Figure 1. Effect of mutations that decrease the quantity of a gene product. The solid vertical line shows the threshold for clinical effects. (a) For this gene, effects become noticeable only when the level of product drops below 20% of normal, therefore loss-of-function mutations will be recessive. (b) Haploinsufficiency: for this gene, effects become noticeable when the level of product is below 65% of normal. People with one nonfunctional copy of the gene will be affected and the resulting condition will be dominant.