A pair of “semi-identical” twins who share about three quarters of their genes has been identified for the first time.

The very rare twins were conceived as a result of two sperm fertilising a single egg, which then divided to form two embryos that lie somewhere between fraternal and identical twins in terms of similarity.

The conception, which is completely new to science, was reported yesterday on the web-site of the journal Nature. Full details have been published in the Journal of Human Genetics.

Such twins are thought to be so rare that scientists do not expect to find another case, as the event and its discovery relied on three occurrences, each of which is by itself unlikely.

First, an egg had to be fertilised by two sperm, and yet develop into a viable embryo. Next, this embryo had to split into two, by the process that normally forms identical twins.

It was then necessary for the children that were born to come to the attention of science, which happened in this case only because one was born with ambigious genitalia.

Charles Boklage, an expert on twinning at Eastern Carolina University in Greenville, North Carolina, said: “There’s value in understanding that this can happen, but it’s extremely unlikely that we’ll ever see another case.”

Vivienne Souter, of Banner Good Samaritan Medical Center in Phoenix, Arizona, who led the team that investigated the twins’ genetic make-up, said: “Their similarity is somewhere between identical and fraternal twins. It makes me wonder whether the current classification of twins is an oversimplification.”

One of the twins was found to be a “true hermaphrodite”, with both ovarian and testicular tissue, while the other is anatomically male.

Genetic tests, however, revealed that both are “chimeras”, containing cells that appear to come from two individuals. Each twin’s body contains some “male” cells with both an X and a Y chromosome, and some “female” cells with two X chromosomes.

The likeliest explanation for this outcome is that a single egg, with an X chromosome, was fertilised by one sperm with an X chromosome and one sperm with a Y chromosome. Such double fertilisations account for about 1 per cent of conceptions, but do not normally survive. Those that do are chimeras, with cells that look like they should belong to two individuals.

In this case, the embryo not only survived, but then cleaved into two. The result was two twins that are more similar genetically than fraternal twins, which share 50 per cent of their DNA, but less similar than identical twins, who share all their genetic material.

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