Human beings have three alleles for the ABO system: i (which is recessive) and IA and IB which are both co-dominant.   This means that both the IA and IB alleles are genes which contain instructions for making proteins for blood cell membranes.  These genes  always work in red blood cells and always cause the phenotype protein to be there, no matter what other protein is there.   The "i" allele doesn't have any protein-making instructions; so when a person has the "ii" genotype that person has 0 (zero) proteins of the A or B or AB blood type phenotypes. BLOOD TYPES (phenotypes) alleles (genotypes)     A IA IA     or       i  IA     B IB IB       or       i IB AB IA IB   O i i Sample problem:  What if somebody who is type A, but her mother was type O, finds a sperm donor who is type B, but his mother was type O.  Show the somebody and the sperm donor genotypes on the borders of the Punnett square, and then show the possible progeny within the yellow box:  Note:  since their mothers were both type O with genotypes ii, we know that both of these breeding mates are heterozygous.     IA    i IB   IA    IB      (AB) i IB      (type B) i   i IA       (type A)   ii       (Type 0)   The Rh factor is easier:  although there are multiple versions of the alleles, all of them are dominant to one recessive allele: Rh BLOOD Factors (phenotypes) alleles (genotypes)     positive (+)  R1 R2     or      R1r    or      R2r     positive (+) R3 R4       or     R3r         or      positive (+)   Rn   ____      negative (-)   rr Explanation:  The R alleles are all different versions of a protein found on blood cell membranes. They all give a positive reaction when blood is tested for Rh factors.  The r allele represents a mutation in which the protein is not there at all in any version. Examples of Problems: If you have AB- (AB negative blood), you really have AB AND Rh negative,  two different pairs of genes.  What is your genotype for this situation? What is your genotype and phenotype if you have O-positive blood? Can a person with O-positive blood type possibly be the father of a baby with A-negative type? THE MN system.  A third pair of genes for blood membrane proteins comes in two co-dominant versions:  M and N.  If you have two "M" alleles, you're type "M."  With two "N" alleles.... The MN system has forensic applications, but transfusion problems are rare; so the Red Cross doesn't test for it. also see http://gslc.genetics.utah.edu/basic/units/blood/types.html