Re: rapid variation

Walter ReMine (wjremine@mmm.com)
Sat, 09 Sep 1995 18:32:19 -0500

Jim Blake wrote:
>Walter wrote:
>
>>Small correction. Since humans are diploid and contain two copies of each
>>gene, the mutation rate per progeny for that gene is TWO in 10000 (not one
>>in 10000, as Glenn calculated).
>
>Shouldn't it be two in 20,000? i.e. 1/10,000 for each copy?

No. For ease, let me call our nominal 1000 nucleotide gene, "gene X". The
mutation rate for gene X is (Glenn's figure) 1/10000 for each copy. Since
there are two copies of gene X in each progeny, the *per progeny* mutation
rate is 2/10000 mutations of gene X.

Let's calculate it again another way. There are two copies of gene X in
each progeny, each one is 1000 nucleotides long, for a total of 2000
nucleotides. At Glenn's stated mutation rate (1e-7) there would be 2000 x
1e-7 = 2/10000 mutations of gene X per progeny. The same result as we
calculated above.

******

>>In addition, Glenn's model of the how genetic variation accummulates in a
>>population is wrong. It does not correctly model
>>the way human populations accumulate genetic variation. A) Humans are
>>diploid. B) Human populations have been in the millions for most (if not
>>all) recorded history. Those two factors were left out of Glenn's model.
>
>I don't think either of these factors affect the statistics of the mutated
>gene *frequency*.

Let me show how diploidy, population size, and gene frequency (all factors
that Glenn has consistently ignored) are all relevant to the problem. Take
a human population size of one million adults, and an allele that has a
frequency of 0.25. (That is an entirely realistic situation.) Because the
population is diploid, there are 2 million copies of the gene, of which half
a million (i.e. one fourth) are of the given allele. That allele (which may
already contain mutations separating it from other alleles) will itself
experience mutations at a rate of 1/10000 (that is Glenn's figure). Thus,
there will be 50 mutations of that allele in merely ONE GENERATION!

Glenn's persistent mistake, despite repeatedly being called on it, is that
he assumes there is ONE AND ONLY ONE copy of the allele in the population.
He implicitly assumes the organism is haploid (when it isn't), and has a
population size of ONE throughout history (when it didn't). My example
above shows how these factors cannot be ignored.

Walter ReMine
P.O. Box 28006
Saint Paul, MN 55128