For the trait that is coded by a recessive allele to be expressed, the must be homology, which means that an offspring has to inherit that recessive allele from both its parents.
A dominant allele will dominate over the recessive allele, so for the trait coded by a dominant allele to be expressed, the offspring only needs to inherit the allele from one of its parents.
For example: "R" is brown eyes, "r" is blue eyes.
Now you get one R from each of your parents.
RR (homozygote) = brown eyes
Rr (heterozygote) = brown eyes
rr (homozygote) = blue eyes
