A gene pair is homozygous when both genes are the same. A homozygous spider ball python would have a gene pair containing two spider mutant genes. The spider ball pythons that we see are not homozygous -- they have a spider mutant gene paired with a normal gene.



If the spider mutant gene is lethal when homozygous, here is the way things would work. When a spider gene is paired with a normal gene, the spider mutant gene does something that is enough to change the snake's appearance while the normal gene does the normal thing enough to keep the snake reasonably healthy. Two copies of the spider gene could make such a big change from normal working that the effect kills the snake.



Different lethal mutant genes affect different organs. Different lethal mutant genes also kill the owner at different times. Death might occur any time between fertilization of the egg and sexual maturity. The final effect doesn't even have to be death; sterility would be adequate.



Nobody knows exactly what effects the spider mutant gene has beyond the effect on the pattern.





Disclaimer: The rest of this post is brainstorming, with no claim of accuracy.



I have read that some spiders show some head tremors. Perhaps the spider mutant gene also has an effect in the brain or nervous system. Two copies of the spider gene might affect the nervous system enough to make the snake's heart stop beating. Death would be the result.



Paul Hollander

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