Codomination
Codominance is a term from genetics that is used to describe the phenotypically visible expression of hereditary characteristics in organisms. A prerequisite for codominance is that an organism is at least diploid, i.e. has at least a double set of chromosomes - this is the case in most animals and plants. In addition, the carrier must have different variants of a certain gene, at least two different alleles , i.e. be heterozygous . Codominance cannot therefore occur if the alleles present are the same, in homozygosity .
One speaks of codominance when the different alleles of a gene - i.e. both the maternal and the paternal variant - have the same effect in the heterozygous state and appear in the phenotype. The gene products of both alleles are expressed and their associated characteristics are expressed independently of one another. This phenotype with expressions of both characteristics side by side with codominance must therefore be differentiated from the phenotype with expressions of an intermediate mixed form, such as occurs in the case of intermediate inheritance.
An example of codominance is the inheritance of blood group characteristics A and B in the AB0 system . If a person has inherited the genetic make-up for blood group A from one parent and those for blood group B from the other parent, his red blood cells have both antigens A and B. His blood group is AB. In contrast, if the alleles are heterozygous, inheritance is dominant / recessive for A and 0 as well as for B and 0. Features A and B prevail over 0, the absence of both antigens. Because the presence of the respective dominant allele on just one chromosome of the chromosome pair is sufficient to form the corresponding antigens.
Another example of codomination is the lesser known MN blood group system .
Individual evidence
- ^ Neil A. Campbell , Jane B. Reece : Biology. Spektrum-Verlag 2003, ISBN 3-8274-1352-4 , pages 303-305.