A recessive allele is a version of a gene that does not express itself when paired with a dominant allele. It only shows its effect if two copies are present in the organism's genetic makeup.
This is the version of a gene that will always express itself when present, even if there's only one copy.
Homozygous: This term describes an organism that has two identical alleles for a particular trait.
This refers to an organism that has two different alleles for a particular trait.