Half-identical twins

[Dr Mordrid]

Link....

Study says some twins are half-identical

WILKES-BARRE, Pa., April 26 (UPI) -- U.S. researchers are investigating a theory that twins can be half-identical.

Researchers at Kings College in Wilkes-Barre, Pa., are looking at the DNA of nearly 25 sets of twins to prove that, in addition to fraternal and identical twins, there are also half-identical twins, the Wilkes-Barre Times-Leader reported Wednesday.

Biologist Ann Yezerski said the finding would explain why some fraternal twins look almost exactly alike.

Identical twins are formed when a single fertilized egg splits in half. Fraternal twins form when two eggs are fertilized by separate sperm cells.

Yezerski said half-identical twins are formed when an unfertilized egg splits in half and each half is fertilized by one sperm, the newspaper said.

[Mehen]

What causes eggs to split in half anyways? Is it just a chance event or is it caused by something else?

On a side note- what is it that causes people on fertility drugs to have so many babies?

[Dr Mordrid]

1. I'd guess genetic + hormonal + environmental similar to identicals, but with an unknown twist that does it before fertilization.

2. multiple ovulations from fertility drugs or multiple fertilized eggs being implanted because most don't usually thrive.

[Mehen]

Ah ok, so from #2 its all fraternal.

[Dr Mordrid]

Not necessarily. Each egg has a chance of splitting regardless of the mechanics of how it got there. It's quite possible to implant 4 fertilized eggs or have multiple ovulations and have at least 1 split.

[Mehen]

Ah ok, all my basic biology is coming back to me now. So really, if anything, its suprising that this issue/theory has not come up before. It seems pretty logical.