The Rhesus (Rh) Blood Group
We are all familiar with traditional blood groups (A+, B-, etc.). In terms of notation, the letter of the blood group gives a person's phenotype on the ABO blood group gene, while the plus (+) or minus(-) sign gives the person's phenotype on the Rhesus gene. We will ignore ABO for now to explain the Rh system.
The fine details of the Rh system tell a very complicated story, but for our purposes here we can assume that there are two alleles, the + (plus) allele and the - (minus) allele. Like the ABO system, the Rh locus codes for an antigen that resides on the surface of the red blood cell. It was discovered in 1940 that blood of individuals with at least one + reacted to an antiserum developed from Rhesus monkeys, hence the name of the locus. Individuals homozygous for the - allele did not react. The + allele was considered dominant because the heterozygote +- reacted to the antiserum.