I can't help you with the mental block. If you're like me sometimes you just have to push the "I believe" button! We know from the example that the two types produce normal animals so the genes are not allelic or on the same loci of a chromosome. What may be tripping you up is that the resulting phenotype appears to our eye as being the same.

Perhaps your T T- example would help. Here the offspring would be:

T t AND T- t-


In each case neonates would have single good copy of a gene for making tyrosinase and another good copy of the gene for proper synthesis of melanin further along the metabolic pathway. At both steps the necessary genetic instructions exist for the synthesis of melanin despite the fact that both parents were albino.

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