Gene Expression

Brown out!

i-62eafb111586f7e5138c69443f4784e5-IMG_2953.jpgThe most common emails I receive are about hair and eye color, and of these the most frequent source seems to be from individuals in interracial relationships. Quite often they are curious as to the possible outcome of their offspring’s phenotype. Sometimes they wonder why their offspring looks the way he or she does. On one disturbing occasion someone was appealing to me to clear up the suspicion of non-paternity because of the unexpected outcome of the offspring’s appearance! Today I received this email:

I have a 19 month old son who is very light skinned, blond hair and blue eyes. My husband (American) is bi-racial. His mother is white and his father is black.

Myself (German), I’m white, same with both of my parents and grandparents. I’m a redhead with greenish brown eyes, my husband has black hair and brown eyes.

What are the odds for this to happen, for us to have a white baby with blond hair and blue eyes?

This question is very common: why does my baby look so white? Succinctly the child’s ancestors are mostly white (American blacks are on average 20% white, so it is likely that the child is more than 75% European in terms of recent ancestors). I’ve offered more detailed expositions on black white twin pairs. The underlying logic is that genetics is discrete, not blending. This is the the insight and power which Mendelian principles introduced to our understanding of evolutionary process. All that being said the above posts and these sorts of jargon-laden assertions really don’t mean much to many people. So I’m going to answer the email above in some more detail below.

But first, one particular issue that I want to bring up. Some of the emails I receive imply strongly that people are surprised at the relatively lack of potency of colored blood, especially black blood. I think what’s being assumed as a background condition is the rule of hypodescent, which assigns a mixed-race child to the identity of the lower status parent. One drop of black blood was enough to render someone a black person in the Untied States. It seems to me that many people are somewhat shocked that an individual who many would say is black, as the man above, could be the father of white offspring. After all, is black blood not strong? Well, I hate to disappoint Afrocentrists but there’s nothing special about black blood. Add enough creme and it will give way. The problem is that quite often mixed-race individuals are identified as if they are the socially subordinate race, so the logic would imply that their own child would inherit that subordinate race. But when the child “looks white,” there is some cognitive dissonance and we have to unpack our assumptions.

This is not to say that the assumption of hypodescent is necessarily racist. Quite liberal people now accept it because it is a social convention to allow someone who is of mixed-heritage to identify as only of the subordinate race. For example, most people accept Halle Berry as a black woman even though she has a white mother. Now, if Berry asserted that she was a white woman because she had a white mother I assume most people would look at her a bit strangely. As it happens Berry is now pregnant and her child will be genetically 3/4 white, and there is a strong likelihood that he or she will be able to “pass” if they so wish (if they insist on being identified as a black American I suppose most individuals will allow them to continue to do this). Consider the daughter of the actress Victoria Rowell, who is biracial. She looks white, and her ancestry is 3/4 white. In the past in the United States individuals such as Maya Fahey, Rowell’s daughter, would have passed into the white population or been absorbed into the black population. But this is not necessarily a choice she needs to make today, and it is not a choice that is even as important as it was in the past. I doubt that with a brother who is 3/4 black Maya Fahey will wish to pass and disappear into the white population and disavow her African American ancestry.

