Ditto.
Let X be the gene for butter. Let Y be the gene for lesser. (For example.) Let - denote random A, C, T, and G's in the DNA sequence. Then it could look something like this.
----------Y-------------- (and so on)
----------X-------------- (and so on)
So when half that DNA strand gets passed on (sperm/eggs carry one strand of DNA), either the X or the Y will get passed on...meaning ALL butters or lessers, but no BEL's.
JonV