We know about identical and fraternal twins – the former being the Olsen twins, and the latter being confusing when you meet them.

What you might not know about are sesquizygotic or semi-identical twins, who are neither fraternal or identical, but something in between.

The reason you haven’t heard about them is that they are incredibly rare. In fact, the Guardian reports that there are only two recorded cases in history.

The second of which saw scientists lucky enough to be able to study the twins in utero.

The team say the boy and girl, now four years old, are the second case of semi-identical twins ever recorded, and the first to be spotted while the mother was pregnant.

The situation was a surprise to the researchers. An ultrasound of the 28-year old mother at six weeks suggested the twins were identical – with signs including a shared placenta. But it soon became clear all was not as it seemed.

“What happened was the mother came back for her routine ultrasound some months later, and we saw one [twin] to be a boy and one to be a girl,” said Dr Michael Gabbett, first author of the report from Queensland University of Technology in Australia. “At that point we started the genetic studies and worked it out from there.”

In the case of identical twins, one egg is fertilised by one sperm, but the resulting ball of cells splits in two, which results in two babies with identical genetic material.

Fraternal twins occur when two eggs are fertilised by two different sperm. The resulting twins are no more identical than children born to the same parents at different times.

Semi-identical twins occur through a sequence of very unusual events:

The situation is believed to arise when two sperm cells fertilise a single egg. In the latest case, one sperm carried an X chromosome among its genetic material, and the other carried a Y chromosome.

After fertilisation the chromosomes from the two sperm cells and the single egg got bundled into three “genetic packages”: one contained chromosomes from both sperm – meaning it contained two sets of genetic material from the father, but none from the mother. The other two packages each contained the same set of chromosomes from the mother, as well as genetic material from one of the two sperm, giving rise to either XX (female) or XY (male) cells.

As the fertilised egg divided and the ball of cells grew, those containing only chromosomes from the two sperm died. However, those containing chromosomes from both the egg and a sperm cell continued to divide.

“Then what happens is that little ball of cells splits into two, and that is why you have twins,” said Gabbett, adding that these offspring have a greater genetic similarity than fraternal twins, but are not identical.

The twins were born and appeared healthy, but there were complications. The baby girl developed a blood clot and had to have her arm amputated. Later, her ovaries were removed because they didn’t fully form.

Other than that they’re fine.

While we’re on the topic of uteruses and babies, have you heard about that other rare condition where a woman has two uteruses which can result in surprise babies?

Check it out. It’s terrifying.

[source:guardian]