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Geneticists report 'semi-identical' twins

Egg and sperm three-way

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Geneticists in the US have discovered a previously unknown kind of twins they have called semi-identical. The twins are identical on their mother's side, but only share half of their father's DNA.

The discovery, which has been published in the journal Nature, has prompted some to suggest that the current classification system for twins is too simple and needs to be revisited.

The twins are technically chimeras: that is, their cells are not genetically uniform. Some cells contain male cells with an X and Y chromosome, others have female cells bearing a double load of Xs. In the journal Human Genetics, the researchers report that the proportion of XY and XX cells varies depending on the kind of tissue being examined.

For the genes to be distributed in this way, two sperm cells must have fertilised a single egg. Some DNA from each sperm is present in each child.

The sequence of events needed for the children to have been born is extremely unlikely: some scientists suggest the pair may be unique.

First, two sperm must fertilise a single egg. This does happen in about one percent of human conceptions. More often than not the fertilised egg does not form a viable embryo. This embryo must then split to form twins, who if they are to be identified as semi-identical, must subsequently come to the attention of scientists.

In this case, one of the twins was born with "ambiguous" genitalia, which brought in the scientists. They discovered that the baby is a true hermaphrodite, with both testicular and ovarian tissue, while the other twin is anatomically male.

Both children, now toddlers, are reportedly growing normally and have no mental developmental abnormalities.

According to Nature.com, in 2003 biologist Michael Golubovsky predicted that this kind of intermediate twin could result from "double fertilisation". He told the news service: There are a lot of unclear situations in the genetics of twins and twinning. We need to keep our eyes open for other unusual scenarios." ®

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