A woman has a recessive genetic condition but neither of her parents has the condition.
Which one of the following could not be true?

Answer:

In order to have the condition, the woman must have two copies of the recessive allele – information
from the question. In order to inherit two copies, she must have inherited a recessive copy from each
of her parents. As neither of them have the disorder, they must be heterozygous, so A is true. Her
parents must have inherited a recessive allele each from their parents (the woman’s grandparents) so
they could all be carriers (D), one of each set of grandparents could be a carrier (C) or they may have
the condition (B). Any of these options is possible.
In order for the woman to develop a condition that requires two copies of the recessive allele,
however, the DNA mutation in one gamete is not sufficient. DNA in the gametes from both parents
would have had to mutate if neither of them possessed the recessive allele to start with. Therefore E
cannot be true.
The answer is E