When we study genetics, we are usually focused on genes and intend to believe that genes control all the traits. In a way this is true but it is not completely true. We all know hen a good breeding crop is grown in poor soil wouldn’t be as good as in a fertile soil. This means environment plays an important role in the expression of the genes, i.e., the phenotype. A single genotype can produce many different phenotypes, depending on the environment. A single phenotype may be produced by various different genotypes, also depending on the environment. This brought up about a great debate on nature vs. nurture theories on human behavior.
Phenotype Variance and Heritability
Phenotype variance is composed of genetic variance (VG) and environment variance (VE), sometimes also covariance of genetically-caused variance and environmentally-caused variance (COVGxE), and genetic-environment interaction (VGxE). The genetic variance VG is made up of three components: VA, additive genetic variance, genetic variance that results from the actions of genes that have additive effect; VD: dominance variance, interaction of genes located on same locus and VI, epistatic variance, interaction of genes on different loci. Broad sense heritability H2 equals to VG/VP; while narrow sense heritability equals to VA/VP.
Given a genotype, the probability of showing a phenotype is called penetrance. Given a genotype, the phenotype are expressed, but may be to a less extend, this is called expressivity. Expressivity is not co-dominance, rather, it is incomplete expression of a phenotype; while co-dominance means both alleles are expressed.
Gene-gene interaction is often seen in modifying gene expression. Some gene’s activity may be suppressed by other genes, which is termed gene suppression. In some cases, expression of one gene can modify the phenotype of another gene, a phenomena called epistasis. In other cases, the position of a gene within the nucleus can also affect the gene expression (position effect). Environment is known to regulate gene expression in numerous conditions, such as temperature sensitive mutants of yeast.