New form of semi-identical twinning identified

4 April 2007

A paper in the journal Human Genetics has reported the discovery of a novel form of genetic twinning. Twins are normally classified as dizygous (fraternal) or monozygous (identical); identical twins develop from the same fertilized egg and share 100% genetic identity, whilst non-identical twins develop from different fertilized eggs and share 50% genetic identity, as any two siblings with the same parents would. Now, a set of twins born in the US have been found to share greater genetic identity than a dizgous pair, but less than monozygous pair [Souter VL et al. (2007) Hum Genet. 121(2):179-85].

Each twin has a chimeric 46 XX / 46 XY chromosomal content, and share 100% of maternal alleles but only around 50% of paternal alleles. The proportion of XX to XY cells varies between and within each twin, and this is presumed to account for the fact that one twin is phenotypically male whilst the other displayed a degree of hermaphroditism (ambiguous genitalia). The authors postulate that the twins developed from a single egg fertilized by two different sperm cells, a mechanism that has previously been proposed as being a theoretical possibility. The twins are reported to be healthy toddlers; the hermaphrodite twin is being raised as a girl.

Comment: This report is of interest in that it suggests that other unusual cases of genetic twinning may occur and go undetected. The twins in this case only came to medical attention because one possessed a significant amount of both ovarian and testicular tissue. Whilst the particular example of ‘semi-identical’ twinning is likely to be an extremely rare event, it shows that highly improbable events can nevertheless occur. This reinforces the necessity to consider such issues, and related examples such as human chimerism or mosaicism, when developing or using applications that assume normal genetic inheritance and cellular genetic composition, such as DNA profiling for criminal investigation or paternity testing.

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