|MadSci Network: Genetics|
First, it's necessary to think about what it means for an allele (a form of a gene) to be recessive or dominant. It's all about the phenotype of the heterozygote. That is, when an individual has two different alleles, what does it look like? Let's say a cat has the genotype Aa. An AA individual has black fur; an aa individual is albino, or white. Is the Aa individual black like the AA one, white like the aa one, or something in between? If an Aa cat is black, then we say that the allele A is dominant and the allele a is recessive. Why does this happen? Genes code for proteins. An allele A, in this case, codes for a protein that catalyzes a reaction (an enzyme). The allele a codes for a modified form of that enzyme that doesn't function. The enzyme catalyzes a reaction that helps to produce the black pigment melanin – if a cat doesn't have any of that enzyme, then it's albino, because it produces no pigment. However, if the cat is Aa, both alleles are expressed; both proteins are produced, the active enzyme and the inactive protein. In this case, and in a lot of cases, one "dose" of enzyme is enough to get the job done – the melanin is made, and the Aa cat is black. Mutations are changes in the DNA. A mutation can change a gene slightly, giving a different allele. The new allele can code for a slightly different protein. If the normal allele codes for an active enzyme, the new allele may still code for the same active enzyme, may code for an inactive protein, or may code for an active enzyme that catalyzes a different reaction. Of these options, coding for the same active enzyme may be the most common, but then we don't usually notice there's been a mutation. Coding for an inactive protein is the next most likely outcome. So, most of the time when there is a mutation that produces any noticeable effect at all, it produces an allele that codes for an inactive protein. A heterozygote Aa produces some active enzyme and some inactive protein. Most often, one "dose" of active enzyme catalyzes the normal reaction enough to produce a normal appearance, so we say that the allele A is dominant, and the mutant allele a is recessive. There are plenty of exceptions. Certain types of dwarfism in humans are caused by a dominant mutant allele, for instance. Still, it's probably true that the majority of mutant alleles are recessive.
Try the links in the MadSci Library for more information on Genetics.