Breed animals with each gene together to get the two-gene combo. Take the two-gene combo and breed it out. If you only get both forms of each single gene but never the two-gene combo or an animal with neither gene, then you have an allele.
Like if you breed a BluEL Lesser/Mojave, you only get Lessers and Mojaves, never more BluELs or wild-types
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The one here is probably the most up-to-date as we aim to keep it so