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In genetics, underdominance, also known as homozygote advantage, heterozygote disadvantage, or negative overdominance," [1] is the opposite of overdominance.It is the selection against the heterozygote, causing disruptive selection [2] and divergent genotypes.
Autosomal dominant and autosomal recessive inheritance, the two most common Mendelian inheritance patterns. An autosome is any chromosome other than a sex chromosome.. In genetics, dominance is the phenomenon of one variant of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome.
In a dominant-recessive inheritance, an average of 25% are homozygous with the dominant trait, 50% are heterozygous showing the dominant trait in the phenotype (genetic carriers), 25% are homozygous with the recessive trait and therefore express the recessive trait in the phenotype. The genotypic ratio is 1: 2 : 1, and the phenotypic ratio is 3: 1.
These classifications are still widely used in Drosophila genetics to describe mutations. For a more general description of mutations, see mutation, and for a discussion of allele interactions, see dominance relationship. Key: In the following sections, alleles are referred to as +=wildtype, m=mutant, Df=gene deletion, Dp=gene duplication.
The quiz randomly chooses terms from the Talking Glossary and asks the user to select a term name to match the definition shown. Hints are available for each question, and at the end of the quiz all users are able to print a "Certificate of Completion" that includes the date the test was taken, number of correct answers, and the user's name.
Non-additive effects involve dominance or epistasis, and cause outcomes that are not a sum of the contribution of the genes involved. Additive genetic effects are singularly important with regard to quantitative traits , as the sum of these effects informs the placement of a trait on the spectrum of possible outcomes.
Figure 1: Inheritance pattern of dominant (red) and recessive (white) phenotypes when each parent (1) is homozygous for either the dominant or recessive trait. All members of the F 1 generation are heterozygous and share the same dominant phenotype (2), while the F 2 generation exhibits a 6:2 ratio of dominant to recessive phenotypes (3).
Dominance and overdominance have different consequences for the gene expression profile of the individuals. If overdominance is the main cause for the fitness advantages of heterosis, then there should be an over-expression of certain genes in the heterozygous offspring compared to the homozygous parents.