D3.2.9—ABO blood groups

one gene determines (ABO) blood groups / one gene for ABO blood groups;
genes have different/alternative forms called alleles;
there are three alleles ( $\mathrm{I}^{\mathrm{A}}, \mathrm{I}^{\mathrm{B}}$ and i ) of the gene for (ABO) blood groups;
(ABO) blood groups are an example (of the effect of) multiple alleles (in this instance three alleles can result in four phenotypes);
each individual has two alleles of the gene but only one is passed to offspring;
alleles that are codominant both affect the phenotype in a heterozygote;
(alleles) $\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}}$ are codominant;
(alleles) $\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}}$ are dominant over i / i is recessive to $\mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{B}}$;
(genotypes) $\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{A}}$ and $\mathrm{I}^{\mathrm{A}} \mathrm{i}$ both give blood group A ;
(genotypes) $\mathrm{I}^{\mathrm{B}} \mathrm{I}^{\mathrm{B}}$ and $\mathrm{I}^{\mathrm{B}} \mathrm{i}$ both give blood group B ;
(genotype) $\mathrm{I}^{\mathrm{A} \mathrm{I}^{\mathrm{B}}}$ gives blood group AB ;
(genotype) ii/homozygous i gives blood group O;
example of a cross involving ABO blood groups;