How can an individual carry a defective gene but not exhibit the defective phenotype

How can an individual carry a defective gene but not exhibit the defective phenotype?

a) An individual may not have that defective gene in every cell, and so appear normal
b) Individuals carry two copies of gene instructions, and one dominant allele could mask the presence of a recessive defective allele
c) Individuals are always homozygous
d) Both a) and c) are true
e) None of the above is true

B

Lili,

The question is a poor one.

For starters, it should say "an individual carry a defective allele"..... NOT a defective gene.

Second, it is possible that an individual is a mosiac of functioning genes. For example, human females have, randomly, either one or the other X chromosome active.... the inactive X chromosome is the Barr body. Thus if there is a "defective" allele on one of the X chromosomes of a female, then.... "a" above is functionally correct.

It's obvious the asker wants "b" as the answer..... but the question is really poor.

b) Individuals carry two copies of gene instructions, and one dominant allele could mask the presence of a recessive defective allele

B