The phenotypic value (P) of a trait is determined by the genotypic value (G) and environmental effects (E). The genotypic value is the combined effect of genes influencing the trait, and represents the phenotypic mean value for individuals with the same genotype. For Jersey cattle, the transferrin locus identifies 3 genotypes that influence milk yield, and the average yield for each genotype provides the genotypic values. The population mean value is calculated based on the genotypic values and Hardy-Weinberg genotype frequencies. Changes in gene frequencies can change the population mean value over time.

2. • The phenotypic value of a given trait or quantitative
trait is for instance the yield of the individual with
respect to the trait. The phenotypic value often has the
symbol capital P.
• The phenotypic value can be measured and is
evaluated in relation to the population mean value.
• The two factors, which determine the phenotypic value
are the genotypic value and the environmental effect,
which have the symbols capital G and capital E.

3. • The genotypic value is the combined effect of all
genes in all the loci which influence the trait.
• The genotypic value is given the symbol capital G.
• The genotypic value is in practise interpreted as
the phenotypic mean value of individuals with
the same genotype.
• The phenotypic mean value of a population gets
the symbol p-bar. The phenotypic mean value is
also called the population average or population
mean value.

4. • An example of a QTL is the transferrin locus in Jersey cattle. The genotype can be
identified by the gel electrophoreses technique. The transferrin locus is a locus,
which have identifiable effect on milk yield in Jersey cattle. There are 3 identifiable
genotypes, capital T capital T, capital T small t and small t small t. For each cow the
milk yield, which has the phenotypic value capital p, and the transferrin genotype
can be calculated for each of the three genotypes.
• In a population the average milk yield of all cows with the genotype capital T
capital T is 1882 Kg, with the genotype capital T small t 1882 Kg, and with the
genotype small t small t 2082 kg. This is the genotypic values of the given
genotypes. The phenotypic mean value is the same as the genotypic value of the
given genotype. In the population the frequency of capital t and small t, is
respectively p 0.67 and q 0.33. The Hardy-Weinberg frequencies of the 3
genotypes are 0.45, 0.44 and 0.11. Then the population mean value is 0.45 x1882
+ 0.44 x1882 + 0.11 x 2082 which gives 1904 kilogrammes of milk.
• The population mean value is determined by the gene frequencies and the
genotypic values. A change in the population mean value can only be done by
changing the gene frequencies. For example will the mean value of the milk yield
increase, if the frequency of the favourable allele small t is increased.