Assessing the Genetic Component of a Phenotype
A phenotype is the appearance of an individual, which results from the interaction of the person's genetic makeup and their environment. Phenotypes can be categorical or numerical. If we are interested in the genetic component of a traitΒ there are different methods we can use for analysis.
We define a trait for analysis, determine what we are interested in studying, and then the methods that need to be used. For example, if we want to know how BMI is linked to gene effects we would need to consider other factors such as age, sex, smoking, etc. Thus, a Multiple Linear Regression might be appropriate, were we can measure the variablity of each factor.
Variance of Phenotypic Traits
π2T = The observed (phenotypic) variability of a trait
π2T = π2G + π2E = The phenotypic variability can be partitioned in to variability due to genetic and environmental effects
π2G = π2A + π2D = The genetic component can be further partiioned into additive and dominance genetic variance
We can write a model for the trait as:Β Β
T = (A + D) + E = G + E
Additive and Dominance Components
Consider a frequency distribution of trait values for two alleles B and b, where B creates a high trait value and b creates a low trait value on a continuous scale:
d is the mean of the Bb group
In an additive model d, would equal 0 (no dominance; dominance variance = 0)
In a recessive model: d = -a