00:00There is a type of twin that cannot exist, and most people don't know why.
00:03That type is identical boy-girl twins. So why is that?
00:07Identical twins share 100% of their DNA. Everything is the same, including their chromosomes.
00:13And this is the key point. Girls have XX chromosomes, while boys have XY.
00:18If twins are identical, the two babies must have exactly the same set of chromosomes.
00:22That means they can only be two boys or two girls.
00:25This means that an identical twin pair consisting of one boy and one girl is biologically impossible.
00:31So how do identical twins form? It all begins with a single fertilized egg.
00:34That egg later splits into two separate embryos, resulting in two babies with completely identical genetic makeup.
00:40Same DNA, same sex, and often even surprisingly similar personalities.
00:45However, there are boy-girl twins, but they are fraternal twins, which form through a completely different process.
00:50Instead of one egg splitting, the mother releases two different eggs at the same time, and each egg is fertilized
00:55by a different sperm.
00:56The result is two siblings who just happen to be born on the same day.
