I have definitely heard this before and we're not even sure what we're having yet. Twin Research and Human Genetics. The biological mechanisms that prompt the single fertilized egg to split in two remain a mystery. Her longest relationship, which lasted 20 years, was with a man she also believes was irritated by her closeness to her family. They share about half of their DNA with each other. The odds of having one set of identity and one set of fraternal twins is one in 10, Twin gender probability Hi Doctor Math.
This topic is discussed in more detail under a separate article.
Is there a more common gender combo with twins?
One study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2. Archived from the original on 17 January One fetus acts as a parasite towards the other. About 1 in 90 human births 1. A year German study  of 8, vaginally delivered twins that is, 4, pregnancies in Hesse yielded a mean delivery time interval of Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be treated medically. I just wondered what the most common combo was.