Genetics issues in ancient India

[Vidyasankar Sundaresan]

>But NB: What I said was that mtDNA studies "may not be as
>relevant" as studies of y-chromosome polymorphisms to the Aryan
>question — not that they "may not be relevant" at all. This
>difference isn't trivial.

Sorry for having been a little unclear. I have read your page
listing the genetics papers, and am aware that you don't make
the mistake I referred to. My post was meant as a preemptive
and cautionary note to those who *would* make such a mistake.
It is certainly tempting enough to make such an assumption,
and to not recognize it as a fallacy at all.

>position. The data published by Strauss in _Science_ 286 (24 Dec.
>1999) suggests that paternal and maternal mtDNA sometimes do
>undergo sexual recombination. How and when this occurs is not
>currently known. What *is* certain is that if this is a normal
>situation, the value of mtDNA studies in investigating

I must apologize for referring only to Strauss's news report
in Science (24 Dec 1999). This journal usually gets some of
its own staff to write a short report on technical papers of
wide interest, to make it accessible to a bigger audience.

The proper scientific reference for mtDNA recombination may
be found in the same issue of Science, in the following paper.

_Linkage Disequilibrium and Recombination in Hominid
Mitochondrial DNA_, Philip Awadalla, Adam Eyre-Walker
and John Maynard Smith, Science, 286 (5449): 2524-2525.

As in all fields of knowledge, basic changes in time-honored
assumptions do not go unchallenged. Many objections have been
raised to the above study, from four different research groups,
and the authors have responded, in the 16 June 2000 issue of
Science, 288 (5473): 1931.

The evidence on recombination in mtDNA is still coming in. In
the 16 June response, the authors conclude, "... recombination
does occur in mtDNA. The next challenge will be to estimate
the rate at which recombination occurs, and test whether
hominids are unique in allowing it to happen." Till the rate
and extent of this recombination are investigated and fully
understood, geneticists will continue to give importance to
mtDNA studies, and to think that maternal inheritance is
perhaps still predominant.

Re: Y chromosomes, there are recombination events that are
even more significant than in mtDNA. There is a small part
of the Y chromosome that is conserved and known to be non-
recombinant, but it is not yet clear whether Y chromosome
studies will be universally accepted as more important than
mtDNA studies. Recombination may be a "sometimes" event for
DNA in mitochondria, but it is well known to be a "more than
often" event for DNA in the sex chromosomes. Simply put (I
pointed this out a year ago on this list), the Y chromosome
in any given man's cells is not an identical copy of that in
his father's cells. It will always have a variable amount of
DNA derived from the X chromosome provided by his mother.
(A similar situation obtains with X chromosomes in any given
woman. They contain DNA derived from the father.) That is
part of the reason why no two siblings of the same gender
look exactly like each other, except for identical twins.

This considerably complicates Y chromosome data. Either one
has to restrict attention to the non-recombinant portion of
the Y chromosome, and ignore the genetic information encoded
in the female half of the population, or one has to account
for uncertainties resulting from sexual recombination events
in each generation, in addition to other reasons for genetic
variability. Incidentally, that is the reason why population
genetics has hitherto tended to concentrate on mtDNA evidence.

So, mtDNA studies will continue to remain more than a little
relevant for migration studies. In the near future, they will
indeed remain important, but a possible paternal inheritance
pattern will have to be factored in. Statistically speaking,
this may modify the current picture only slightly. Or it may
end up having a huge effect. We don't know for sure yet. The
whole thing is still very, very new. I envisage a substantial
period of conflict regarding the evidence before it becomes
quite clear how significant this factor will be. Meanwhile,
the recognition that paternal inheritance in mtDNA may not
be absolutely zero means that Y chromosome studies will start
to receive more attention, but these are not going to replace
mtDNA studies en masse, any time soon. Moreover, as of now,
all these issues are based on statistical analyses. Only if
there is experimentally observed evidence of mitochondrial
recombination during fertilization will this become accepted
universally.

On another note of caution, those who eagerly look forward
to genetics solving the problem of Aryan origins (on both
sides of the debate), should be prepared to be disappointed
somewhat, with respect to time scales. It is not going to
be easy. The science usually talks in terms of many kilo
years (kya). As far as I can see from the current state of
research, the conclusions of this field of science are more
definitive about trends from 6000-10000 years ago (and more)
as compared to events from 3000 years ago. Genetic evidence
tends to be more reliable for periods over which evolutionary
changes take place, and also for extremely short time periods,
spanning 3-4 generations, but they can be sometimes quite
inconclusive for the ~5000 years of recorded human history.

Vidyasankar
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