Type A people have either AA or AO genotypes. Type B people have either BB or BO genotypes. Novel phenotypes often result from the interactions of two genes, as in the case of the comb in chickens. The single comb is produced only by the rrpp genotype. Bateson reported a different phenotypic ratio in sweet pea than could be explained by simple Mendelian inheritance.
Siamese cats are darker on their extremities, due to temperature effects on phenotypic expression.
Expression of phenotype is a result of interaction between genes and environment. This is caused by an allele that controls pigment production being able only to function at the lower temperatures of those extremities. Environment determines the phenotypic pattern of expression. Many traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes. Polygenic traits are not expressed as absolute or discrete characters, as was the case with Mendel's pea plant traits.