Text 1972, 105 rader
Skriven 2006-06-06 13:10:28 av hap newsom (1:275/311)
     Kommentar till en text av DAVE SACERDOTE
Ärende: Re: squirrels
=====================
Hey Dave!

-> DSh> And there is an area in North East Ohio (e.g. Kent State University
-> DSh> campus) where the only squirrels that you see are black.
-> 
-> JW> Those black squirrels are the same species as the grays. They are
-> JW> quite rare except in a very few places.
-> 
-> Note to JW:  Black squirrels are a lot more common than you may
-> think. 
-> 
-> DSh>   We were told that the melanistic trait was actually 
-> DSh>   due to a recessive gene, but if that were true then 
-> DSh>   I wonder how they could have become the dominant 
-> DSh>   variety on campus.
-> 
-> Longmeadow MA also has a population of black squirrels and they
-> are beginning to replace the common greys.  This is actually
-> quite possible given the way recessive characteristics work:
-> 
-> Let's say that the grey coloration is carried by gene X (dominant), 
-> and the black coloration is carried by gene x (recessive.)  Each
-> squirrel must carry 2 coloration genes, one passed to it by
-> "dad" and one by "mom".  Two genes make up one chromosome, which
-> would color the squirrel as follows:
-> 
-> XX = grey
-> Xx = grey, carrying black
-> xx = black
-> xX = grey, carrying black
-> 
-> Now, follow the gene distribution in a table showing every possible
-> combination of the above genes:
-> 
-> XX + XX = XX  XX  XX  XX (four "true" grey)
-> XX + Xx = XX  Xx  XX  Xx (two "true" grey, 2 grey carrying black)
-> Xx + Xx = XX  Xx  xX  xx (one "true" grey, 2 grey carrying black, 1 black)
-> Xx + xX = Xx  XX  xx  xX (one "true" grey, 2 grey carrying black, 1 black)
-> xX + xX = xx  xX  Xx  XX (one "true" grey, 2 grey carrying black, 1 black)
-> xX + Xx = xX  xx  XX  Xx (one "true" grey, 2 grey carrying black, 1 black)
-> xx + xx = xx  xx  xx  xx (four black)
-> 
-> So, out of 28 offspring you would have:
-> 
-> 10 "true" grey
-> 10 grey carrying the black gene
->  8 "true" black
-> 

Here's where I find an error, you missed the cross of XX+xx which will give 
you all 4 offspring being grey with the black gene, and the cross of Xx+xx 
which will give you 4 offspring 2 grey with black gene and 2 black.
Your totals then would be:
10 "true" grey
16 grey carrying the black gene
10 "true" black
with the ratio of grey to black being 2.6:1
All things being equal that ratio would remain essentially the same regardless
of the population size.

-> If the black coloration is a trait unto itself, the black squirrel
-> population will always be stable or growing slightly since 
-> carriers + blacks always outnumber "true" greys.  But if the 
-> black pigmentation gene is associated with another trait favorable
-> to survival (intelligence, perhaps, or greater aggression) it 
-> would be well within probability to see the black squirrel
-> population grow as more squirrels begin to carry 2 recessive genes.
-> The greys would likely never die out, but would indeed become a
-> minority.
-> 
Here's where you threw the monkey wrench into the machinery! If the black 
gene is associated with another trait that is genetic in nature the 2.6:1 ratio

will be different, taking into account the new variable. If the survival of
black 
squirrels is dependant upon some outside factor (ie; black squirrels are preyed

upon less than greys due to coloration) then the ratio becomes less 
importand because  natural selection will eventually weed out the undesirable 
trait even if it's a dominant one. But if it's left to "simple" genetics the
greys 
would always be dominant at a 2.6:1 ratio.

-> [Warning: anecdotal evidence below - this is not tested data]
-> In Longmeadow, in the area near Baypath College and St. Mary's
-> RC church, the population of black squirrels which was small 
-> when Lynn was a kid at St. Mary's School is now the dominant
-> squirrel population.  In the past 15 years or so, I have only
-> seen 1 black squirrel carcass on the side of the road, but many
-> dead greys. That could be an indicator that the blackies are
-> more savvy of automobile traffic, or that they are more
-> territorial and don't stray from their turf so much.
-> 
-> Human intervention could be a factor as well.  Black squirrels
-> are unusual to most people, and many people find the sable
-> coats attractive.  People seem more willing to feed blackies,
-> who then adapt, lose some of their fear of humans, endear 
-> themselves further to their patrons, get fed more, and so
-> on (again, giving them a survival advantage.)
-> 
Those are good observations, anecdotal or not! 
chat with you soon!
hap 
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