`**" "`Polygenic inheritance is the inheritance of a characteristic that is controlled by more than one gene.
`**" "`polygenic inheritance can produce unexpected phenotypes (i.e., phenotypes that don't fot woth Mendel's expected outcomes based on independent assortment of dominant and recessive alleles).
`**" "`Two examples of polygenic inheritance are : (1) skin colour in humans, and (2) Height in humans.
Example 1 : Human skin colour : Polygenic inheritance can cause a trait to have continuous variation, which means the characteristic does not have discrete forms and instead varies gradually between two extremes. Human traits that show continuous variation include height and skin colour. In humans there are at least three genes coding for skin colour. Each skin colour gene two forms, one form codes for high levels of melanin production, and the other form coded for low levels of melanin production.
Melanin is a brown coloured pigment that protects us from the sun's harmful UV rays. the more melanin you have, the darker, and better protected your skin will be.
The skin colour genes are interesting because they exhibit incomplete dominance. This means that a person who is heterozygous for all 3 skin colour genes (Aa, Bb and Cc) has 3 units of pigment, whereas a person homozygous for all 3 skin colour genes (AA, BB and CC) has 6 units of pigment. The table below shows that there are seven shedes of darkness in people, based on genetics. In reality there are far more than seven skin colour shades due to individual differences between people as well as environmetal effects such as sun tans.
Table 1 : The various shades of human skin colour
