Final answer:
Haplosufficient genes are not dosage sensitive, as one copy is enough to produce a normal phenotype, and they do not need to exhibit dominance nor are they caused by gain of function mutations.
Step-by-step explanation:
Haplosufficient genes are genes for which one copy is sufficient to produce a normal phenotype in diploid organisms. This means that they are generally not dosage sensitive, as the presence of one functional allele provides enough of the gene product to exhibit the normal trait. They can be dominant over haploinsufficient genes, where one allele does not provide enough product for a normal phenotype, and thus two copies are essential for normal function. Gain of function mutations, which can lead to dominant negative mutations, are different and not necessarily related to haplosufficiency.
So, the answer to the student's question would be: Haplosufficient genes are B) not dosage sensitive. Options A) and C) do not correctly describe haplosufficient genes, as they do not have to be dominant over haploinsufficient genes by default, and they are not necessarily caused by gain of function mutations.
Haplosufficient genes are genes that can perform their function properly even if only one copy is present in the cell. This means that these genes are not dosage sensitive and their function is not affected by the presence or absence of an additional copy. Haplosufficient genes are usually dominant over haploinsufficient genes, which are genes that require both copies to be present in order to function properly.