Review of some key genetic terminology

Gene: The hereditary determinant.Carrier of Genetic information
Allele: Forms of a particular gene.
Examples: R or r; D or d; Y or y (Round or wrinked, Tall or dwarf, Yellow or green)
Genotype: Genetic constitution of an organism or cell.
Can refer to the entire genetic complement or to only the specific alleles in question.
Examples: RR; Rr; rr
Homozygous: A genotype in which pairs of alleles are the same.
Examples: RR or rr, YY or yy
Heterozygous: A genotype in which pairs of alleles are different.
Examples: Rr, Yy
Phenotype: Observable properties of an organism.
Usually with reference to the trait of interest.
Note: phenotype does not necessarily reveal the genotype.
Example:  Round = RR or Rr.  Usually the genotype does reveal the phenotype, although there are exceptions.