Colour loci in corn

Multiple Alleles (Based on Classical Concept of Allelomorphism)
Multiple alleles and isoalleles 
Colour loci in corn
Skin colour in rodents
Eye colour in Drosophila
Self sterility in Nicotiana
ABO blood groups in humans

Several series of multiple alleles are known in corn. Two such series are briefly described.

C locus on chromosome 9. C locus on chromosomes 9 controls colour and has atleast three alleles including CI, an inhibitor of colour, O responsible for full colour in presence of A and R, and c expressing colourless phenotype. Sometimes they are also designated as CI, C and c. These alleles are also described to be closely linked genes, rather than alleles.

R locus on chromosome 10. This is a complex locus, controlling anthocyanin colour of seed and the plant. Following four alleles were earlier designated as Rr, Rg, rr, and rg. But later, due to their two independent functions, they were designated as SP, Sp, sP and sp respectively.

Rr = SP - coloured seed and coloured (red) plant
Rg = Sp = coloured seed and green plant
rr = sP = colourless seed and coloured (red) plant
rS = sp = colourless seed and green plant

The new designations were based on the assumption that S and P are two closely linked genes rather than forming a series of multiple alleles. But since alleles later called pseudoalleles, can undergo crossing over between them, the concept of multiple alleles may apply in this case.