The Clooneys have welcomed twins, but what's the science behind having a girl and boy?

Amal and George Clooney
Girl-boy twins are more common than you might think. [Photo: PA]

Amal and George Clooney have just welcomed twins into the world, and not just any twins – one boy and one girl.

The fraternal twins, who were given the classic names of Ella and Alexander, are “healthy, happy and doing fine” according to a family statement given to People.

We often think of twins as being identical and the same gender, despite the fact that fraternal twins – and one boy and one girl – are the most common type of twin.

Fraternal twins usually occur when two fertilised eggs implant into the uterus at the same time, which are then fertilised by two different sperm cells.

Fraternal twins
Fraternal twins are essentially normal siblings [Photo: Pexels]

So fraternal twins essentially look as similar as any two siblings would and can be different sexes, as each sperm cell contains the chromosome which will determine the child’s sex.

While eggs contain sex chromosomes too, it’s always an X, while the sperm can be either X or Y – so it just depends whether the sperm that fertilises each egg carries an X (for a girl) or Y (for a boy).

Identical twins, which come from the same egg split into two, share 100% of their DNA, and are therefore the same in appearance and sex.

There’s no denying twins are more of a handful [Photo: Pexels]

‘Non-identical’ twins Ella and Alexander will share around 50% of their DNA.

Having fraternal, or non-identical, twins isn’t that uncommon. According to Twins UK, they occur in about 1 in 70 pregnancies, and the chance increases for mothers over the age of 35.

Since Amal is 39, this means her chance of bringing an extra life into the world was probably slightly higher.

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