>I think in your example A and B would be different genes not just different locations on some chromosome for the same gene in different species.

Correct, A and B are different genes. One codes for one thing (perhaps tyrosinase), and the other codes for something else. If both function properly, the snake looks normal. If the A gene pair is not functioning, the snake does not have black pigment. If the B gene pair is not functioning, then there is no black pigment. If neither the A pair nor the B pair is functioning, there is no black pigment.

Tyrosinase does not make melanin all by itself. Tyrosinase catalyses two of the chemical reactions in the synthesis of melanin. There are a number of other chemical reactions before melanin is formed. Then the melanin is laid down in a protein matrix to form the pigment granule. Maybe A codes for tyrosinase production and B codes for one of the steps in the production of the protein matrix. I don't know. But I do know that the making of normal coloration requires a lot more than one gene pair.

Paul Hollander

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