There is no linkage between the I locus and the C locus. Both can produce a white chicken. They work in different ways on the cell level. The dominant white locus (I) does not allow for the production of eumelanosomes ( little structures in a cell that hold black pigment) but do produce phaeomelanosomes ( little structures in a cell that hold red pigment). The follicle cells of a dominant white chicken do not have the structures that hold the black pigment therefore the black pigment can not be produced in the follicle and placed into a feather. The black pigment has to be made inside the melanosomes.
The recessive color allele c+ is effective at inhibiting all color ( almost all of the time) while the I allele is only affective at inhibiting black pigment (most of the time). The recessive white allele is a mutation of the color allele (C). The recessive white allele contains a virus insertion which disrupts the molecular structure of the gene. Ultimately this incorrect molecular structure does not allow for the correct molecular structure ( shape) of the tyrosinase enzyme. If the tyrosinase enzyme is not the correct shape it can not help make pigments. The tyrosinase enzyme is needed in the first two biochemical reactions that produce pigments. If the first two reactions do not occur, then no pigment is produced.
WOW! Thanks. In many cases I know WHAT a gene does, but HOW it works is usually something to which I haven't been exposed. Now I need to copy and re-read a few dozen times and figure out any questions to learn more...