The MN Blood Group
 Genotype   M molecules   N molecules   Phenotype 
 
 
LM LM + - M
LM LN + + MN
LN LN - + N
 
 
LM LN   X   LM LN
↓
1/4 LM LM
1/2 LM LN
1/4 LN LN
 
The MN blood group exhibits codominance; a heterozygote expresses both allele products.

The phenotype is determined by the production of 2 molecules located on the surface of red blood cells.

Crossing heterozygotes results in a 1:2:1 genotype and phenotype ratio.