AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

VALUES AND MEANS

• Genetic properties of a population are expressed in terms of gene frequencies and genotype frequencies.
• To understand the connection between gene frequencies and the quantitative differences exhibited in a quantitative/metric character, the concept of value is introduced.
• A given quantitative trait is characterized by a mean value and a standard deviation expressed in metric units by which the character is measured.

Phenotypic value (P)

• The phenotypic value of a given quantitative trait is the yield of the individual with respect to the trait.
• The phenotypic value symbol is P.
• The phenotypic value can be measured and is evaluated in relation to the population mean value.
• The phenotypic value (P) of an individual is determined by the combined effect of the genotypic value (G) and the environmental deviation (E)

P = G + E

Genotypic value (G)

• Genotype is the sum total of genes possessed by an individual in pairs and environment is all the non-genetic circumstances that influence the phenotypic value.
• For any individual the genotypic value (G) is determined by the combined effect of all genes in all the loci which influence the trait.
• The genotypic value symbol is G.

Environmental deviation (E)

• The environmental deviation represents the combined effect of all non-genetic factors that have influenced the phenotypic values.
• The environmental deviation symbol is E.
• The genotype will confer certain value on the individual whereas the environment causes a deviation from this in one direction or the other.
• For a single locus, the mean environmental deviation in the whole population is taken to be zero. So the mean phenotypic value is equal to the mean genotypic value.