Polygenic (“many genes”) traits controlled by more 2 or more genes. This is the case for most traits! produce a wide range of phenotypes. Ex: Skin color Evidence indicates that skin color in humans is controlled by 3 genes – each of which have a dominant allele and a recessive allele. We represent the 3 genes as A, B and C, take a look at the example:

this example shows 2 individuals who are heterozygous for all 3 genes and have an intermediate skin color are crossed. A Punnett square for this cross would produce 64 possibilities for the offspring. What statements can be made about this graph?

The graph shows the probable outcomes of offspring. These individuals are most likely to have offspring with an intermediate skin color but can have offspring with very light skin or very dark skin. Most polygenic traits show a pattern like this – most individuals show a phenotype that is somewhere in the middle.

Figure: 11-12a Title: Continuous Variation and the Bell Curve Caption: (a) Mendel’s pea seeds may have been green or yellow, but traits such as human height do not have this either/or quality. Human heights exist in a range, with no fixed increments between heights of individuals. Such “continuous variation” results from polygenic inheritance, in which each of several genes contributes a small additive effect to a character. University of Connecticut students in the picture have been arranged by height to show how continuous variation works in this one human trait. Note that the group as a whole takes on the shape of a bell. The students’ heights are distributed in a pattern that creates a bell curve.

Gene interaction map