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Preimplantation Genetic Diagnosis: The Who, the What, the Why and the How


 

Genetically identical twins born with different eye and skin colour

28 February 2016

By Hannah Somers

Appeared in BioNews 841

Two baby girls from County Durham are thought to be the first genetically identical twins in the UK to be born with different eye and skin colour.

Doctors had previously told the girls' parents they would be so similar when they were born they would need to be 'marked with ink' to be told apart. However, when they arrived, Amelia was born with dark skin and brown eyes, while Jasmine had blue eyes and fair skin.

Mother Libby Appleby said: 'When they were born, we were flabbergasted – even the doctors couldn't believe it. They look like they're different races. Amelia is the spitting image of her dad, while Jasmine is a mini version of me.'

Before their birth, placental sampling had shown the twins were monozygotic – they had developed from the same fertilised egg and share the same DNA sequence.

Dr Claire Steves, from the Department of Twin Research at King's College London, said that although this means they would mostly share the same genes for skin and eye colour, this isn't always strictly the case.

'An exception might be when a change in one of these happens after the twins separate in very early development – a so-called somatic mutation,' Dr Steves said.

For example, a study from 2013 showed that SNPs, single nucleotide polymorphisms, can be found in the genome of one twin yet not in the other. These variations likely arise very early in development but after the separation of the embryo into two. Such changes could explain the difference in skin and eye colour between the girls.

'Alternatively, sometimes markers on the DNA which influence the extent to which the DNA is expressed can be different in the twins. We do have some evidence that skin colour is subject to this kind of "epigenetic'" control,' Dr Steves added.

These epigenetic modifications do not alter the DNA sequence, but affect whether the gene is expressed or not. However, further study would be required to identify any actual epigenetic differences between the twins.

Dr Steves said: 'Despite these possibilities, which might explain this intriguing phenomenon, the vast majority of identical twins have very similar skin colouring and this case is very unusual indeed.'

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