Alleles are different variations of a gene. Every person has two alleles - one from each of their parents. Depending on whether it is expressed, an allele can be dominant or recessive.
A dominant allele is the one that is expressed even if the individual has one dominant and one recessive allele. This means that dominant alleles mask recessive alleles.
An example of a dominant allele is the one for brown eyes. This means that you need only one allele of this type to have brown eyes, which is why brown eyes are the most common.
