Re: prenatal environments in twins

[Michael MacLean]

An article that I (as a non-expert in the area) found very informative on this issue was Phelps et al. (1997).  They describe that there are two different chorionic arrangements that occur with MZ twins.  The large majority cases are monochorionic, meaning they share a placenta, blood flow, etc.  A subset, however, are dichorionic, meaning that they apparently never exchange blood.  The most striking aspect of their review of the literature (which is still relatively new) is that in many ways similarity between twins is much stronger for the monochorionic twins than the dichorionic twins.  They suggest that until twin studies control for this chorion factor, MZ twin similarities that are due to having the same prenatal environment (monochorionics) will be falsely attributed to genetic effects.

I have found this useful to address in class whenever discussing twin research, as it is not mentioned in most texts (yet anyway).

-Mick
 

Phelps, J. A., Davis, J. O., & Schartz, K. M.  Nature, nurture, and twin research strategies.  Current Directions in Psychological Science, 6 (5), 117-121.

Jim Clark wrote:

Hi

On Wed, 17 Nov 1999, Andrew Swihart wrote:
> It is my understanding that MZ twins share a significantly more similar
> prenatal environment than DZ twins.  Could someone briefly explain to me
> the mechanisms which render MZ prenatal environments more similar than
> those for DZ, and direct me to a reference source(s) with the details?

I can't help with a reference, but my understanding from a quick
look a few years ago was that MZ are more likely to share a
single placenta and a single chorion (enclosure within womb).  I
have long wondered how one controls for this in studies of
heritability or whether it has been.

Best wishes
Jim

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James M. Clark                          (204) 786-9757
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Michael G. MacLean, Ph.D.
Assistant Professor
Department of Psychology
SUNY College at Buffalo
1300 Elmwood Avenue
Buffalo, New York  14222

Email:  [EMAIL PROTECTED]
Web: http://www.buffalostate.edu/~macleamg/
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