Hi.
Well, it isn't that easy...
The similarity of pheromones seems to be one important reason for vanishing stocks. Real degenerations need several to multiple generations to break through. However, every stock is inbred to some extent. The problems arise only when you're breeding with just one of two pairs in every generation. Then you work almost with "clones" (that should mean a low allele diversity). In order to avoid problems, one should breed with at least 6 to 12 pairs in every generation.
Regards,
Christian