General Genetics/Dominant and Recessive Genes
When [[General Genetics/Mendelian Inheritance|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