User:Melissahwong/sandbox

Additive genetic effects
Additive genetic effects, discovered in 1910 by Nilsson-Ehle, are quantitative changes certain alleles, called additive alleles, cause in a phenotype regardless of any other alleles that are present or absent. Alleles for an additive trait often come from different loci, and contribute to the quantitative trait of an individual such that the phenotype is equal to the sum of the effects of individual alleles, or proportional to the number of alleles. The environment, however, also has an effect on individuals' phenotypes, such that the mean phenotype of all individuals with one genotype is dependent on the number of alleles, but individuals with the same genotype do not all have identical phenotypes.

Types of additive traits
There are three main types of additive traits: continuous traits, such as height that exist on a continuum; meristic traits, that fall into discrete classes (such as counts); and threshold traits, which are either present or absent depending on the number of additive alleles in the genotype.

Calculation of additive effects
'''Additive effects are often calculated by genotyping and phenotyping offspring of a genetic test cross. If there are two alleles at a gene locus then the additive effect is half of the difference between the mean of all cases that are homozygous for one version of the allele (a/a) compared to the mean of all cases that are homozygous for the other allele (A/A)'''. Sometimes, however, the genotype cannot be directly determined, so it may be estimated using the individual’s breeding value: double the difference in mean phenotype between a population the individual is randomly mated to (multiple mates) and the the offspring produced.

Heritability, selection, and additive alleles
Additive genetic effects where there are several loci in the genome produce heritable variation in phenotype, and are thus responsible for parent-offspring resemblance. The variance in additive alleles in a population (denoted VA) divided by the variance in phenotype in the population (denoted VP) produces the proportion of phenotypic variance caused by additive genetic effects, called narrow-sense heritability, h2.

This heritable variation between individuals in a population or species provides a basis for selection. Narrow-sense heritability (h2) is a predictor of a trait's response to selection; increased variance in phenotype (due to strong genetic effects) causes the rate of selection to increase.

Additive Genetic Effects
Additive genetic effects are when alleles from different loci, called additive alleles, contribute to the quantitative trait (see quantitative genetics) of an individual such that the phenotype is equal to the sum of the effects of individual alleles, or proportional to the number of alleles. The additive genetic effect of an allele is an estimate of the quantitative change in a trait that is associated with substituting one allele (one genotype) with that of another allele within an interbreeding population (this sentence is from the original stub).

Types of Additive Traits
Here I will talk about continuous additive traits, such as height that can be measured on a continuous scale, and meristic additive traits, which are discrete "counts" such as number of leaves.

Calculation of Additive Effects
Here I will paste the rest of the stub, talking about how additive effects are calculated from test crosses. It gives an example for how the additive effect of a trait controlled by one locus with two alleles is calculated, so I will see if I can find how more comlex traits are calculated if they are not too complicated for me to understand and for the purposes of this project.

Heritability and Additive Alleles
Here I will talk about heritable variation, and how a trait must be additive to be heritable.