Okay, the crush has relaxed; I think I'll have more time to spend on this today... Bill Thurston wrote:
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.
That's a lower bound and not so far from the true answer, for humans. Human chromosome 22 (the shortest; they're numbered in order of length) averages one crossover per meiosis, while chromosome 1 averages four. I was hoping to include some information about the probability distribution function for these numbers, but the guy I'd ask -- Simon Meyers, one floor down from me, and author of last year's Science paper with the best map of recombination across the genome to date -- just left for a week in Hawaii.
[...] 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.
This is actually a pretty bad assumption because of the extent to which recombination events cluster: about 80% of recombinations take place in about 10-20% of the sequence -- even more for some people, since recombination probabilities depend on your genotype. This gets in to hard biology, though, so we should probably drop it. (The paper in question is Science 14 Oct 2005 pp. 321-324, "A Fine-Scale Map of Recombination Rates and Hotspots Across the Human Genome", http://www.sciencemag.org/cgi/content/abstract/310/5746/321 if you're at an institution with access to Science on-line. But it's not terribly relevant to most of the present discussion; biologists usually focus on recombination rates on scales much smaller than full chromosomes.)
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.
My answer ("yes, you can tell easily") was based on Jim's hypothetical in which we assumed that we really knew the full sequence of each of the two haplotypes in each individual -- that is, for each person you had two sequences, the "from mom" and "from dad" ones (though unlabelled). This is what's really present in the cell, but *not* what you generally get when you genotype someone, as we discussed yesterday. The answer to Jim's question depends on what you're assuming about the data you're able to gether. --Michael Kleber -- It is very dark and after 2000. If you continue you are likely to be eaten by a bleen.