Final answer:
Haploinsufficiency refers to the condition where one copy of a gene is not sufficient to carry out the normal function that two copies would, often leading to disease or abnormalities.
Step-by-step explanation:
The inability of one copy of a gene to perform the function of the normal two copies of the gene is known as haploinsufficiency. This occurs because a single copy of the wild-type allele is not always sufficient for the normal level of function required for survival or regular functioning of the organism. This often happens with essential genes, where the nonfunctional allele can be present in the population as long as there's a functional copy. However, haploinsufficiency can lead to abnormalities and susceptibility to disease, particularly when the essential level of gene product is not achieved. In contrast, dominant negative mutations affect the normal gene product adversely within the same cell, leading to competitive inhibition of normal protein functions, which is not the case with haploinsufficiency.