General Genetics/Dominant and Recessive Genes
When Mendel studied peas, one of the phenotypes showed complete dominance over the other one. If we look at pea height, and denote the gene for short as s and the gene for tall as S, then as every plant has two sets chromosomes each has two genes at this locus.
So there are three possibilities: SS, Ss, ss (order doesn't matter)
In this case S was fully dominant over s, so Ss individuals were phenotypically identical to SS individuals. Only ss pea plants were short. The S gene would be said to be Dominant While the s gene is said to be Recessive.
The Molecular Basis of Dominance
As has already been mentioned, all diploid organisms have two homologous chromosomes. At a specific locus on each homologous chromosome, there are homologous alleles for a particular trait. For example, the gene that codes for a dominant tall pea plant could be labeled A2 and for a short recessive pea plant could be labeled A1.
Alternative Patterns of Inheritance
Not all loci show this simple dominance. If we represent phenotype on a plot then Complete Dominance would be like this:
AA/Aa aa Complete Dominance
Other types are:
AA Aa aa No Dominance
AA Aa aa Incomplete Dominance
Aa AA aa Over Dominance