Starch synthesis in pea seeds is controlled by one gene with two alleles (B and b). It is cited in the text as an example of incomplete dominance (when considering starch grain size as the phenotype), not multiple alleles . Other options are correctly matched:

• ABO blood grouping is a classic example of co-dominance (where and express together) .
• Grasshoppers exhibit XO type sex determination .
• T.H. Morgan coined the term linkage to describe the physical association of genes on a chromosome .