In any case, enough sociology. What are the chances that two non-blue-eyed people will give rise to a blue-eyed child? First, let us remind ourselves that children tend to grow darker with age, and often blue-eyed babies turn brown-eyed at some point in their lives. But this child is nearly 2 years old so let’s take development off the table. Eye color is controlled mostly from one gene, so let’s assume that it is a single locus trait. The child has blue-eyes, but neither parents have blue-eyes. So I would say that the likelihood here is on the order of ~1/4, give or take, that future children will also have blue eyes. It’s a mostly recessively expressed trait, so the ratio would emerge naturally since we know parents are not blue-eyed, so they are heterozygotes. The mother is a red-head, so it is likely she has loss of function on MC1R. The child’s hair has a blondish cast. If the father’s white parent was fair-haired then the combination with a red-haired women can naturally resulted in the emergence of a fair-haired offspring. The genetics is a little tricky here, I’m not comfortable with a single gene approximation. That being said, red-hair implies very little production of eumelanin, so any moderate skewed sampling toward the loss of function alleles which the father likely carries from his white parent could result in a fair-haired child. If I was a betting man I’d give the odds at around 1/3 or so, with most of the variation in hair color coming from the sampling of the father’s genes. Finally, in regards to skin color the red-hair and fair skin of the mother means that this is likely again decided by sampling of the father’s genes. If he is mixed-race he likely carries functional and non-functional copies of SLC24A5, SLC45A2 and TYR in heterozygote genotypes. There’s a 1/8 chance that on these three loci only European variants will be passed along and a 1/8 chance that only African ones will be passed along. There is another gene or two of some significance, and it is very likely that the child has a non-functional MC1R from the mother already. The child doesn’t seem as fair as the mother, so let’s assume that it received one functional copy from the three genes listed above, that gives 3/8 shot for that outcome. There’s a 1/2 shot that the child could be darker on these genes (a 1/8 chance it could be lighter, receiving no functional copies).

I throught out some general probabilities based on inference of genotype from phenotype, what I know about family history, and the sketchy outlines of the genomic architecture which we’re starting to understand now. For example, normal human variation for skin color is mostly controlled by 4-5 genes, so I generally just pretend as if it’s a binomial distribution. Eye color is 3/4 controlled by one locus, so I pretend it’s single locus, though that’s mostly useful with blue vs. non-blue distinctions. Recent work on hair color has added a lot more detail which I should have cranked through, but I think the rough estimates would work. Of course, in the near future the parents would just get sequenced on the appropriate loci and they could get a phenotypic expectation and variance.

In short, I don’t think that the baby is that unlikely.

Note: The correspondent encouraged me to share their story and post the photo. Just so you know!

Comments

  1. #1 David B
    November 20, 2007

    The child’s hair will probably darken as it gets older. There is an amusing photo somewhere of a very large English family with the children (at least 12 of them) arranged in order of age. Their hair colour ranges in a near-perfect cline from light blonde to dark brown.

  2. #2 Salamander
    November 20, 2007

    I had a friend who was a blue-eyed blonde Nordic type, and her husband was 3/4 black, 1/4 Irish. Their first child looked pretty much like you’d expect a biracial child to look — coffee-colored skin, dark brown curls, hazel eyes. Their second looked pure Irish — white, white, white skin and flaming red curls and pale blue eyes.

    My own children are all of mostly European descent (with a smattering of Cherokee and Choctaw) and they have such wildly divergent colorings that people often assume they had different fathers (they do not).

  3. #3 pconroy
    November 20, 2007

    My wife has 2 grandmothers who are Sicilian – both dark brown eyes and dark brown hair – and 2 grandfathers who are German – both blue eyes and blonde hair. I recently saw a photo of herself and her 3 siblings were they were young kids, and ALL had blonde hair and blue eyes. Today her brother has dark brown hair and blue eyes, my wife has red-brown hair and green eyes, her next sister blonde hair and blue eyes, her youngest sister, light brown hair and blue eyes.

  4. #4 Henri
    November 20, 2007

    Reminds me of this study from 2003 which concluded that in Brazil physical features are a poor predictor of African ancestry.

    http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=140919

    Usually I would link to a more readable news article, but the ones on this study were very confused.

  5. #5 Ryan
    November 20, 2007

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    http://aigbusted.blogspot.com

    -Ryan

  6. #6 tertian
    November 20, 2007

    Both my parents are black African. My elder brother and I are brown skinned, but my younger sis and bro are very light-skinned. They both had blue eyes and looked white at birth. Of course, the eyes have browned with age.

