Final answer:
Polygenic inheritance describes when a single trait is affected by multiple genes, with each dominant allele contributing to the trait's expression. This introduces a range of phenotypes, rather than the strict dominant-recessive patterns postulated by Mendel's laws of inheritance.
Step-by-step explanation:
When a single trait is affected by more than one gene locus, this is called polygenic inheritance. This concept is an exception to Mendel's original model of inheritance where he postulated traits are controlled by single pairs of alleles exhibiting dominance and recessiveness. However, polygenic traits are controlled by multiple genes, each contributing to the phenotype.
In contrast to monogenic traits, which follow the simple patterns of dominance and recessiveness described by Mendel, polygenic inheritance involves the cumulative effect of multiple genes on a single trait. Examples of human traits that exhibit polygenic inheritance include height, skin color, and eye color. Each dominant allele adds to the expression of the phenotype, resulting in a gradient of phenotypic expression.
Extensions of Mendelian genetics such as codominance, incomplete dominance, and epistasis further illustrate the complexity of inheritance. Codominance occurs when both alleles in a heterozygote are fully expressed, such as in human blood types AB, and incomplete dominance results in an intermediate phenotype, as seen in the blending of flower colors. Epistasis involves the interaction of different gene loci where one gene can mask the expression of another.