How to find how many alleles control a trait

Quantitative genetics is the study of the inheritance of traits that show a continuous distribution of phenotypes in a segregating population. Traits that are controlled by many genes also exhibit quantitative inheritance as each gene segregates in a Mendelian fashion. Even when there are only a few genes involved, the trait variation will show a continuous distribution due to the results of measurement error and environmental effects. Quantitative or biometrical genetics is considered to have been founded in the early 20th century, notably by R.A. Fisherâ™s article which showed that the inheritance of continuously varying traits is consistent with Mendelian principles. The basic principle of quantitative genetics Simply put, the phenotypic value (P) of an individual is the combined result of its genotype (G) and the effects of the environment (E): P = G E Genotype refers to the total genetic variation. This includes not only the effects of nuclear genes, but also the effects of mitochondrial genes and the interactions between genes. Genotypic variation can be partitioned into additive and dominance variation: Additive variation represents the cumulative effect of individual loci, therefore the overall mean is equal to the summed contribution of these loci. Dominance variation represents interaction between alleles. If a trait is controlled by a dominant allele, then both homozygous and heterozygous individuals will display the same phenotypic value. Quantitatively varying traits are also affected by the environment (E). This can be further subdivided into pure environmental effect and interaction between genes and the environment. In other words, how different genotypes respond in different environments. Finally, interaction (I) between different genes can modify the observed phenotypes. This is called epistasis, or non-allelic interaction, distinguishing it from dominance. So, the equation above can be better written as: P = A D E I However, since geneticists are interested in studying and identifying variation and its source in a given population, this equation is better described below. The total phenotypic variation (V) of a population is the sum of the variation in additive (A), dominance(D), gene-interaction (I), environmental (E)and gene-environment interaction (GE) effects: VP = VA VD VI VE VGE #QuantitativeGenetics #inheritance #phenotypes #Genes #Genotype #AdditiveVariation