During a second trimester pregnancy examination, the following images were obtained:

In view of the findings, the parents elected to terminate the pregnancy and these are two post-mortem images that were obtained:

Of course, these are thoracopagus twins. And for sure there is some sort of obstruction such as a cloacal atresia. But what is truly unusual in the discordance of gender. The twin on the left side has female genitalia, while the twin on the right has male genitalia. Unfortunately, the parents refused all tests after birth and there is thus, no finale proof of how this could have happened.

How is this possible?

1) There are a few unlikely hypothesis such as that of Polar Body Twinning. This is an event in which the half genetic material contained in the primary oocyte—which has laid dormant since fetal life in the dictyate stage of the prophase of meiosis—become activated and is expulsed as a polar body or that later the remaining secondary oocyte has it undergoes meiosis 2 expulses the second polar body. Fertilization of the polar body would result in "half-identical twins" since they would share the paternal DNA and half of the maternal DNA each. The evidence for this mechanism is not only flimsy since only 3 articles in Medline refer to the process, but these articles date back from when genetic knowledge was not what it currently is.

1. Harlap S, Shahar S, Baras M.  Overripe ova and twinning. Am J Hum Genet. 1985 Nov;37(6):1206-15.   2:  Bieber FR, Nance WE, Morton CC, Brown JA, Redwine FO, Jordan RL, Mohanakumar T.   Genetic studies of an acardiac monster: evidence of polar body twinning in man.  Science. 1981 Aug 14;213(4509):775-7.   3:  Goldgar DE, Kimberling WJ.  Genetic expectations of polar body twinning. Acta Genet Med Gemellol (Roma). 1981;30(4):257-66.

I think this hypothesis has not current support and thus can be left aside.

2) A related theory is that of an early division of an ovocyte by 2 distinct spermatozoid "Sesquizygotic twins". A single author is responsible for both articles on the subject:

1. Golubovsky M.  Paternal familial twinning: hypothesis and genetic/medical implications. Twin Res. 2002 Apr;5(2):75-86. Review.2. Golubovskii MD, Golubovskaia IN.  [Possible cytogenetic mechanisms of direct paternal influence on the human twinning tendency and their consequences: a hypothesis]  Genetika. 1984 Jun;20(6):1043-51. Review. Russian.

3) Hormonal problems are hard to justify: why one fetus develop ambiguous genitalia and not the other in an obviously shared circulation?

4) Fusion theory: As you know the prevalent theory on conjoined twins is that they are not conjoined, but impartially fused (this is referred as the "Fission theory". Rowena Spencer has argued in a very recent book (Conjoined Twins: Developmental Malformations and Clinical Implications) in favor of the fusion theory instead of the fission theory. Her arguments are well-defended but not supported by much embryological evidence (note the review by Beckwith).

5) Finally the most common explanation for these type of twin was set forth by Pr. Opitz in 1983 (Opitz, J. 1993. Blastogenesis and the "primary field" in development. In Blastogenesis: Normal and Abnormal (J. M. Opitz, ed). Birth Defects 29: 3 - 37.)  In this theory mitotic non-disjunction results in heterokaryotypic monozygotic (see Sonography of multiple gestations   Under Articles > Multiple Pregnancy > Reviews, page 11).

I show this case to Prof Opitz who agreed that this was the most likely explanation:

"Mon très cher Philippe,

The image does not contradict a hypothesis of a MZT event with simultaneous non-disjunction so that one twin is 45,X (female genitalia) and the other 46,XY (male genitalia).

I reviewed the photos on the conjoint Jordanian twins once more and conclude that the most likely explanation is simultaneous nondisjunction so that one fetus was 45,X and the other 46,XY.

All the best in 2004.

Faithfully,

John"

Since Pr. Benirschke is also a "god" in all twin matters I asked his opinion and he concurred in the following.

This was also the opinion Kurt Benirschke:

"Thanks for the note. I do not believe that Rowena is correct and you need to read the book review by Bruce Beckwith as well. He also disagrees.

Conjoined twins with allegedly "different sex" could only be explained by the loss of a Y chromosome in development. Many surviving Turner-like patients are mosaics and it is not at all difficult for me to understand how this can happen before partial splitting of an XY genotype. Only skin cultures would prove or disprove this idea. Kurt Benirschke."

Vivienne Souter has recently reviewed dizygous monochorionic twins and her article is well-worth reading.