In genetics , a distinction is made between dominant ( Latin dominus , 'master' ; 'dominating, covering') and recessive alleles and inheritance patterns . In individuals who are heterozygous with regard to one characteristic , only the characteristic of the dominant allele is pronounced ( dominant inheritance ), the characteristic of the recessive allele, on the other hand, is not found in the phenotype .
- If a person has the allele for blood group B from one parent and the allele for blood group 0 from the other parent, then he has blood group B because the allele for B is dominant over the allele for 0.
- A classic example and formative for the term were Mendel's attempts to inherit traits in the pea . One of his examples was the flower color: the pea plant has red and white flowers . If you cross peas with red flowers with those with white flowers, all offspring (in the first generation of offspring) have red flowers. Today we know that the white-flowered peas lack a certain enzyme for the formation of the dye anthocyanin because it is no longer synthesized due to a mutation in a regulatory gene (a transcription factor ). Due to the loss of the red flower pigment, the flowers turn white. Plants that have at least one functional allele can develop the dye normally; their flowers are colored red. This is independent of whether the second allele, inherited from the other parent, enables the production of the dye or is also mutated. If the allele that leads to dye production is symbolized with A and its mutated variant, which produces only white flowers due to the lack of dye, with a, the combined alleles AA and Aa both lead to red flowers ( indistinguishable in phenotype ).
scope of application
If both alleles have an effect on the phenotype, mixed-inherited (heterozygous) individuals are different in their characteristic expression from pure-genetic (homozygous) individuals, and the individuals in their characteristic expression are often between those of the pure-blooded representatives. Different cases can be distinguished: With codomination , both features are expressed side by side (example: in white and red-flowering plants, the offspring have red-white spotted flowers). If the dominance is incomplete, a phenotype is created that appears as a mixture in terms of the characteristics (example: in white and red-flowering plants, the offspring have pink flowers).
Dominant and recessive are not absolute properties of the alleles or inheritance traits involved, but only apply to the combination of factors examined in each case. A certain mutant can be dominant over a second but recessive over another, independent one. In very many cases a certain trait is determined by more than two alleles. In the ABO blood group system, for example, allele A (just like B) is dominant over 0 (the combination of alleles A0 results in blood group A, the combination B0 blood group B), but alleles A and B are codominant (blood group AB then results).
- Elisabeth Günther: Outline of Genetics . 2nd Edition. Gustav Fischer, Stuttgart 1971, DNB 456838384 .
- on the molecular identity of the classical Mendelian genes cf. James B. Reid & John J. Ross (2011): Mendel's Genes: Toward a Full Molecular Characterization. Genetics 189 (1): 3-10. doi: 10.1534 / genetics.111.132118