Twins (was: Re: Introduction)

Irina Rempt ira at rempt.xs4all.nl
Fri May 5 02:37:42 EDT 2000


On Fri, 5 May 2000, Paul Andinach wrote:

> Nothing wrong with 'identical' boy-girl twins. *Shakespeare* used
> identical boy-girl twins.
> (Anyway, are they actually impossible, or just horrendously unlikely?)

They're actually impossible. Identical twins result from the egg
splitting in two after fertilization - when it's already XX or XY.

Fraternal twins result from two eggs being fertilized by different
sperm, giving equal changes of boy/boy, boy/girl, girl/boy (the same
outcome but a different instance) and girl/girl. 50% of fraternal
twins are boy-girl.

This means that boy-girl twins are always fraternal, and because
identical twins are much less common than fraternal twins, same-sex
twins are often fraternal as well.

That said, my avowedly fraternal twin daughters (an ultrasound in the
13th week showed them each in their own membrane) look so much alike
that I'm tempted to have them tested :-)

   Irina

-- 
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