Homozygous dominant (AA) – dominant allele expressed

Homozygous recessive (aa) – recessive allele expressed

Heterozygous (Aa) – dominant allele expressed

Codominant alleles: I^A & I^B are A & B proteins, I⁰ = no proteins 

I⁰I⁰ = blood group O

I^AI^A or I^AI⁰ = blood group A

I^BI^B or I^BI⁰ = blood group B

I^AI^B = blood group AB (codominance)

Parents AABB and aabb > F1 will all be AaBb > F2 will give 9:3:3:1 ratio of phenotypes