In the 150 years since Mendel's first experiments, geneticists have discovered that genetic traits usually don't follow simple inheritance patterns. Some traits, like human eye color and stature, are produced by the interplay of many sets of genes; these are known as "polygenic traits." Other traits show a phenomenon called intermediate expression, in which a heterozygous genotype will produce a phenotype that's different from homozygous dominant genotypes. In snapdragons, for example, a homozygous dominant genotype makes red flowers, homozygous recessive makes white flowers, and heterozygous individuals are pink.
In addition, some genes have more than two different versions. This type of inheritance is called multiple allelic inheritance. Our blood types are transmitted in this way. In the ABO blood group there are three alleles; two of the alleles represent varieties of antigens -- substances that trigger an immune response. One allele, IA, represents the A antigen. Another allele, IB, codes for the B antigen. The third allele, i, indicates the absence of either antigen. The body makes antibodies against any antigen that it doesn't have. Combinations of these three antigens produce the four different blood types -- A, B, AB, and O.
Now that you know a little about blood types, let's learn about the genes that code for them.