Although many characteristics are determined by alleles at a single place on the chromosome, some characteristics are determined by an interaction of genes on several chromosomes or at several places on one chromosome. This condition is polygenic inheritance.
An example of polygenic inheritance is human skin color. Genes for skin color are located in many places, and skin color is determined by which genes are present at these multiple locations. A person with many genes for dark skin will have very dark skin color, and a person with multiple genes for light skin will have very light skin color. Many people have some genes for light skin and some for dark skin, which explains why so many variations of skin color exist. Modifying only one of those genes changes the skin color only slightly.