Europeans descended from just 50 African migrants
less than 50 people founded the entire population of Europe, according to a new and accurate way of reading history from the human genome. Scientists previously believed that the 500 million people living in Europe today are ancestors of about 10,000 people who left Africa around 100,000 years ago.
Scientists at the Whitehead Institute at Massachusetts Institute of Technology ( mit ), usa, believe all Europeans probably descended from fewer than about 50 people who interbred together over about 30 generations. This select group may have left Africa about 60,000 years ago. They have found genetic evidence of inbreeding.
With the availability of a dense genome-wide map of single nucleotide polymorphisms ( snp s), a central issue in human genetics has become whether it is now possible to use linkage disequilibrium ( ld ) to map genes that cause disease. ld refers to correlations among neighbouring alleles, reflecting 'haplotypes' descended from single,, acquired chromosomes. The size of ld blocks has been the subject of considerable debate. Computer simulations and empirical data have suggested that ld s exist only a few kilobases (kb) around common snp s, whereas other data have suggested that it can extend much further, in some cases greater than 100 kb. Researchers at mit have now studied ld s extensively to, for the first time, to narrow down on disease causing genes.
The data for the research comes from maps of snp s, which are the single letter differences in dna that can exist between people. The discovery is good news for medical researchers. The unexpectedly low levels of genetic variation will make it far easier to isolate the genes that may lie behind some common diseases ( Nature , Vol 411, p119).
In every generation dna sequences are shuffled by recombination, when genomes from sperm and egg are chopped into chunks and strung together. As one generation is followed by another, the repeated shuffling makes it increasingly unlikely that sets of snp s are shared by individuals.
Researchers used this knowledge to trace the genealogy of Europeans. This technique is far more accurate than the previous method of measuring the mutation rate in mitochondrial dna , because recombination must occur in every generation whereas mutation rates can vary. From archaeological evidence, scientists expected to see snp s correlated over about 3,000 base pairs.
But David Reich, researcher at the Whitehead Institute, found the genome was preserved in much larger chunks, each about 160,000 base pairs long. This pointed to a far more dramatic population bottleneck.
Reich's discovery, therefore, means it will be easier to find the genes that underpin common diseases. This is because comparing snp maps from populations with and without the diseases will reveal the sections that differ and these should contain the suspect gene.
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