Quantitative genetics

The value of a real character will usually be influenced by the individual's environment as well as its genotype - by nurture as well as nature. If the character under study is something to do with size, for example, it will probably be influenced by how much food the individual found during its development, and how many diseases it has suffered.

Measuring environmental effects and genotype effects

Both these environmental effects and genotype effects can be measured. The phenotype can then be expressed as the sum of environmental (E ) and genotypic (G ) influences (where P is the phenotype):

P = G + E

This, simple as it is, is the fundamental model of quantitative genetics. For any phenotypic character, the individual's value for that character is due to the effect of its genes and environment.

(In quantitative genetics, the value of a character in an individual is always expressed as a deviation from the population mean. For example, if the average beak size in a population were 0.875 cm, then a beak phenotype measuring 1 cm would be written as +0.125 cm.)