General Genetics/Dominant and Recessive Genes

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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[edit]

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[edit]

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

The Arbitrary Nature of Recessiveness and Dominance[edit]