Gene Expression

Twins of different races, 2009

i-a307f07faa210e2c88e5c7205cd00498-article-1103391-02EC61C2000.jpgEvery now and then British newspapers publish the story of twins born to an interracial couple who seem to resemble only one of the races of their parents. The irony being that siblings may appear to be of different populations. I’ve commented on several of these stories before. Now we have another one to ring in the new year, Mixed-race couple give birth to black and white twins – for the second time:

Seven years after having one black twin and one white twin, the 27-year-old mother has given birth to a second set of twins with different coloured skin at odds of one in 500,000.

When the couple’s first daughters arrived in 2001, they were astonished to see that Lauren had her mother’s blue eyes and red hair, while her twin Hayleigh had dark skin and hair, like her father, Dean.

So when Miss Spooner, from Fleet, in Hampshire, found out she was pregnant again this year, her friends and family joked that they ought to take a bet out on the same thing happening again.

Doctors who delivered the sisters early, because of fears for their health, were relieved to find them well, but also amazed that the second set of twin girls were born with different skin tones.

As her parents discovered when the girls were laid side by side in the hospital cot, Miya resembles her father and Leah has inherited her mother’s looks.

It is rare enough for two sets of twins to be born to the same parents, but the chances of them inheriting different skin and hair colour from their mother and father are just two in a million.

I would take issue with several points in the article:

1) The “black” father and daughters are of mixed appearance. In fact, by this photo the complexions of the black father and daughters are far lighter than the typical person of West African ancestry (the father is West Indian). It seems certain that the father is of mixed European and African ancestry. His “black” daughters exhibit features of this mixed ancestry. In contrast, his white daughters do not, having inherited genetic variants of a European nature from their mixed-race father.

2) There are several genes where the variation between Europeans and Africans is disjoint. In other words, almost every person of African and European ancestry have the same variant within population, and different between. One example of this is SLC24A5. It is possible that the father is a heterozygote, carrying one variant for light skin, and one variant for dark skin. In that case, each one of his children has a 50% chance of carrying the light skin variant, and a 50% chance of carrying the dark skin variant. Since the mother is European, the potential combinations on this locus would be heterozygote, or homozygote for the light skin variant. Expand this process to 3-4 other loci, and you see that the variance within the sampling process means that the differences you see among these siblings are not surprising.

3) Twinning runs in families. I suspect that the probability generated above does not take into account the fact that someone who has twins naturally once is more likely than the norm to have twins in the future, because the fact of having twins initially suggests a greater likelihood that the individual may produce twins. On other words, I doubt the 2 in 1 million number takes into account conditional probabilities….

Comments

  1. #1 Judith
    January 2, 2009

    Hi, we’ve been talking about this over on

    http://www.studentmidwives.co.uk/phpBB3/viewtopic.php?f=9&t=58158 your posts on previous twins have been very helpful.

  2. #2 bioIgnoramus
    January 2, 2009

    I’ll just celebrate the fact that a British couple are still together after 7 years.

  3. #3 Melinda Barton
    January 2, 2009

    Thanks for this. I know it gets all the press when twins are involved, but these factors affect many people of mixed race.

    Both of my parents are mixed white/Native American. My father has dark hair, skin, eyes, etc. My mother (since her father is mixed race and mother white) is blonde and blue-eyed with light skin. I have some rather obvious (though softened) Native American features, but am otherwise colored somewhat like my mother.

    You have no idea the nonsense people give me when I say I’m biracial, because they assume that a mixed race person will always be of the darker color. On the other hand, many people familiar with what Native Americans look like often ask me what tribe I am before being told that I’m mixed race. Strange.

  4. #4 Wendy Levy
    January 3, 2009

    You wait til you see the movie ‘SKIN’ starring Sam Neill and Sophie Okonedo, it will blow your socks off! These twins and these kinds of things help us to understand how racism and any kind of prejudice is so stupid!
    ‘Skin’ is a true story about a black girl born to 2 white parents, I saw it at the LA film festival and I have not stopped thinking about it since!
    A must see movie if you have a mother and a heart!!!

  5. #5 Charyl
    January 4, 2009

    The first time I heard this I was amazed that something like this could happen. I think that it is amazing. It not only happens with children whose parents are of different ethnicities. My husband and I are both African American and our twins look more like distant cousins than brother and sister. I am fair skinned, while my husband is dark skinned. Our daughter takes after my husband, while my son takes after me.

  6. #6 Huxley
    January 4, 2009

    “‘Skin’ is a true story about a black girl born to 2 white parents, I saw it at the LA film festival and I have not stopped thinking about it since!”

    I remember reading somewhere that a child can’t be darker than the darker of his two parents and that in such instances the biological father is not the putative father. I have no idea if that’s true or not.

  7. #7 razib
    January 4, 2009

    I remember reading somewhere that a child can’t be darker than the darker of his two parents and that in such instances the biological father is not the putative father. I have no idea if that’s true or not.

    this is wrong. the genetics are pretty straightforward, so i’ll blog it soon….

  8. #8 deadpost
    January 5, 2009

    “I remember reading somewhere that a child can’t be darker than the darker of his two parents and that in such instances the biological father is not the putative father. I have no idea if that’s true or not.”

    Perhaps biological mother may not be the putative mother.
    Let’s not be sexist.

    Joking

  9. #9 S. Friske
    January 5, 2009

    Just goes to show there’s only ONE race, the human race!

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