Quantitative genetics is concerned with characters influenced by many genes, called polygenic characters.
Nurture and nature:
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.
Fundamental model of quantitative genetics:
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, such as beak size in Darwin'sGeospiza finches, 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.)
The image opposite is one of the finches made famous by Darwin: Geospiza magnirostris, whose beak size is the result of genetic and environmental effects.
How can we measure environmental and genotype effects?