Mendel’s laws of inheritance also apply to humans. Indeed, the principles of segregation and independent assortment account for the transmission of certain human traits. Human blood type is one of the most familiar Mendelian traits. Blood type has three phenotypes—A, B, and O—based on three alleles of a single gene. If only the A allele or both the A and O alleles are present, the phenotype is A. If only the B allele or both the B and O alleles are present, the phenotype is B. If both A and B are present, the phenotype is AB. If neither A nor B is present, the phenotype is O. Note that O is recessive to both A and B, while A and B are codominant. Codominance means that instead of one allele masking the other, the products of both alleles are observed. Additional examples of Mendelian traits, or those controlled by a single gene, include Huntington’s disease, widow’s peak, cystic fibrosis, sickle cell anemia, Tay-Sachs disease, hemophilia, and red-green color blindness. OMIM, or Online Mendelian Inheritance in Man, hosts an online database of almost 5,000 Mendelian human traits. It is important to note that the majority of human traits are not controlled by a single pair of genes. More often, a single gene can have multiple effects. Even more commonly, multiple genes are needed to produce a single effect. These are referred to as polygenic traits. Most human traits are polygenic, not Mendelian. A good way of determining if a trait is polygenic is to assess whether the trait can be measured. Traits that can be measured, such as height or weight, are polygenic. Also, traits that have a wide range or lots of variability and can be affected by environmental factors are probably polygenic. The survival of a species depends on genetic diversity and variation. If there is a reduction of a gene pool due to geographic isolation or other environmental factors than a species is at risk of extinction.