On Saturday, August 26, 2017 at 2:03:45 PM UTC-7, taf wrote: > On Saturday, August 26, 2017 at 11:47:34 AM UTC-7, wjhonson wrote: > > > In actuality the statistics on this point is not correct. > > Or rather it's both correct and not correct. > > > > The genes do not half. You don't inherit a half-gene. > > Actually, they sometimes do, and you sometimes do. There are two types of crossovers that drive the 'halving', and one of them takes no notice whatsoever of where gene boundaries are - it is completely random. The other is not random, having hotspots and deserts, but it isn't entirely clear what defines these - it is not boundaries between individual genes, which for the most part don't exist, with the possibility of the same DNA being part of two different genes, but may be the boundaries of 'regulatory blocks' that include several full genes - that is just a guess, but it may be a valid one. > > > The segments, on average, and viewed globally, can be inherited in half > > pieces, however there is a limit to that. There is not going to be a case > > where you inherit, in a string of ten base pairs, one base pair from > > randomly disconnected ancestors. > > In fact, the smallest divisible segments are probably in the 10s to 100s of thousands of bases, which is what puts a definite limit on the number of generations over which autosomal is likely to be informative without a huge amount of luck (for every ancestor from 1600 from whom you have a detectable preserved block, you have many more ancestors from whom you inherit no DNA whatsoever). These blocks pass intact for an incredibly long time, the block that includes the gene determining the most common form of blue eyes is about 150,000bp long and to have passed largely intact for more than 12,000 years. This presents a problem on two sides - relatively close relative may not share the block at all. If two people do share the block, it shows they are related, but perhaps too distantly to be genealogically relevant. > > taf I actually agree with the point that *you* may have no preserved DNA some particular ancestor, however that misses the point of the group to which you match. For each ancestor you have, you have a group of descendants who have tested. Even if you have no preserved DNA for that ancestor, one of the members of your group will. The further back in time the ancestor, the more people in the group of descendants. So as the chance of you having DNA from that ancestor goes to zero, the number of descendants goes to infinity. Essentially, the further back in time you want to go, the more people you need to have had testing done, and matching to you in such a way that the ancestral pyramid can be build stone-by-stone as it were.