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    Hey, so I have been studying about lethal alleles in my genetics module.

    In the yellow mice cross, where the yellow is dominant but lethal if two alleles are obtained, why do we use Ay to represent the yellow allele (dominant) and just A to represent the darker coloured allele?

    Can't I use A to represent the yellow allele, which is dominant, and and small a to represent the darker coloured allele? Thanks.

    No, because lower case a refers to a non-agouti allele which is separate. Both A and Ay give rise to agouti, and both are dominant alleles. It's almost like three tiers, homozygous recessive a produces a non-agouti mouse, homozygous dominant A produces an agouti/wild type mouse and heterozygous A/Ay gives rise to the yellow coat, and then AyAy is lethal. Hope this makes sense!
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