Answer:
The correct answer is: C) The alleles have an additive effect on the phenotype, intensifying the genes.
Explanation:
Polygenic traits (such as skin, hair and eye color) are those phenotypes that are <em>influenced by more than one gene</em>, meaning that the whole genetic information needed for these traits is located in multiple loci.
Aside from consisting of many alleles participating in the expression of some characteristics, polygenic inheritance differs greatly from the classical mendelian inheritance by also having <em>continuous variation</em> (which means that there aren't only two possible outcomes, but a whole spectrum) and a much more complex inheritance pattern.
Even when the genetic information for these traits plays an important role on the expression of these genes, the environment can be of great impact on these phenotypes (such as how the diet of an individual can affect greatly on a person's weight).
Polygenic traits are also of <em>variable expressivity</em> (people with the same genotype can express the characteristic with different intensities) and of <em>incomplete penetrance</em> (even when having the same genotype, some people may not actually develop the phenotype associated with it).
The alleles, however, don't always have an additive effect as they sometimes do the opposite job and suppress the alleles of other genes.
