Alleles can be either dominant or recessive.
Dominant alleles show their effect even if the individual only has one copy of the genes, For example, the allele for brown eyes is dominant, therefore you only need one copy of the 'brown eye' allele to have brown eyes ;although, with two copies you will still have brown eyes.
If both alleles are dominant, it is called co-dominance. The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of co-dominance of the A and B dominant alleles.
Recessive alleles only show their effect if the individual has two copies of the allele. For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the 'blue eye' allele.
Illustration showing the inheritance of dominant and recessive alleles for eye colour.
