When twins are born, they are either the product of separate eggs that are fertilized by separate sperm (fraternal twins) or they are the result of one egg plus one sperm that divided to produce two babies (identical twins). However, a set of twins has been discovered that is very likely unique: they are the result of one egg and two sperm that gave rise to two babies.
In short; these twins have identical maternal genes and different fraternal genes.
Already toddlers, the twins were discovered because one of the twins was discovered to have sexually ambiguous genitalia, with both ovarian and testicular tissue, while the other child is anatomically male. Even though one is anatomically male, genetic tests reveal both children are “chimeras”, with some male cells, which have an X and Y chromosome, and some female cells, which have two X chromosomes.
The most likely explanation for how they were formed is that two sperm cells — one with an X chromosome and another with a Y chromosome — fused with a single egg.
Charles Boklage, an expert on twinning who works at Eastern Carolina University in Greenville, North Carolina, said: “There’s value in understanding that this can happen, but it’s extremely unlikely that we’ll ever see another case.”