Hermaphrodite reveals previously unknown type of twinning.
Researchers have discovered a pair of twins who are identical through their mother's side, but share only half their genes on their father's side.
The 'semi-identical' twins are the result of two sperm cells fusing with a single egg — a previously unreported way for twins to come about, say the team that made the finding. The twins are chimaeras, meaning that their cells are not genetically uniform. Each sperm has contributed genes to each child.
"Their similarity is somewhere between identical and fraternal twins," says geneticist Vivienne Souter, of the Banner Good Samaritan Medical Center in Phoenix, Arizona. "It makes me wonder whether the current classification of twins is an oversimplification."
Such twins are probably very rare. Their existence and discovery relies on three unusual, and possibly unlinked, events: first, that an egg fertilized by two sperm develops into a viable embryo; second, that this embryo splits to form twins; and third, that the children come to the attention of science.
"There's value in understanding that this can happen, but it's extremely unlikely that we'll ever see another case," says Charles Boklage, an expert on twinning who works at Eastern Carolina University in Greenville, North Carolina.
Souter and her colleagues investigated the twins' genetic makeup because one was born with ambiguous genitalia. This twin turned out to be a 'true hermaphrodite', with both ovarian and testicular tissue. The other twin is anatomically male.
Genetic tests revealed that each twin contained some 'female' cells with two X chromosomes, and some 'male' cells with an X and a Y. The proportion of each type varies from tissue to tissue in each twin, the researchers report in Human Genetics1.
The babies, now toddlers, were conceived and born normally, and each twin's growth and mental abilities appear normal.
Just the two of us
Fraternal twins are formed when two eggs meet two sperm in the womb. Each is fertilized independently, and each becomes an embryo. With identical twins, one egg is fertilized by one sperm, and the embryo splits at some later stage to become two.
Occasionally, two sperm are known to fertilize a single egg; this 'double fertilization' is thought to happen in about 1% of human conceptions. An embryo created this way doesn't usually survive, but a few cases are known to have made it — these children are chimaeras of cells with X and Y chromosomes.
"The number of these cases is very small, but before they were reported, most people would have said this could never happen," says geneticist David Bonthron of the University of Leeds, UK.
The new case seems to be a sub-type of double fertilization that involves the extra step of twinning. There are two possible ways this could have happened. The first possibility is that an egg cell divided, without separating, and each cell was then fertilized with a single sperm. The genes could have then been muddled up before the egg fully separated. But egg division before fertilization is thought to be extremely rare, says Souter.
The second, more likely scenario is that two sperm cells fused with a single egg, creating an embryo with three sets of chromosomes. Then, perhaps at the embryo's two-cell stage, one cell would have shed the chromosomes from one sperm, and the other cell the other sperm's genes.
Curiosities
It had been previously predicted that this might happen. Biologist Michael Golubovsky, for example, now at the University of California, Berkeley, suggested in 2003 that twins intermediate between identical and fraternal might result from double fertilizations2. The new discovery confirms his theory.
"There are a lot of unclear situations in the genetics of twins and twinning," Golubovsky says. "We need to keep our eyes open for other unusual scenarios."
"Whether these things are academic curiosities, or whether we've overlooked something significant is hard to say," says Bonthron. "A lot of what we know about fertilization is deductive, because we can't observe these events in humans."
'Semi-identical' twins discovered 
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