Final answer:
Dominant mutant alleles can affect an organism by enhancing the gene product function, changing its distribution in the body, or through a dosage effect compared to the wild-type allele. They can also contain gain-of-function mutations, as seen in oncogenes.
Step-by-step explanation:
Dominant mutant alleles can exert their effects in a few different ways. One of the ways is by enhancing the function of the wild-type gene product or changing its distribution in the body. For instance, in the Antennapedia mutation in Drosophila, the mutant allele causes an expansion of the gene product's distribution, resulting in the development of legs on the head where antennae should be. Another way dominant alleles can express themselves is through the effect of 'dosage' where the wild-type allele provides the appropriate amount of gene product, and the mutant allele provides a lesser amount or none, affecting the phenotype. Finally, dominant alleles like oncogenes may possess gain-of-function mutations that result in a constitutively activated protein, which will be dominant over the normal allele's product.