Re: What makes a dominant gene dominant?

All of the genes that are labeled as dominant or recessive produce protein 
molecules.  Recessive alleles are most commonly due to a "loss of 
function" mutation.  This means that the normal function of the protein 
molecule is lost.  This is why recessive alleles need to be present in the 
homozygous condition for expression.  In the heterozygous condition, if 
you have one allele with a loss of function, you still have the second 
copy that will produce a normal functioning protein. Dominant alleles are 
usually due to a "gain of function" mutation.  This new protein activity 
will be expressed in both the homozygous and heterozygous condition.

One other factor to consider in the discussion of dominant and recessive 
alleles is the actual location of the protein product.  There can be 
varied phenotypes depending upon whether the protein product from a gene 
has a fixed location or is soluble and can move throughout the cell or 
body.