Twins Info

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 reported the first known case of semi-identical twins. They 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. It 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.