University of Arizona
University of Arizona: Blood Types Tutorial
This site from the University of Arizona describes how blood type is passed on from parent to child. The tutorial covers the genetics of the ABO blood groups as well as Rh factor. There are even practice problems and blood type calculators!
University of Utah
University of Utah: Learn Genetics: Genes and Blood Type
For a concise summary of human blood types, this is a great resource. Discussion includes the topics of genotype, phenotype and agglutinins. There is also a nice pictorial table that summarizes the three blood types.