Codominance
Encyclopedia
Codominance refers to a relationship between two alleles of a gene. It occurs when the contributions of both alleles at a single locus are visible and do not overpower each other in the phenotype.
For instance, in the ABO system, the IA and IB alleles are co-dominant in producing the AB blood group phenotype, in which both A- and B-type antigen
s are made. Another example occurs at the locus for the Beta-globin
component of hemoglobin
, where the three molecular phenotypes of HbA/HbA, HbA/HbS, and HbS/HbS are all equally detectable by protein electrophoresis
. (The medical condition produced by the heterozygous genotype is called an incomplete dominant, see above). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed
into RNA
.
Co-dominance and incomplete or semi-dominance are not the same thing. For example, in some plant species, pink flowers may be the product of a mixture of red and white pigments (co-dominance on the pigment level, no dominance on the color level), or the result of one allele that produces the usual amount of red pigment and another non-functional allele that produces no pigment, so as to produce a dilute, intermediate pink color (no dominance at either level).
For instance, in the ABO system, the IA and IB alleles are co-dominant in producing the AB blood group phenotype, in which both A- and B-type antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
s are made. Another example occurs at the locus for the Beta-globin
HBB
Beta globin Beta globin Beta globin (HBB, β-globinprotin that, along with alpha globin (HBA), makes up the most common form of hemoglobin in adult humans. The normal adult hemoglobin tetramer consists of two alpha chains and two beta chains.-Gene locus:...
component of hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
, where the three molecular phenotypes of HbA/HbA, HbA/HbS, and HbS/HbS are all equally detectable by protein electrophoresis
Protein electrophoresis
Protein electrophoresis is a method for analysing the proteins in a fluid or an extract. The electrophoresis may be performed with a small volume of sample in a number of alternative ways with or without a supporting medium: SDS polyacrylamide gel electrophoresis Protein electrophoresis is a method...
. (The medical condition produced by the heterozygous genotype is called an incomplete dominant, see above). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
into RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
.
Co-dominance and incomplete or semi-dominance are not the same thing. For example, in some plant species, pink flowers may be the product of a mixture of red and white pigments (co-dominance on the pigment level, no dominance on the color level), or the result of one allele that produces the usual amount of red pigment and another non-functional allele that produces no pigment, so as to produce a dilute, intermediate pink color (no dominance at either level).