POPULATION GENETICS
Mitochondrial DNA and Y-chromosomes are genetic markers
inherited directly from one's mother and father, respectively. Being non-adaptable
and non-recombining with very slow mutation rates renders them useful in
detecting ancestry and tracing population movements. However, autosomes, which
use multiple ancestry informative markers to paint a more complete picture,
are beginning to gain ground and are thought by many to be superior to single
locus analyses. Note that markers used in the field's infancy, like restriction
enzymes, HLA genes, sickle cell and other blood groups, have now been recognized
as unreliable.
Mitochondrial DNA
mtDNA, inherited maternally,
consists of European haplogroups H, I, J, K, T, U, V, W and X;
Asian haplogroups A, B, C, D, F, G, M and Z; and African haplogroups
L1, L2 and L3. Asian- and African-specific lineages exist at very low
frequencies throughout Europe.
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Y-chromosomes
Y-chromosomes, inherited
paternally, are made up of African Groups I, II and III and Asian
Groups IV-X; European lineages are all variations of these Groups.
In the more common Jobling/Tyler-Smith nomenclature, we have prominent
European haplogroups 1, 2, 3, 9, 21, 22 and 25; Asian haplogroups
4, 10, 12, 16 and 28; and African haplogroups 6, 7 and 8. African-specific
lineages are virtually absent in Europe, while Asian-specific ones are
concentrated in the Northeast.
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Autosomes
" 'Genes on the mitochondrial
genome or the Y chromosome don't unambiguously allow you to infer population
history,' notes Andrew G. Clark, a biology professor at Pennsylvania State
University. 'That's because there's a lot of stochasticity, a lot of chance,
that goes on in sampling of those genomes from generation to generation.
What the autosomal genes get us is many more realizations of genes passing
through history. If we look at enough of them we'll be able to get a good
call on the true population history.' Especially ripe for examination, Clark
adds, are autosomal regions with low rates of recombination, which are just
now being identified. ...more autosomal studies are crucial for advancing
the field of molecular anthropology: The 22 autosomes, after all, harbor
the lion's share of polymorphisms."
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"Earlier we emphasized the importance
of using a large number of loci in the study of human evolution. This is
because (a) the interpopulational genetic variation is very small compared
with intrapopulational variation and (b) the evolution of a single gene (or
mtDNA) is subject to large stochastic errors (Nei and Livshits 1989; Livshits
and Nei 1990). In this study, using gene frequency data for 29 genetic loci,
we could reconstruct an evolutionary history of human populations that seems
likely to be less controversial and more enduring than some current alternatives."