Pairs of alleles can be either heterozygous or Homozygous. An allele are a pair of genes (section of DNA).
Heterozygous Heterozygous means that there are two different alleles, one being dominate and the other is recessive. This is represented by letters For example: Rr, Gg, Tt The capital means dominate gene, while the lower case means recessive.
Homozygous Means that both alleles are exactly the same. For example: FF, GG, TT
Dominate and recessive A dominate allele will always be expressed over a recessive allele. For Example: Brown eyes are dominant to blue eyes, thus brown eyes are given the letter B, as blue eyes are given the letter b. If a person has the alleles Bb, they will have brown eyes, BB they will have brown eyes, bb they will have blue eyes.