sentences:  Yet, the C4 protein derives from a simple two-locus allelic model, the C4A-C4B genes, that allows for an abundant variation in the levels of their respective proteins within a population.Originally defined in the context of the Chido/Rodgers blood group system, the C4A-C4B genetic model is under investigation for its possible role in schizophrenia risk and development.One of the earlier genetic studies on the C4 protein identified two different groups, found within a human serum, called the Chido/Rogers (Ch/Rg) blood groups.