Basic Allele Types

This short section deals with some general types of alleles that can exist for almost all genes and which we will see often in this course. An allele is defined as an alternate form of a gene, which simply means that it has a different DNA sequence than another allele of the same gene. The difference in DNA sequence between two alleles leads to a difference in the products (protein or RNA) coded by the alleles which then results in a different effect on the phenotype. This is why different genotypes can yield different phenotypes.

As you might suspect there is an enormous amount of possible variation that could exist between alleles - there are many ways in which two sequences can be different! And, of course, each gene will have different possible variants (alleles). Despite this, there are some common types of alleles that geneticists have observed and we will look at three of these here.

These are the major allele types:

• Loss-of-Function alleles


• Gain-of-Function alleles


• Co-dominant alleles