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