Genetic effects are broadly divided into two categories: additive and non-additive. Additive genetic effects occur where expression of more than one gene contributes to
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
(or where
alleles
An allele is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.
Alleles can differ at a single position through single nucleotide polymorphisms (SNP), but they can also have insertions and deletions ...
of a
heterozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mos ...
gene both contribute), and the phenotypic expression of these gene(s) can be said to be the sum of these contributions.
Non-additive effects involve
dominance or
epistasis
Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes. In other words, the effect of the mutation is depe ...
, 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. Quantitative traits are commonly polygenic (resulting from the effects of more than one
locus).
Heritability
Broad sense heritability
Variation in phenotypes across a population arises from the interaction between environmental variation and genetic variation between individuals. This can be stated mathematically as:
V
P = V
E + V
G, where the terms refer to variation in phenotype, environment, and genotype respectively.
Broad sense heritability (H
2, or H
B) refers to the phenotypic differences arising from all genetic effects, and can be described as the ratio of genotypic variation to that of phenotypic variation in the population, or: H
2 = V
G / V
P.
The genotypic variation from the above equation can be subdivided into V
A (additive effects), V
D (dominance effects), and V
I (epistatic effects).
Narrow Sense Heritability
Narrow sense Heritability (h
2 or H
N) focuses specifically on the ratio of additive variance (V
A) to total phenotypic variance (V
P), or: h
2 = V
A / V
P.
In the study of Heritability, Additive genetic effects are of particular interest in the fields of
Conservation, and
Artificial selection
Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant ...
. The effects of dominance and epistasis are not reliably transmitted to progeny (see
Mendelian inheritance
Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...
, laws of segregation and independent assortment). This means that h
2 represents the phenotypic variation that is reliably passed from one generation to the next and which can be used to predict changes in mean fitness between generations.
Fisher's Fundamental Theorem
Fisher's Fundamental Theorem asserts a direct correlation between the amount of genetic variation in a population and the possible variation in mean fitness for that population. This Theorem suggests that where a trait affects individual fitness, the amount of variation due to additive genetic effects will decline with each successive generation, and all other things being equal, will approach zero.
References
Genetics
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