Re: can identical twins be of opposite genders?

Dear Charles -

When I first read your question, I immediately though "no it can't happen." Fortunately, I poked around a bit on MEDLINE, a large collection of biological and medical articles. The answer leans more towards a "sort-of yes" than a definitive "no."

First, let me tell you a little about how our "genetic" information is packaged, and how it relates to the possibility of having identical twins with opposite genders.

People have 23 sets of "chromosomes" (46 total). A chromosome is like a part of blueprint - it contains a portion of the instructions needed to make a person. You need all 23 sets to make a healthy person. Of these 23 sets, one set determines a person's sex - we call this special set the "sex chromosomes." We have two sex chromosomes, called X and Y. Women have two X chromosomes for their set, while men have an X and a Y for their set.

Identical twins (generally) share the same genetic information. Such twinning happens when a single fertilized egg splits into two embryos early in the course of development. Under normal circumstances, a fertilized egg that is XX (female) will not to suddenly give rise to an XY twin, just as an XY egg that splits cannot give rise to an XX twin.

However, I said under "normal circumstances..." There have been rare reports of male and female "identical twins" (see references below). Truly, none of the twins is genetically identical as they have a different complement of X/Y chromosomes. Their sex chromosomes differ, but their non-sex chromosomes are otherwise the same. I found no instances where both twins had the normal set of 23 chromosomes. Either one or both had genetic anomalies.

How do these very rare events occur?

We can't always tell for certain, but it appears the original fertilized egg may have a significant genetic defect such as extra chromosomes, or fragments of sex chromosomes that become stuck to one of the non-sex chromosomes. In some cases the defects may arise early in development - after fertilization, but either before or after the twinning occurs. In all cases, both twins arose from a single fertilized egg.

Fraternal twins, twins arising from two separately fertilized eggs, are commonly different genders, though they can also be the same gender.

Hope this helps..

- Lynn Bry, MD/PhD
Department of Pathology
Brigham & Women's Hospital
Harvard Medical School
Boston, MA.

References:
Prenat Diagn 1999 Jan;19(1):72-6 'Identical' twins with discordant karyotypes. Nieuwint A, et al.
Am J Med Genet 1994 Oct 15;53(1):52-5 Monozygotic twins of different apparent sex. Yokota Y, et al.
Am J Med Genet 1991 Nov 1;41(2):239-45 Monozygotic twins of discordant sex both with 45,X/46,X,idic(Y) mosaicism. Fujimoto A, et al.