Extremely Rare Semi-Identical Twins In Australia

Semi-identical twins are extremely rare, with only one other case documented before.

Transcript

00:00Not all twins are fraternal or identical.

00:02In Australia, semi-identical twins were born.

00:05What does that mean?

00:06So normally, a developing embryo will get one set of chromosomes from the mom's egg

00:15and the other set of chromosomes from the father's sperm.

00:18In fraternal twins, two separate sperm cells fertilize two separate eggs and those develop

00:22into the twins.

00:23So the babies share about 50% of their mom's genes and 50% of their dad's genes.

00:28In identical twins, a single sperm fertilizes a single egg, which means that that fertilized

00:34egg then divides into the two twins and they end up sharing 100% of both their mom's and

00:38their dad's genes.

00:40So in semi-identical twins, something different happens.

00:42Two separate sperm fertilize the same egg.

00:45So then these babies share 100% of their mom's DNA, but they only share a proportion of their

00:51dad's, in this case, 78%.

00:53This is extremely rare.

00:54This has only been documented once before in twins that were born in the U.S.

00:59The twins are now four and a half years old and though there were a few complications

01:02after they were born, they're now developing normally.

01:05They are a boy and a girl, which actually doesn't happen in identical twins.

01:08In identical twins, since they share 100% of their genes, it's either a girl and a girl

01:12or a boy and a boy.

01:13However, semi-identical twins are so rare that researchers don't think routine testing

01:17is warranted.