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