I've produce hundreds of spiders and spider combos over the years. Out of those there have only been 3 that come to mind as being "Failure to thrive" issues. I can't however attribute that to the spider gene. We all make decisions about the genes that we choose to keep or breed. Is there some due diligence that come with breeding. Intentionally breeding for animals or combos that we know (with empirical information) are tragic failures, in my mind is irresponsible.