This raises the question about what happens if the fused ova are of the same sex. Presumably their karyotypes will be either 46XX or 46XY, as unremarkable people, clinically speaking, and therefore of no interest.
Until recently I could not find any references in the literature, until I discovered a paper from the New England Journal of Medicine(1). A 52-year-old woman with three sons, had renal failure and was considering a living related donor kidney transplant.
After genetic tests were performed on her family members, as possible donors, she was told that she was "excluded" as the mother of two of her three children. The picture was confused by the presence of an undetected second cell line in her blood. More extended testing, using cells from her skin, hair, thyroid and buccal mucosa, confirmed that she was a true XX/XX tetragametic chimera,. Moreover, it extended her options for transplantation.
In-vitrio fertilisation involves the artificial insertion of a sperm into an ova, and usually two sperm and ova are used. It worth noting that concern has recently been expressed in the press that it might lead to a greater incidence of hermaphroditism.
Method of karyotype.
Karyotyping consists of taking cells and culturing them so that they divide. They are treated with a chemical that traps them at the point where the chromosomes become visible. They are extracted, stained with a dye such as Giemsa, and placed on a microscope slide. A photograph is taken, then individual chromosomes cut out and arranged in order. If mosaicism is suspected, eight or ten separate karyotypes may be taken using different cells.
Uehara's subject had cells of two different blood types, which, we understand, is quite common in these cases, and also with mosaicism.
Karyotyping is only carried out in cases where the appearance of the external genitalia is seriously in doubt. More extensive genetic analysis is time-consuming and expensive.
For those that are interested, the tests carried out by Uehara et al included: ABO blood typing, enzyme sub-typing of RBC's, and RFLP of the pseudo-autosomal region on the sex chromosomes.
1. Kruskall, M.S., Uhl, L., Neng Yu, Sahid Hussein, Edmond J. Yunis, E.J., Knoll, J, (2002) Misinterpretation of HLA typing in a woman with congenital chimerism due to postzygotic fusion of two embryos New England Journal of Medicine, 346: 1545-1552
Bland, J., (2003) About Gender: Questions Raised.
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04.01.99, 16.01.02, 14.06.03 Last amended 02.11.03, 27.03.12