastomas in this individual?

A. A new mutation in the unaffected individual, which has corrected the disease-causing mutation
B. Highly variable expression of the disorder
C. Incomplete penetrance
D. Multiple new mutations in other family members
E. Pleiotropy

Answer 1

The absence of retinoblastomas in the 20-year-old man is most likely due to Incomplete penetrance, which allows him to carry the genetic predisposition to the disease without displaying its phenotypic characteristics.

Step-by-step explanation:

The most likely explanation for the absence of retinoblastomas in the 20-year-old man, despite his offspring and other family members being affected, is Incomplete penetrance. Incomplete penetrance occurs when not all individuals with a disease-causing genotype exhibit the phenotype; that is, even though an individual may carry the gene for a disorder, they do not necessarily display the disease characteristics. This differs from variable expression, where all carriers show symptoms, but with varying degrees of severity.

In this case, the man may have inherited a predisposition to retinoblastoma but does not manifest the tumors, while his offspring have inherited the predisposition and do exhibit multiple tumors. The fact that the man's father and one of his siblings also had retinoblastomas further supports the argument for a genetic predisposition with incomplete penetrance, rather than multiple new mutations or other explanations.