Since human cells have two copies of each chromosome, they carry two versions of each gene. Different gene versions are known as alleles. Depending on their associated characteristics, alleles are either dominant or recessive.
Dominant alleles display their effect even when an individual only carry one copy of the allele. This condition is referred to as being heterozygous. For instance, since the allele for brown eyes is dominant, an individual only need one allele for the brown eye to have brown eyes. An individual with two copies of the ‘brown eye’ allele will still have brown eyes.
Recessive alleles only display their effect if an organism has two copies of the allele. This condition is also known as being homozygous. For example, blue eyes allele is recessive. Therefore, for an individual to have blue eyes, he/she needs to have two copies of allele for the blue eyes. Another example is Cystic fibrosis which is caused by a recessive allele. To have Cystic fibrosis, an individual need to have two copies of the faulty allele.
Sometimes, both alleles can be dominant. The condition where both alleles express equally is called codominance. Blood types are good examples of this condition. The blood group AB is the result of codominance of the A and B alleles.