Twins are normally either identical—the result of a single sperm fertilizing a single egg, which then splits and duplicates itself—or fraternal, developing from two separate eggs fertilized by two separate sperm. But last March, doctors reportedthe first known case of semi-identical twins.
“When we did some detailed genetic analysis, we discovered that there was a single maternal contribution to the twins but they had a mixture of two paternal contributions,” says Melissa Parisi, a doctor on the gender-assessment team at Children’s Hospital and Regional Medical Center in Seattle. “So that suggests that there were two sperm involved but maybe only one egg.”
With these twins, whose identities were not revealed, each paternal contribution contained a different sex chromosome. This resulted in the conception of a boy with normal genitalia and a girl who was developing both ovarian and testicular tissue. It was the sexually ambiguous genitalia that brought her to doctors’ attention. After several years of studying genetic material from the twins and their parents, the team came up with two possible scenarios. In the first, a single egg divided on its own and was then fertilized by two separate sperm from the dad. The second theory proposes that the egg was fertilized by two sperm and then divided.
“This sort of event could occur more frequently than we realize,” Parisi says. “If the two sperm contain the same sex chromosome, you would have no reason to necessarily do any more extensive testing because the baby would not have any genital anomalies.”
On a related note, a previous study found that 20 percent of hermaphrodites carry a mixture of female and male cells in their bodies, just like the sexually ambiguous twin who was later assigned the female gender. (The other twin, who has more Y chromosome-containing cells, developed as a male.) This suggests that some cases of hermaphroditism may result from two sperm with differing sex chromosomes fertilizing one egg.
“My hope is that by understanding these complicated mechanisms,” Parisi says, “we will be able to provide better information in the future for parents who have children with disorders of sex development.”
