Re: How are alleles of genes dominant and recessive?

What a good question! Hope this helps to explain it better. If a mutation in a gene knocks out its normal function (lets say it makes the gene product inactive), then most often this would be an example of a recessive allele. When you bring in a second copy of the gene as a normal allele, you only see the normal or wild type function because the mutant allele is non-functional and essentially silent.

Now for an example of dominance. Lets say that the mutation now changes the function of the gene product in some way to make it different from the wild type. When you have this mutant allele along with a wild type allele together in a cell, you would now see the effect of the mutation regardless of the presence of the wild type.

A simple example is a mutation to antibiotic resistance. The antibiotic streptomycin acts to inhibit a ribosomal protein important in protein translation. When you add the antibiotic the cells can't do translation and die. It is possible to isolate a resistant allele of that gene making the same functional ribosomal protein that is not inhibited by streptomycin. In a diploid where you have the mutant resistant allele and the wild type allele, if you add streptomycin the cell will survive because the mutant allele will allow protein translation to continue. Hence it is dominant.

So the short answer is: if a mutation confers a new or changed property on a gene product such that this new property can still be detected in the presence of the wild type, then the mutation is generally dominant. If the mutation inactivates or changes the gene product such that the wild type product takes over, then the mutation is recessive.