365 Days of Twinipedia - Day 71

Day Seventy-one of Twinipedia:

Fact #71

Most people believe that there are two types of twins, Identical and Fraternal. But a theorized third type of twinning occurs when an unfertilized egg splits into two equal parts (like the beginnings of identical twins) but is then fertilized by two separate sperm. These twins, called Polar body twins, share about 75% of their genetic markers. The embryos share the mothers genes since she produced the one egg that initially split but their genetic make-up comes from two different sperm so they are therefore not identical. They share less genetic similarities than identical twins but more than fraternal twins.

Another outcome when Polar body twinning occurs is when the woman ovulates, and as her egg matures, a portion of the egg called the first polar body separates from the egg. Because this polar body contains the same genetic material as the egg itself, theoretically it could be fertilized by a sperm cell (usually, however, the polar body just disintegrates). But if both the egg and the polar body are fertilized by separate sperm and two babies develop from this event, polar body twins are formed as explained in the first paragraph.

"Other Terms for Polar Twins - half twins, half-identical twinning"

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