Complex Inheritance of Traits
Numerous traits are inherited following Mendel’s basic rules of dominant and recessive. However, many others follow much more complex patterns, including traits of plants, animals and humans.
Incomplete Dominance
When two traits are crossed and the offspring show what appears to be a mix of the two parents, it is incomplete dominance. In a Punnett square for incomplete dominance, only capital letters are used. One version of the trait is the letter. The other trait is indicated with the same letter and a ‘ mark (called prime).
Codominance
When two traits are seen at the same time but are not blended together, it is called codominant inheritance. When doing Punnett squares, only capital letters are used. One trait is one letter; the other trait is a different capital letter. When two different capital letters come together, both traits are seen, usually in an alternating pattern.
Multiple phenotypes from multiple alleles
Remember each trait is controlled by only two alleles. You get one from the male and one from the female. However, for some traits, there may be three or more (even 100!) possible alleles (in the population), each resulting in a different phenotype. Usually, one allele is dominant over all others, others are dominant over certain ones and recessive to the others and one is recessive to everything.
Polygenic inheritance
Some traits (such as height, hair color, and skin color) are controlled by several genes at once. Most of these traits follow the simple dominant or recessive pattern, but the effects from each gene are added together.
Polygenic inheritance is recognized when the traits seen show a wide range of variation.
Example: Dark hair color is dominant over light hair. People with very dark hair have inherited all dominant genes for hair color. People with light hair color have mostly recessive genes for hair color.
AABB is very dark. AaBb would be medium. aabb would be light.