How did the fusion of Chromosome 2 get passed on to future generations?
http://richarddawkins.net/discussions/642203-how-did-the-fusion-of-chromosome-2-get-passed-on-to-future-generations
Comment #3 covers some territory the a little primer :
The base-pairing rules of DNA due to hydrogen bonds cause the two strands of DNA to be complementary (A against T, C against G). This allows DNA to reproduce by splitting into two separate strands (an enzyme breaks the hydrogen bonds), each of which is a template on which a complementary strand is grown. But this specific AT/CG pairing also allows genes on one chromosome to attract to similar genes on another chromosome. When chromosomes pair up in mitosis and meiosis, this mechanism works despite the fact that the chromosomes differ slightly because the mother's and father's inherited DNA are not identical. The allelomorphic forms of a gene differ at a very small fraction of their places if they differ at all, as does happen in a significant percentage of genes (a condition for that locus called heterozygosity). When 2A & 2B fuse together, it creates a super-chromosome which attracts a 2A and 2B to the right places. This means that chromosome fusion doesn't affect either mitosis or meiosis. It is effectively a neutral mutation, which means it has a chance of becoming the norm in the population. The probability of this happening for any one such mutation and the number of generations it will take for that mutation if it does happen are respectively inversely and directly proportional to the population size. Theoretically, if you know how often chromosomes fuse you can calculate how often in 100 million years a chromosome fusion should become "fixed" in a species
Comment #23 follows on with chances of a fused mutation increasing when talking smaller
numbers of individuals as in a bottle neck situation we know happened several times , very probable situation
The small number of trisomy 21 individuals due to a fusion of chromosome 21 with another chromosome in one of the parents is relevant here. The fused chromosome will never go to 100% in the current population of almost 7 billion, but if there was a global catastrophe that reduced the human population to just a few thousand, as happened 70,000 years ago with the eruption of Toba, and if the fused chromosome or a new one arose in a few of the individuals, then neutral drift allows it to go to dominance by just chance .You don't need two individuals with the chromosome fusion mating. Just one is enough, as happens with any genetic variation occurring in small populations
evol meme |
