1) In the nucleus of the cell, there are pairs of chromosomes which carry the same genes in the same places. 

2) Alleles are different versions of these genes.

3) For example, a person may have an allele for brown eyes on one chromosome and an allele for green eyes on the other. 

4) Dominant alleles only require one copy for it to be expressed, whereas recessive alleles need two copies: they are only expressed if both alleles are recessive (no dominant alleles).