Semi Identical TwinsTwo sperm cells fused with a single egg
Types of Twins
With fraternal twins, the most common type, the mother contributes two eggs that each are fertilized in the womb by two different sperm cells from the father. They are genetically as similar as any ordinary siblings.
With identical twins, one egg from the mother is fertilized by one sperm from the father, and then very early in development the embryo splits and two fetuses grow. These twins are very similar genetically.
The 'semi-identical' twins are the result of two sperm cells fusing with a single egg, before becoming two embryos. An embryo created this way doesn't usually survive, but a few cases are known.
Semi Identical Twins
These twins are also chimaras, meaning that their cells are not genetically uniform. Each sperm has contributed genes to each child -- a very rare and previously unreported way for twins to come about.
New Semi Identical Twins
Normally, chimeras form essentially when two separate fertilized eggs that would normally make fraternal twins fuse early in development and make a single baby that ends up with a mix of tissues from each egg. Most of the time it goes totally unnoticed, and only shows up either if the two eggs have different genetic genders and the baby ends up with a mix of male and female gonads, or if it happens to be picked up on a genetic test that's done for other reasons (ex. paternity tests).
New type of Semi Identical Twins ?
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.
New Semi Identical Twins
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.
With these babies, they at some point had two sets of DNA that could potentially have formed separate semi-identical twins just like what happens with fraternal twins, except that they either never completely separated, or else separated and then got mixed together and, they ended up making two babies that both have both types of cells.
The new case came to light because one of the twins had an abnormality in sexual development -- ambiguous genitalia -- and was considered a hermaphrodite with both ovarian and testicular tissue.
This child is being raised as a girl. The other twin is a boy. The 'semi-identical' twins were evaluated at a U.S. hospital initially a few years ago.The twins were conceived completely naturally, noting that in vitro fertilization or infertility treatments were not involved.
Dr Melissa Parisi, a paediatrics professor at the University of Washington in Seattle and co-author of the research, said, "I can tell you that in my last contact with the family, the twins were doing very well - healthy, growing well, developing normally".
"Their similarity is somewhere between identical and fraternal twins," said Vivienne Souter, lead author of the study, reported online by firstname.lastname@example.org, the online site of the science journal Nature.
The study initially appeared in the Journal of Human Genetics, published in Britain, researchers said the new 'semi-identical' twins are more genetically similar than fraternal twins, but less similar than identical twins. Observation suggests the existence of other similar twins that have not yet been, and may never be, identified.
"It's extremely unlikely that we will ever see another case." said Charles Boklage, an expert on twinning at Eastern Carolina University in North Carolina.
This phenomenon has been related to 'vanishing twin' phenomenon, where an embryo or fetus dies in the womb and disintregrates, often leaving no trace, but there can be signs on the placenta. The sole survivor of a twin pregnancy may carry the genetic code of his or her twin in blood or other tissues.
Althea Hayton, Project Director, wombtwin.com
In this case there may have been a vanishing triplet, a female, whose blood and stem cells carried in the blood, mixed with her identical twin brothers' blood before she died and faded away.