The inheritance of polygenic traits depends upon the interaction of two or more genes. This is sometimes confused with the idea of multiple alleles, which are just different forms of the same gene. However, the two concepts are related when discussing the probable phenotype resulting from a particular genotype. Such diseases as diabetes mellitus or heart disease are not the consequence of single gene inheritance, likewise physical traits such as height, weight or even behavior are all examples of quantitative traits whose expression depends upon several different factors. These include the number of genes involved, the number of alleles each gene has, and how much the phenotypic variability depends upon environmental interactions. Unlike qualitative traits such as blood type or other multiple allelic genes that show an unambiguous phenotype, quantitative polygenic traits show a range expression. The distribution of height, for instance...