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RE: What will I get?

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Posted by: chrish at Tue Oct 9 00:21:03 2012  [ Report Abuse ] [ Email Message ] [ Show All Posts by chrish ]  
   

Nope, Scott is correct here.
Stripe is sort of a co-dom gene. It is formed by breeding a rufescens kenyan sand boa to a kenyan sand boa. Therefore, when you breed a stripe to a normal you will typically retain a bit of the rufescens influence which results in the tiger morph.


Then the striped pattern is a result of incomplete dominance of the dark-backed pattern with the normal pattern. But if there is a "tiger" pattern that is intermediate between the stripe (het) and normal (homozygote) the characterstic is not inherited by simple mendelian allelic patterns. It would have to be polygenic/polyallelic or there would have to be something else happening (epistasis?).

i.e. If RR is dark-backed, and NN is normal, and RN is striped, how do you make a tiger? It can't work this way.

Genes can't be sort of codominant. Codominance is a very specific situation where two different alleles express a product and the heterozygote shows both characteristics of the homozygotes. Incomplete dominance is where the het is intermediate between the two homozygous characters. Neither of those explains this situation.

We as snake breeders are too quick to label something codominant just because it isn't a simple dominant/recessive allele pair. The stripe seems unlikely to be inherited through co/incomplete dominance. In fact, when you look at the larger picture (that the same pattern inheritance seems to occur in miliaris) it gets more potentially complex. The development of a dark stripe on the back which obscures the normal pattern occurs in more than one species of sandboa (depending on your views of Eryx whitakeri, maybe three species). In fact I remember hearing or reading about dark-backed E. jayakari as well somewhere years ago, so maybe four?

There is the possibility that the tigers are just a variant expression of the striped genotype (i.e. some stripes are more complete than others), but in that case striped x striped crosses would produce tigers, and stripes, and normals, and dark-backed snakes. Do they? If not, it isn't incomplete dominance.

Anyone have a decent pedigree we could look at?


The coin flip analogy doesn't really fit here as the chances that the anery gene is passed on to the offspring is not 50/50 because both parents have a copy of it.

I think you missed my point. My point had nothing to do with the actual probability, just the nomenclature.

My analogy was that just because a coin has a 50% probability of turning out heads when flipped doesn't mean that coin should be called a 50% heads coin. But that's what we are doing with these snakes.

The normal looking (non-anery) from this cross offspring have a 66% percent chance of being heterozygous. But no individual can be 66% percent heterozygous, therefore it makes no sense to call an individual a 66% het.

I understand that the name is implying that there is a 2/3 chance that the snake is heterozygous, but that isn't what it actually says and it is also not how some people interpret it. I know because I have had to explain it to them.

My problem isn't the statistics or the genetics, it is the terminology.

Chris

-----
Chris Harrison
San Antonio, Texas


   

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