Q: What happens when two sperm fertilize one egg?
A: Semi-identical intersexed twins.
I stumbled upon this TIME article from 2007 while doing some research on fraternal twins. Fascinating!
"There are two basic twin types now, identical and fraternal, but a third type is the focus of a new discovery. The story of these one-of-a-kind twins begins after natural conception and an uncomplicated term pregnancy, when the newborns were brought to the attention of science because one is anatomically male and one has sexually ambiguous genitalia.
For the first time, researchers have identified twins that are identical on their mother’s side, but share only half of their father’s DNA. The twins, now toddlers, have been described as “semi-identical” — caught somewhere between identical twins (the result of the cleaving of an egg fertilized by one sperm) and fraternal twins (the result of two eggs meeting two sperm). Lead investigator Dr. Vivienne Souter says that while the term semi-identical provides some idea of how the twinning occurred, it is “an oversimplification.”
According to a study published in the Mar. 28 issue of Human Genetics, two sperm fertilized one egg and created the twins. The phenomenon occurs in about 1% of the population, but most embryos created in this way — called triploids because they have three sets of chromosomes — do not live. Says Dr. Mary Jane Minkin, a clinical professor of obstetrics and gynecology at Yale University: “This confirms that two sperm can get into an egg.” Normally the cell dies. But Minkin makes a valid point: “Never say never in medicine and biology.”
Read the rest here: http://www.time.com/time/health/article/0,8599,1603799,00.html#ixzz0w4S0q3HS