I've been told by people who are seriously involved the rule of thumb that the number of crossover events ~ the number of chromosome pairs, so that each chromosome of a child on average is divided into two segments from different parents. Two siblings share on average the same amount of genetic material as a parent and child, but that relationship is easily distinguished because the sharing is unevenly distributed over the genome --- about 1/4 of the length has nothing in common, 1/2 shares one copy, and 1/4 shares both copies. Furthermore, assuming the rule of thumb above for the "small integer", the siblings autosomal matches are broken into about 3*22 segments, while the parent-child is broken into about 2*22. Not all crossovers would be directly observable from just the sibling genomes, though, so I'm not sure how much this would (theoretically) help. When Jim posted his original question I thought I saw asymmetry between the mother and daughter, as Michael wrote, but I no longer see this. Unfortunately I've deleted those messages, but: the entire length of their genomes share one identical copy. We could label them so that aaaaaaaa matches cccccccccc ... I don't see what more information there is. Bill Thurston On Jun 29, 2006, at 6:03 PM, Michael Kleber wrote:
Wow! I had no idea that there was so much interest in genetics-fun simmering just below the surface here.
I only have a little time now, so I'll blather about one thing here, and try to get to the rest of the discussion soon.
Jim Propp wrote:
This fact about the asymmetry of the parent-child relationship suggests a more general question: How much can you deduce about the precise way in which two blood-relatives are related simply from looking at their genes?
I don't know the general answer to the question "what pairs of familial relations map to isomorphic genetic comparisons," but this begins to get into the trickier things that we happily ignored in Jim's first question; I'm not even sure I could figure out all that apply. The key, obviously, is the mixing between the two copies of the parental genome, and while biologists have a good basic model of it, even this is imperfectly understood -- recent research is just starting to reveal how frequent some things are that were previously believed to be rare (eg loss of heterozygosity).
But even with the baby model, things are tricky. Your genome is broken up into 22 pairs of autosomes (chromosomes other than X and Y), one of each from each parent. The mixing happens by these pairs exchanging corresponding sections, and the number of such crossover events is constrained, by cell biology, to be a small but nonzero integer. The result is that the amount of your genome that you get from each grandparent is 25% on average, but that there is much more variation than you might think. (And even more if you're, say, a possum, with only 8 pairs of chromosomes to our 22.)
Before I finish not answering, let me make two key points in an attempt to keep us aware of what's realistic and accurate:
1) We're talking about what information we could obtain, theoretically, if we knew both haplotypes of an individual's entire genome. That's completely unrealistic. First, almost all ways of obtaining genetic information mix the two haplotypes together, so you really get a mixture of the signal from the two parents' copies. Second, when you do eg a paternity test, they don't look at the, say, ten million places where your genome differs from someone else's. Instead they look at some 20 locations where people's genomes tend to be highly variable -- say, a spot where there's a repeated section that goes "acgacgacgacg..." somewhere between 5 and 11 times.
2) I'd like to point out that we're talking about looking at someone's genome, *not* at their "genes". You have about 20,000 genes -- that is, pieces of DNA which are used for building proteins. These together make up perhaps 2% of your genome. A total of about 5% of your genome is DNA which appears to be useful in *some* way, though we don't know entirely what. The other 95% -- the part that isn't acted on by selective pressures -- is the part that is the most free to vary among individuals; mutations easily creep in over time. If you're trying to work out the ancestry of all living things, that's the information gold mine. (But if you're trying to learn about *important* events in evolution, then you *do* want to look at the genes.)
--Michael Kleber
-- It is very dark and after 2000. If you continue you are likely to be eaten by a bleen.
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