A dominant allele is always expressed, even if the individual only has one copy of it.
For example, the allele for brown eyes is dominant.
You only need one copy of it to have brown eyes.
Two copies will still give you brown eyes. 
A recessive allele is only expressed if the individual has two copies of recessive and does not have the dominant allele of that gene.
For example, the allele for blue eyes is recessive.
You need two copies of this allele to have blue eyes. 
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