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Genetics question
Steve
It is possible to make the explanation simpler by using a little math. It is really a simple problem in probability theory. In your example, each individual gets ten chromosomes from each parent, and so in its progeny, there is a probability of 0.5 that a given chromosome came from a given parent. Therefore, with your numbers, you can compute exactly the probability that a given individual has virtually no genes from a given grandparent: 0.5 raised to the tenth power, or 0.0009765625. Given that a given mating of orchids may produce hundreds of thousands on seeds, it is probable that there are some progeny that have no genes from a given grandparent. You could go even further and compute the probability that there are no progeny that have no genes from one of the grandparents, or the probability that there are progeny lacking any contribution from one maternal and one paternal grandparent (0.00000095367431640625: I'll leave the details of getting this answer to the reader ;-). Of course, if funds are not limiting, one can determine the relative contributions from the more distant ancestors using DNA fingerprinting, but that is a whole other story. K...., I know I didn't tell you one thing about genetics that you didn't already know. Somewhere you took Helmut Rohrl to mean something more than he did, I think. If he was quoted accurately, then he overstated his point. Last of all, I know genetics gets more complicated with genes having ways to migrate to different chromosomes so that one chromosomes will actually contain genes from more than one parent. There, also are the plants that end up as 4n or 6n; multiple copies of each chromosome and more potential to carry genes from more ancestors. That doesn't matter much. The above is still valid. I just hope I didn't confuse everyone. I'm no teacher! Actually, it does matter plenty. Work the numbers and you'll see that increasing polyploidy significantly reduces the probability that a given individual lacks a genetic contribution from one of its grandparents, or more distant ancestors The effect of crossing over (a process where a pair of chromosomes exchange genes) is much less significant, unless it is much more comon that I have understood it to be. K. I *think* I'm having trouble because I'm used to animal genetics where a limited number of ovae are fertilized and offspring without genes from a grandparent might not survive past the blastula stage (*G*). However in This is a possibility regardless of the genetic contributions of the ancestors and the number of offspring. orchids, hundreds of thousands of eggs can potentially be fertilized. Therefore the mathematical realm of possibilities (bell curve or gene segregation ) can be demonstrated. Well, I am not sure I'd put it this way. The probability of the event, in a given indivudal, is not affected by the number of progeny, only the probability of observing it in a given population at a given time: these are two different probabilities. I am not sure that the bell curve applies. I would have thought a poisson or a binomial distribution more likely; but the I haven't had enough interest in the question to actually work out the correct probability distribution for independant assortment. I still think, however, that in practice the lack of genetic material would lead to the decreased fertility seen in complex hybrids (ie its not unususal for some plants to be poor parental stock or for some crosses to yeild a minute number of seed or flasks). And that therefore there would indeed be genetic material from all parental stock in the exisiting offspring , no matter how far removed..... I doubt that a lack of genetic material is to blame here. Rather, I suspect if cultivars with different numbers of chromosomes are involved in the cross, the probability of the cross having an odd number of chromosomes increases, and then the cross becomes less fertile being the process of segregating the chromosomes being two gametes no longer works well. This is why sexual reproduction tends to decrease in importance and asexual reproduction tends to increase in importance as the degree of polyploidy increases: but highly polyploid plants tend to be incredibly robust and difficult to kill. But none of this affects the probability that in surviving progeny of a cross that is known to have low fertility will have a contribution from a given ancestor: only the probability that it will be observed in a given population at a given time. Am I wrong? Or am I right in why I'm wrong? I expect to be wrong because Rohrl is much more brilliant than I am. And when I was in school genetics was taught simplistically to us undergrads and DNA theory was in its infancy (the one gene one trait idea is now out the window, for example) Now if you were my student, I would yell at you for such a remark. DON'T SELL YOURSELF SHORT! And don't over estimate the capability of alleged "experts"! I can tell you, as a theoretical ecologist, every population genetics model I have seen leaves just about everything to be desired, and are really useful ONLY for the purpose of explaining some basic processes to undergraduate students. Real life is much more interesting and complex than they suggest, but I have not met a geneticist who is really up to the challenge of developing a population model that is of practical utility. There are few ecologists or environmental scientists who are up to that either, but I am working hard on changing that. :-) I met one recently who, even though working in an engineering aspect of environmental science, was so mathematically challenged that she couldn't see the obvious way to compute the spatial distribution of contaminants from a linearly distributed contamination source (think fumes from cars an roads)! Neither could her graduate students. And yet SHE is regarded as an expert! I have two earned doctorates, and so I know what it takes to become an "expert". Trust me when I tell you that barring the most severe developmental handicaps, anyone can do it with just a little effort. Sorry, about this rant, but I find self deprication most irritating (except, of course, when it is me reviling myself: but then, it is OK since I REALLY am a moron :-). Cheers, Ted |
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