Answer
A dominant negative gene interferes with the normal gene, causing a loss of function phenotype even with the normal gene. A gain of function mutation makes the gene activate at the wrong times, and as such typically affects the phenotype.
Work Step by Step
In the presence of a dominant negative, the normal gene and allele is rendered essentially recessive. This is what makes the mutation dominant. A gain of function on the other hand takes a gene and makes it active at the wrong times, thus affecting the offspring's phenotype.
