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.
OK, but then they say:
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.
That would make them identical twins surely? If each twin got the dna from a different sperm, then they’d be half-identical.
Or perhaps the proportion of cells of one type and the other varies from twin to twin as well as from tissue to tissue.
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.
The Chimera aspect of this is more interesting than the twinning. For example, what would happen if the sperm were from two different men? Can a person have two fathers? Who would pay child support?