To: "Exploratorium Pinhole" <pinhole@exploratorium.edu>
Date: Tue, 27 Apr 1999 07:55:01 -0700
From: "BenP" <bpitt@n2.com>
Subject: Sister heads; Brother tails but not 50/50
I can't toss my hat in the 50/50 ring, because of one aspect of this sibling thing. I agree that the chance of fertilization with an X or a Y is 50/50, because meiosis in the male results in two X's and two Y's. Whether an X or a Y finally gets the apple is pretty much a horse race. However, my disagreement with the final 50/50 tally comes in the form of identical twins.
There's a chance this woman has an identical twin, and no chance (0/100) that the identical twin of a woman will be a brother. (a fraternal twin would still have the 50/50 male/female chance) So it seems that the 50/50 would tip just a bit toward her having a sister sib.
My handy-dandy science text says that about 70% of twins are fraternal, and 30% are identical; but it doesn't give any statistics of the frequency of twins overall. Another source (off the web - trust it as you want) says that about 1 in 250 live births are identical twins; but it doesn't say whether a set of twins is counted as one or two live births.
I also understand that with modern technology the survival rate of multiple births is increasing. But who knows how this affects the numbers or the equation.
Ultimately, I don't have the numbers to crunch, and I do have lesson plans to crunch, so I'll just close by saying that if a female has a sibling, it seems that there is a bit more than a 50/50 chance that her sibling is female. I leave it to you twins out there to come up with the numbers.
Ben Pittenger
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