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.
* * *
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.
* * *
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."
* * *
"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."