Extraordinarily Rare' Semi-Identical Twins Born in Australia

By: Yassmine ElSayed

CAIRO, Mar. 1 (SEE) – In an extremely rare case, A pair of twins born in Australia share all of their mother's genes, but only 78 percent of their father's, according to a new case report published in The New England Journal of Medicine.

Twins can be fraternal, identical — and in extremely rare cases — semi-identical.

According to lead author Dr. Michael Gabbett, the diagnostic genomics course coordinator at the Institute of Health and Biomedical Innovation in Brisbane, Australia, the first set of semi-identical twins was identified in 2007 in the U.S., when they were infants; this is the first time semi-identical twins have been identified in the womb.

Initially, the mother of the twins described in the case report thought that she was pregnant with identical twins, based on an ultrasound early in her pregnancy. Later in the pregnancy, however, the woman's doctors were surprised to see that the twins were a boy and a girl. Because identical twins share all of their genes, they can't be of opposite sexes like fraternal twins can.

Normally, a human's DNA comes from two sources: one set of chromosomes comes from the mother's egg and one set comes from the father's sperm. In fraternal twins, two sperm fertilize two separate eggs, yielding twins that share half of their mother’s genes and half of their fathers; in identical twins one sperm fertilizes a single egg, which splits up into the twins that share all of their mother and father’s genes. But in the semi-identical twins, one set of chromosomes came from the egg, and the second set was made up of chromosomes from two separate sperm, Gabbett told Live Science.

But how does this happen? Gabbett hypothesizes that the mother's egg was fertilized by two separate sperm, each carrying its own set of chromosomes –– one set of chromosomes from the mother ended up combining with two different sets of chromosomes from the father.

Once an egg is fertilized, it becomes impenetrable to other sperm, so Gabbett believes that the two sperm must have arrived at the egg at the same time.

To get a better understanding of how rare sesquizygosis twins are, the researchers went on to examine the genes of nearly 1,000 other twins and didn't find another case of semi-identical twins, nor did they find evidence of the condition in a previous study from another research team that examined DNA from more than 20,000 twins.