In looking over offers on the web of snakes for sale, I keep running across breeders claiming that their snake is 66% het for some recessive trait. These claims come from a simple misunderstanding of the laws of chance and their application to genetics.



I am pretty sure that they are making their claim using the following logic. If they cross the F1 generation from a wild type and a recessive, then they know that both snakes being breed are het for the recessive trait. If one does a pundent square for their offspring, you expect to see a ratio 1:2:1, with 1/4th of the offspring homo for the recessive trait. Since this is an obvious phenotype, the only offspring whose genetic makeup will be in doubt are the wild types. It is here that the error comes in. Since they will have no doubt about the homo recessive snakes, they think that by subtracting that that portion from the mix, what is left are mix where 1/3rd will be wild and 2/3rd’s will be het. The fact is that just because you can tell which snakes turned out homo recessive has no influence on the genetics of any of the unknowns. The original odds were that 50% would be hetro, 25% would be homo recessive, and 25% would be homo wild types. Knowing the outcome for some of the offspring does not tell you anything about the other offspring that will be showing the wild phenotype. Each event is independent.



This is the same basic mistake that most of us make with the classic coin toss problem. If one flips a “fair” coin 99 times, and each times it comes up heads, what are the odds that it will come up tails on the next flip. Most of us would say it was a sure thing to come up tails because what are the odds of it happening 100 times in a row. We would think that it had to be due to come up tails. But the fact is that the odds are still the same, 50/50 because each previous event has no influence on the next toss. Each toss is an independent event.

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