Traits like human height and skin color shows a wide range of phenotype, as opposed to just two of three. From the standpoints of genes and alleles, explain why is this so.

Answer 1

Thus a polygenic trait refers to a trait which is controlled by two or more than two genes.
Polygenic inheritence can explain the large variation in the phenotypes of the organisms.
As the trait is controlled by more than one gene, it leads to a wide variation between extreme phenotypes.
Some examples of polygenic inheritance are: human skin and eye color; height, weight.
Human skin color is a good example of polygenic (multiple gene) inheritance.
Assume that three dominant capital letter genes (A, B and C) control dark pigmentation because more melanin is produced. The recessive alleles of these three genes (a, b & c) control light pigmentation because lower amounts of melanin are produced. A genotype with all "dominant" capital genes (AABBCC) has the maximum amount of melanin and very dark skin.
AABBCC→ Very black (*****)

A genotype with all "recessive" small case genes (aabbcc) has the lowest amount of melanin and very light skin.
aabbcc→ pure white
Each "dominant" capital gene produces one unit of color, so that a wide range of intermediate skin colors are produced, depending on the number of "dominant" capital genes in the genotype. For example, a genotype with three "dominant" capital genes and three small case "recessive" genes (AaBbCc) has a medium amount of melanin and an intermediate skin color. This latter genotype would be characteristic of a mulatto.
AaBbCc→ Mulatto