Affected by many different factors, alleles may interact in many different ways, there might be more than just two alleles for a gene, or there might be many different genes that all affect one trait
Incomplete dominance
Neither allele is completely dominant or completely recessive, when plants that are homozygous for red flowers are crossed with plants that are homozygous for white flowers, the offspring have pink flowers, neither of the phenotypes of the parents can be seen separately in the offspring
Codominance
Both alleles of a gene are expressed completely, and neither is dominant or recessive, the heterozygous phenotype would have some red areas and some white area
Codominance
Human blood types, I^A and I^B produce a protein called an antigen on the surface of red blood cells, I^A and I^B are codominant, allele i is recessive and does not produce an antigen
Codominant alleles are neither dominant nor recessive, the ABO blood types result from codominant alleles
Many genes have more than two alleles
Polygenic traits
Traits produced by two or more genes, eye color and skin color are both determined by the interaction of multiple genes, at least three genes affect eye color
Epistatic gene
In mice, at least five different genes interact to produce the fur color phenotype, one of the genes can prevent the expression of all of the other genes, albinism is the result of epistasis
The environment the conditions surrounding an organism
Affect phenotype
Genes and environment interact to determine human traits
Identical twins can have height and size differences depending on environmental conditions such as nutrition and health care
