Twin Sisters

There are two mechanisms by which this might happen:
- Polar twins (or "polar body twins"), where two sperm fertilize an ovum, one of the two fertilizing a polar body; or where an ovum splits into identical copies, one containing a polar body, prior to fertilization, allowing it to be fertilized by two different sperm.
- Sesquizygotic twins, where two sperm fertilize the one ovum, forming a triploid, and then splitting.