A phenotype is a set of one or more physical characteristics possessed by an organism (eg., blue eyes, brown hair, or type O blood). A genotype is the set of biological factors that control a particular organism's phenotype. In the nineteenth century, Austrian botanist Gregor Mendel hypothesized that most phenotypes are controlled by two factors, one inherited from each parent. His research suggested there are two types of factors: dominant, which generate a particular phenotype if present, and recessive, which generate a particular phenotype only if the dominant factor for the same trait is absent. His theories were confirmed by the discovery of DNA in the twentieth century, but the new field of genetics also uncovered a wealth of complexity at which Mendel's famous garden had barely hinted.
The common blood types known as A, B, and O illustrate Mendel's theory of inheritance, but with an added twist: A and B are co-dominant traits, meaning both phenotypes will appear in the organism if both factors (now called alleles) are present. The presence of an A-allele generates A-antigens on the surface of red blood cells, while the presence of a B-allele generates B-antigens; the red blood cells of a person with both A- and B-alleles will contain both antigens. Only type O is recessive, producing neither antigen.
This Demonstration illustrates the possible phenotypes and genotypes of human offspring based upon the genotypes of their parents. The colors red, blue, and yellow denote A, B, and O antigens and alleles, respectively. Orange and green represent the presence of O-alleles in the genotypes of children with blood types A and B, while purple denotes the presence of A- and B-alleles and antigens in type AB blood.