Posted by:
aberlour
at Wed May 24 03:06:03 2006 [ Email Message ] [ Show All Posts by aberlour ]
Here goes my attempt to answer this.... hope it helps.
First things first you need to adjust your focus to the chromosome level of genetics. One of the first things you’re going to need to understand is how chromosomes segregate during meiosis. I'm going to simplify this a lot because I don't really want to make the post super long but let me remind you this is not a simple case of inheritance that should be calculated out on a punnett square.
In the Elaphe decedents such as Cornsnakes and Kingsnakes they have a total chromosome count of 36. During the formation of the gamete cells these cells start to divide the chromosomes will separate into maternal and paternal chromosome sides 72 in all and recreate the missing maternal or paternal side of the pairs, then segregate into 2 parts mixing the information from the individual maternal and paternal chromosomes at random into 2 new cells each containing 36 pairs. The next step these 2 secondary cells divide again into 4 haploid gamete cells containing 18 chromosome halves which contain both chromosomes from the maternal and paternal sides. Again there is no real set order to which of the parental lineage’s chromosomes will end up in the final gamete cell. There is roughly a 50% chance on each individual chromosome half that it will make it into one of the 2 new cells this is then further reduced by a second 50% chance on the final split.
Secondly while this is happening there is also the chance for chromosomal crossover. When chromosomal crossover occurs a section of the two matching parental chromosomes break off and rejoin the opposite parents chromosome which creates two new chromosomes that further serves to intermix the genes of both parents. It is due to this chromosomal crossover that makes it impossible to say that you can breed back to 100% the odds just aren’t there. This is because each of the 36 chromosomes has the possibility of crossing over and can do so at any of the base pairs present in the chromosome. Generally, each chromosome will usually have at least one and no more than a few crossovers in each occurrence of meiosis. This exponentially increases the possible genotypes from a single pairing.
This will make a little more sense as I continue as I will show you the odds without the effects of chromosomal cross-over because I don’t know the number of base pairs in each chromosome. The number of possible combinations just out of meiosis is roughly N = number of chromosomes ((N/2)^(N/2)) which gives about 262,144 chromosome combinations for just one haploid gamete cell. If you take that calculation and then express it as ((N/2)^(N/2))^2 which you get the total number of possible combinations possible in the offspring of any two parents 68,719,476,736 if we are talking about hybrids out of that 68.7 mil 2 of the combinations would be pure. Which would give you roughly these odds, 1 in 34,359,738,368 in getting a pure animal back in a F1 x F1 breeding.
The information related to 50/50, 75/25 percent mixes is just a lineage based estimate which is not an accurate indicator of what the genetic mix really is. I would say that it would be better to say that a F2 hybrid is from a F1 50/50 x F1 50/50 cross then just saying it is a F2 50/50. Accurate information is important in any breeding program. The more information you give when selling an animal the better of the breeder receiving the animal will be.
Nature does not have a fail-safe to revert hybrids but it will use any additional genetic information and incorporate it the best it can into the populations genetic diversity.
When you are establishing breeds what you are basically doing is trying to fix certain genes in the population while removing others. If you want to know how long it will really take your going to need to do a lot of reading in population genetics. The process of getting a hybrid to breed true is basically in removing the excess variations out of the breeding program. What you essentially do is look for the dominant and hom recessive traits and by breeding those together repeatedly you artificially set up an isolationist breeding program to increase the frequency of those genes within the breeding population. As this happens, the phenotype of the hybrid genome will begin to breed true just like it would in an evolutionary occurrence which produces genera, species, and so on. Eventually the phenotype will become the hybrids genome and when this happens you will have a hybrid cultivar.
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Adam
Hybrid Breeders Association
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RE: best answer I can give right now - aberlour, Wed May 24 03:06:03 2006