Patterns of inheritance that follow Mendel's laws are said to follow Mendelian inheritance. These are patterns of inheritance that follow Mendel’s laws of dominance and recessiveness, showing that some genes appear more often than others, and independent assortment, which suggests that genes are inherited randomly. However, scientists now know that many other patterns influence both genotype and phenotype. Patterns of inheritance that that do not follow Mendelian patterns are said to follow non-Mendelian inheritance. These traits have four basic underlying causes:
- alleles for a trait are neither completely dominant nor completely recessive
- more than two alleles exist for a particular gene
- one gene produces multiple phenotypes
- multiple genes code for a single phenotype
Additionally, many physical, behavioral, and physiological traits are influenced by the organism's environment, either during development or throughout the organism's life, or both. For example, a person's maximum height is determined by genes, but nutrition, injury, and other environmental factors can cause the person to be shorter than specified by their genes.
Incomplete Dominance and Codominance
Codominance in Chickens
More Than Two Alleles for One Gene
Human Blood Type
Polygenic Inheritance and Epistasis
Often, many more than two phenotypes exist for some traits. Consider hair color in humans. Even ignoring the various shades of each color, human hair can be black, brown, red, or blond. Polygenic inheritance is when more than one gene codes for a single trait. In humans, height, hair color, eye color, and skin color are all the result of polygenic inheritance. Further, traits such as height and skin color are also heavily influenced by environmental factors, and even eye color and hair color can change over a person's lifetime.Polygenic inheritance can sometimes result from epistasis. In epistasis, one gene "activates" another gene. In other words, the expression of alleles of one gene is dependent on alleles of a completely different gene. When this happens, the "activator" gene is said to be epistatic to the genes it activates. For example, dogs of the Labrador retriever breed have three variants of coat color: black, chocolate (brown), and golden (yellow). The gene that codes for golden fur is epistatic to the gene that codes for black or chocolate fur. The gene for golden fur is denoted E, with the homozygous recessive (ee) giving the gold color. If either homozygous dominant or heterozygous alleles are inherited, the second gene for black (B) or chocolate (b) fur becomes activated. Thus, the Punnett square (which can have more than one set of genes) must show both genes for a single cross.