Extremely Rare Semi-Identical Twins In Australia

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

Transcript

00:00 Not all twins are fraternal or identical.

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

00:05 What does that mean?

00:06 (electronic music)

00:09 So normally, a developing embryo will get

00:13 one set of chromosomes from the mom's egg

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

00:18 In fraternal twins, two separate sperm cells

00:20 fertilize two separate eggs,

00:21 and those develop into the twins.

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

00:26 and 50% of their dad's genes.

00:28 In identical twins, a single sperm fertilizes a single egg,

00:32 which means that that fertilized egg

00:34 then divides into the two twins,

00:35 and they end up sharing 100% of both their mom's

00:38 and their dad's genes.

00:39 So in semi-identical twins, something different happens.

00:42 Two separate sperm fertilize the same egg.

00:45 So then these babies share 100% of their mom's DNA,

00:49 but they only share a proportion of their dad's,

00:51 in this case, 78%.

00:53 This is extremely rare.

00:54 This has only been documented once before

00:57 in twins that were born in the US.

00:59 The twins are now four and a half years old,

01:01 and though there were a few complications

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

01:05 They are a boy and a girl,

01:06 which actually doesn't happen in identical twins.

01:08 In identical twins, since they share 100% of their genes,

01:11 it's either a girl and a girl or a boy and a boy.

01:13 However, semi-identical twins are so rare

01:15 that researchers don't think routine testing is warranted.

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