Multiple allelomorphs, that is, a series of different grades of a single factor.
"Applied Eugenics" by Paul Popenoe and Roswell Hill Johnson
The factor for this eye color is another allelomorph of white.
"A Critique of the Theory of Evolution" by Thomas Hunt Morgan
In such cases each member acts as the allelomorph of any other member, and only two can occur in any one female, and only one in any male.
"Sex-linked Inheritance in Drosophila" by Thomas Hunt Morgan