A very curious case happened in China. A man in fact asked for the DNA test before recognising the twins brought to light by his wife. The motivation? One of the two kids didn’t look like him at all. The test results left everyone amazed: the twins actually have two different fathers!
What happened is a very rare event. It is in fact a case of heteroparental superfecondation, described in the literature only for very few confirmed cases.
There are many ways for a twin birth to take place: in fact, we speak of mono zygotic and di-zygotic twins. These definitions refer to the mechanism that leads to the formation of the two embryos. In the first case, that of mono zygotic twins, a sperm fertilises an egg. After the formation of the zygote, the first cell agglomerate, this splits into two, leading to the formation of two individuals. These obviously share 100% of the genome.
The second case, the most common one, is represented by the di-zygotic twins. In this situation two different spermatozoa fertilise as many egg cells. The egg cell, in order to be fertilised, must be at a precise moment in its maturation cycle: ovulation must have occurred and the cell must be inside the Fallopian tubes. This time window is quite short and, generally, if a twin di-zygote pregnancy is established, the two egg cells are fertilised following the same sexual relationship.