What makes a mixed race twin white or black?
Genetically a mixed race and European couple, who are expecting twins, have about a one in 500 chance that the babies will have different skin colours. Fifty years ago these twin births were almost unheard of, but with the number of interracial relationships increasing, so too are the number of cases.
Around 12,000 sets of twins are born each year in Britain, but only a few are born with a very different skin colour to each other.
"When I think of the reality, I'm like 'that's my daughter there and she's got a totally different skin tone,' and it's just really bizarre that I created her," says Shirley Wales, who gave birth to non-identical twins.
Her son Leo has black skin and her daughter Hope, has white skin.
Shirley, who lives in West Yorkshire, is mixed race and the father of her twins is white. She describes her daughter as the "double" of her father, but is adamant that she should not forget her mixed race heritage.
"Her skin tone is white," she says, "but she is mixed race, and if I were to ever fill out a form, as much as she is white, it's 'no my daughter is mixed race,' because I want her to be proud."
In order to discover how this genetic phenomenon occurred, Shirley took a DNA test to find out more about her own genetic profile.
She was adopted when she was four years old, and her birth mother is Afro-Caribbean and her British birth father was white. Her DNA tests revealed that, genetically, she was exactly 50% African and 50% European.
This is very unusual, and the results suggested that Shirley's mother had pure African roots, and that her ancestors must have moved from Africa to the Caribbean quite recently.
On average Afro-Caribbean people are around one fifth European in their DNA, due to the history of mixed race births dating from the 17th Century and British colonialism in the Caribbean.
Dr Jim Wilson, a population geneticist at the University of Edinburgh, who analysed Shirley's test results, claims the difference in the skin colour of Shirley's twins can be attributed to her ancestry and the chance inheritance of different gene variants that Shirley carries.
"Our skin colour is determined by a number of gene variants - at least 20 variants, I would say, probably quite a few more than that," says Dr Wilson.
"Some of these we know, and some of them we don't yet know, and at each of these genes, that are influencing the colour of our skin, there tends to be two or more variants. One of which is producing a darker skin tone, and one of which is producing a lighter skin tone."
These gene variants control the amount of melanin or pigment produced in the skin. However the particular genes that a child inherits from their parents and ancestors is actually a chance process.
"I think of it as a deck of cards," says Dr Wilson. "Imagine you are at the casino and you have been dealt a hand of cards, some will be black, and some red. [Shirley] happened, by chance, to have passed more of the European skin colour variants, almost all of them, on to Hope, whereas more of the African ones, just by chance, on to Leo."
According to Dr Wilson, genetically a mixed race and European couple, who are expecting twins, have about a one in 500 chance that they are of different skin colours. This chance only applies to non-identical twins, because they are conceived from two eggs fertilised by two sperm.
As in a painter's palette, in the skin the presence of pigment dominates the absence of pigment, so the fact that Hope is white is very unusual.
Shirley's DNA test results also showed that she carries the gene for red hair, so there is the chance that Hope could be a red-head when she grows up.
As well as the genetic differences, there are emotional and psychological factors to also consider.
Twelve-year-old Moesha from Glasgow struggles to fit in. She is white and her twin sister, Ebony and mother Stacey are black.
"I wish I was more like my mum," she says, "because she's a nice colour and I want to be the same colour as her."
Stacey feels that her daughter's difference in skin colour has had an affect on her self-esteem and body image.
"Moesha's got no confidence whatsoever, it's definitely all front," she says. "Ebony is very, very self-assured. What you see is what you get. Moesha puts on this act for everybody. Moesha is just that bit more vulnerable. She tries to fit in but I don't think she knows where she fits in."
Moesha was bullied at primary school for not being the same colour as her mother, which led to her feeling a sense of distinction from her family.
"She's said it before, 'I'm not part of this family, I'm not the same colour,'" says Stacey, "and I just tell her, 'you take your colouring from your dad.'"
'We're just brothers'
One in every 10 children in Britain is mixed race and this number is increasing all the time. The likelihood is that black and white twins could become more commonplace in society and not such a rarity.
"Social attitudes will evolve as this phenomenon increases," says Dr Wilson.
Twin brothers, Thomas and Wesley Charnock, 29, from Manchester have been subjected to racism in their life because of their different skin tones.
Thomas, a naval airman, says his brother Wesley, who is white, has found it harder to deal with racist remarks, because people do not physically notice his mixed race parentage:
"It's like a hidden thing for him," says Thomas. "If someone was to look at him walking down the street, they wouldn't realise his mum was black and he has this African heritage. That's in my opinion why he gets more het up about it."
Wesley, an HGV driver, says that such racist comments are just "small mindedness".
"We're just brothers at the end of the day," he says, "and we don't think of ourselves as any different to anyone else."
Find out more in Twincredibles, which will be broadcast on Monday 10 October at 21:00 BST on BBC Two, as part of the Mixed Race Season.