how opinionated we are about 'the propriety of various sexual relationships -regardless of how little we know about 'their naturally occurring physiologies'. -p
Mar 31 2007, The Economist magazine
Science & Technology
Semi-identical twins
Double trouble

Twins that have cells of both sexes

MARMOSETS are not the only animals to produce unusual twins, it seems (see article). Most human twins are either identical (ie, sharing all their genes because they started as a single fertilised egg) or fraternal (coming from two eggs fertilised independently at the same time). However, Vivienne Souter and her colleagues at the Banner Good Samaritan Medical Centre in Phoenix, Arizona, have discovered a pair of human twins that are neither identical nor fraternal. Instead, they share all of their maternal genes but only half of their paternal genes. This genetic discordance is manifested in their sexes. One twin is male and the other is hermaphrodite—technically both male and female.
   The twins, now toddlers, had been referred to Dr Souter because one had “ambiguous genitalia”. This twin had separate urinary and vaginal orifices, an enlarged clitoris and a suspected testicular mass. Researchers then tested both twins for sex-specific hormone levels, took samples of their genes to determine their parental origins and performed investigative surgery on the twin who had ambiguous genitals.
   The surgery showed that the ambiguous twin possessed female genitalia on the right-hand side and male genitalia on the left, a condition that is known as true hermaphroditism. The other twin had only male genitals, but some of the cells of his skin and his immune system had two X chromosomes in their nuclei, the arrangement normally found in females, although others had the pairing of X and Y that is characteristic of males.
   Knowledge of how such twins form is still sketchy. What seems most likely is that an unfertilised egg cleaves prematurely into two. Each daughter egg is then available for fertilisation by a sperm cell. The result is an embryo with two sets of paternal genes. Further on in its development, but at a point when the individual cells are still at their most basic and have the potential to give rise to any cell in the human body, this embryo splits to form twins.
   What is also unknown is how common such “semi-identical” twins are. This pair would not have been noticed if neither had been hermaphrodite. Such genetically unusual individuals may thus be more numerous than is currently realised.

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