Principles of Genetics and Breeding

11.1.3 Quantitative phenotypes

11.1.3 Quantitative phenotypes ( Polygenic traits)

The branch of genetics dealing with the genetic model for quantitative traits and its applications is called quantitative genetics.

• Quantitative phenotypes are those that are measured, such as length, weight, and fecundity. The important production phenotypes are quantitative ones, although some qualitative phenotypes are quite important, and can greatly increase the value of a crop.
• Because quantitative phenotypes are measured each phenotype is a single category, such as length.
• Individuals do not get segregated into alternate phenotypic categories, such as long vs short instead, they are distributed along a continuum, and differences among individuals are determined by the unit of measure that is used to assess the phenotype; millimeters, grams, etc.
• Because each fish’s phenotype is determined by a measurement, quantitative phenotypes form what are called “continuous distributions”, which are described by the population’s mean and the distribution about the mean (variance and standard division).
• In a population, these phenotypes form what are called “normal” or “bell-shaped” distributions.
• The reason why quantitative phenotypes exhibit continuous distributions and why individuals do not get segregated into descriptive categories is that quantitative phenotypes are far more complicated genetically than qualitative phenotypes.
• Each quantitative phenotype is controlled by dozens to hundreds of genes, each of which makes a small contribution to the production of the phenotype. The exact number of gene is usually never known.
• Additionally, each phenotype is strongly influenced by the environment (e.g., date of birth, access to food, age of mother, etc), and the influence varies both from family to family and from individual to individual.
• The simultaneous action of many genes and the environmental effects creates single phenotypic categories (e.g. length), in which the only way to describe an individual is to measure it.
• Because each quantitative phenotype is controlled by numerous genes, as well as environmental variables, the only way to work with these phenotypes is to analyze the phenotypic variance that exists in the population and to divide the phenotypic variance into its component part.

Phenotypic variance (V_P)

• Phenotypic variance (V_P) is the sum of the genetic variance (V_G), environmental variance (V_E), and genetic-environmental interaction variance (V_G-E) components.

V_{P ­}= V_G + V_E + V_G-E

When conducting a breeding programme, a geneticist tries to exploit V_G. Three distinct types of genetic variance combine to make V_G , and it is important to know what they are, because different breeding programmes are needed to exploit each type.

Genetic variance

• Genetic variance is the sum of additive genetic variance (V_A), dominance genetic variance (V_D), and epistatic genetic variance (V_I):

V_G = V_A + V_D + V_I

These components of genetic variance do not refer to additive, dominance, and epistatic gene action; they refer to specific components of phenotypic variance that are produced by the entire genome, not that produced by one or two genes. The major components are V_A and V_D. Epistatic genetic variance is usually considered to be unimportant because it is difficult to exploit and improvements that occur by exploiting V_I plateau quickly.

Additive genetic variance

• Additive genetic variance is the component that is due to the additive effect of all the fish’s alleles taken independently i.e., the sum of the effects that each allele makes to the production of phenotype.
• Additive genetic variance is the most important component of V_P, and the percentage of V_P, that is controlled by V_A is called “heritability” (h²).

h ² = V_A / V_P

• Additive genetic variance is the genetic component that can be exploited by selective breeding programmes.

Dominance genetic variance

• Dominance genetic variance is the other major component of V_G. Dominance genetic variance is the component that is due to the sum of each interaction that exists between the two alleles at each locus. Because V_D is produced by the interaction of the alleles at a locus, V_D cannot be inherited from either parent.
• Dominance genetic variance is a function of the diploid state (a function of the paired gene), and offspring inherit alleles that exist in the haploid set of chromosomes from the mother pairs with the haploid set of chromosomes from the father. When this occurs, each gene exits in the paired state, and V_D created.
• Consequently, V_D is destroyed by meiosis, and it is recreated by a new and indifferent combinations at fertilization.
• The breeding programme needed to exploit V_D is cross breeding. When h² is ≤ 0.15, crossbreeding is often prescribed to exploit V_G.
• The major reason for determining the heritability of a quantitative phenotype is that it can be used to predict the results of a selective breeding programme by using the following formula,

R=S h²

Where R is the response to selection (gain per generation), S is the selection differential (the superiority of the select broodstock over the population average) and h² is heritability.