Question

One of two major forms of a human condition called neurofibromatosis (NF 1) is inherited as a dominant gene, although it may range from mildly to very severely expressed. Which of the following is the best explanation for why a young, affected child is the first in her family to be diagnosed?

(A) The condition skipped a generation in the family.
(B) The child has one more chromosome than either of the parents.
(C) One of the parent has a mild expression of the gene.
(D) The mother carries the gene but does not express it.

Answer 1

(C) One of the parent has a mild expression of the gene.

Step-by-step explanation:

Neurofibromatosis (NF 1) is inherited as a dominant allele but requires the presence of two copies of the NF-1 alleles to be present in the individual to stimulate the tumor formation. If an individual carries only one copy of the NF-1 allele, tumor formation will not be triggered and the person may not be diagnosed with the disorder.

However, when a child from two heterozygous individuals obtains one NF-1 allele from each of the parents, tumor formation will be triggered and the child will be diagnosed as the first one to have the disease irrespective of the fact that both the parents have one NF-1 allele.