Message-Id: <199904251755.MAA28241@subcellar.mwci.net>
From: "eric nielsen" <entfolks@mwci.net>
To: <AECSoldat@aol.com>, <eiger@montana.edu>, <pinhole@exploratorium.edu>
Subject: Re: Re: pinhole probability
Date: Sun, 25 Apr 1999 11:56:08 -0500
Pinholers --
being both a biologist & a statistician, I think I'm qualified to take a
crack at this one.... Steve below does *not* understand (no offense
Steve)
toss two coins, coin A and coin B. As B lands, you cover it so that you
cannot see the result. let's say that A comes up heads. What is the
proability that B is also heads, Pr(B=heads) ??? This probability is 50%,
or Pr(B=heads) = 0.50. They are independent events, and the result on coin
A has no impact on the result on coin B.
Now let's move on to real-life critters. Mendelian genetics begins with
the model that genes (and correlated traits) associate randomly during the
production of gametes. Under this most basic model, the probability of the
second sib having the same gender as the first is exactly 50%. They (the
two sibs) are independent events, just like the coins tossed above.
Period. Full stop. Do not procede further until this makes sense. Even
your question is essentially *statistical* then the answer is in the
paragraph which you just completed!!
the following integrates the stats with a bit of real-world biology....
This is merely the beginning model, however -- two genes which reside on
the same chromosome do *not* associate randomly!! They are linked, both
physically & probabilistically. I should note that this "genetic linkage"
caveat does *not* apply to gender, because gender is determined by the
presence or absence of an entire *chromosome,* so genetic linkage cannot
cause a deviation from the Mendelian prediction of a gender ratio in
siblings/offspring. I *do* know of the existance of genes which
influence the gender of offspring in some animals, such that the sex ratio
of offspring is skewed away from an expected Mendialn ratio such as 50/50.
Assuming that analogous genes exist in the human population (I believe a
teneable assumption), you could argue that -- for example -- the presence
of a one male sib increases the probability that there is a "male-favoring"
trait in the family linage, therefore the probability that other sibs would
be male as well is greater, but I think that it's rather grasping at
straws. Realistically, I doubut you would detect such "gender biasing
genes" without a sample size in the at least 100's if not 1000's of
families. Might be an excellent thesis in here somewhere -- could work
from genealogy club notes & historical records to establish a longitudinal
study....??
ok, I've spent quite a bit of time on this; hopefully it clarifies. any
questions: ask. by the way, gender can be determined by environmental
vaiables as well as genetic composition -- e.g. cichlid fishes.
Eric
scienceguy.webjump.com
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From: Steven Eiger <eiger@montana.edu>
To: Pinhole Listserv <pinhole@exploratorium.edu>
Subject: Re: pinhole probability
Date: Friday, April 23, 1999 12:02 PM
>Hey pinholers, a probability question:
>
>Let's pretend that you meet a person and she says that she has only
>one sibling. What is the probability that her sibling is a boy? How
>about a girl? I'm pretty sure that it is not 50:50....
>
>(So, in real life, if you meet a woman with one sibling, is there
>really a better chance of her having a brother than a sister?)
>
>Stumped in Oakland
>
>--
>‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡‡
>
>Ian Bleakney;
I am not a statistician. But my guess is to treat this like flipping
coins. There are four possibilities two heads, two tails, a head followed
by a tail and a tail followed by a head. Since we have eliminated one by
knowing the sex of one sib, and we do not know whether she was the first or
second sibling, I would guess that there is twice the likelihood of her
sibling being of the opposite sex, relative to the same sex. I would also
guess that there is a 50% chance I am right about this. I have had this
explained to me several times and it never seems to stick. Steve Eiger.
Steven Eiger, Ph.D.
Departments of Biology and the WWAMI Medical Education Program
Montana State University - Bozeman
Bozeman, MT 59717-3460
Voice: (406) 994-5672
E-mail: eiger@montana.edu
FAX: (406) 994-3190
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