  7. #7 Kristin
    November 21, 2007

    My father is Irish with blue eyes and light brown hair.
    My mother is Japanese.
    I look almost full asian, while my brother has white skin, curly red hair, hazel eyes and freckles?
    Why is this? And why do i look so asian, while he is in contrast to me?

  8. #8 y gorzolla
    November 21, 2007

    Kristin,

    We also have the same types of parents as you do, and one sister is almost hispanic looking while the other looks very asian, and just like Mom, however the youngest looks just like Dad (with no asian influence). The genetics and results are very mind blowing. We are trying to see if Mom has any white in her ancestry. I tried to put a picture up showing us all, but I am not sure if it worked. can you post a pic. too?–although I understand if you don’t want to.

  9. #9 Sandgroper
    November 22, 2007

    Kristin – because genetics is discrete, not blending. We can be fooled into believing it is blending by people like my daughter, who has half-epicanthic folds and appears Chinese to European people and European to Chinese, but she has half-Chinese half-Irish friends with red hair, green eyes, golden skin and eyes with full epicanthic folds.

    My daughter started at a new school recently – one of the European girls sidled up to her and said “Erm…you’re not 100% Chinese, are you? I can tell because you have freckles.” My daughter was like “Oh really?”, trying not to laugh. She said it makes quite a pleasant change to be Chinese now, at her last school she was European, which was OK but it was getting a bit boring to be only one half of herself all the time.

  10. #10 tom bri
    November 23, 2007

    My two girls are neatly blended of my British ancestry and Mom’s Japanese. People say they look Hispanic!

    But Mom’s family doesn’t look typically Japanese, her mom is told by other Japanese that she looks south sea islander.

    I would love to see a gene map and I can’t wait to see what my kid’s kids look like.

    I have seen a lot of half Japanese half American Blacks, and they tend to be fairly African in appearance.

    Half Caucasian half Japanese can look like any combination of the two. Some look like full Japanese, some like quite European.

    The Japanese themselves are a very mixed race people, though they don’t like to admit it. They range from milk white to very dark, with very different facial structures etc.

  11. #11 razib
    November 23, 2007

    The Japanese themselves are a very mixed race people, though they don’t like to admit it. They range from milk white to very dark, with very different facial structures etc.

    japanese genetics.

  12. #12 Monado
    November 23, 2007

    I recall reading somewhere that only 3 out of our 23 pairs of chromosomes were involved in appearance, so they could be distributed one way or the other by chance, more easily than if they were on 10 chromosomes.

    I knew two cousins in school who were both half Chinese. One look fully Chinese and the other looked fully Caucasian with black hair.

  13. #13 Ran
    November 24, 2007

    The kid seems to have turned out pretty well.

  14. #14 Saul
    November 26, 2007

    But Mom’s family doesn’t look typically Japanese, her mom is told by other Japanese that she looks south sea islander…….

    The Japanese themselves are a very mixed race people, though they don’t like to admit it. They range from milk white to very dark, with very different facial structures etc.

    If she is darker than the average japanese and looks malay or polynesian, she might be descended from the old japanese elite. Take a look at a photograph of the first japanese emperor of whom we have a photograph, the Emperor Meiji:

    http://content.answers.com/main/content/wp/en-commons/thumb/9/9b/270px-Meiji_Emperor.jpg

    And of his grandson Emperor Hirohito:

    http://en.wikipedia.org/wiki/Image:Hirohito_in_dress_uniform.jpg

    They look more malayo-polynesian than east asian mongoloid.

  15. #15 Saul
    November 26, 2007

    The biracial “black” guy in that picture looks stereotypically arab by the way. Besides the middle-east, that skin tone and that phenotype is also quite common in south america, south asia, Malaysia/Indonesia etc..

  16. #16 Maxien
    December 3, 2007

    I am mixed race, my father was jamaican and my mother is white (english). I mrried a white man and had 4 children, 3 girls and 1 boy. All 3 of my girls are light skinned, they would pass for being white, one of them has blonde hair and bluey green eyes. My son is darker skinned, and very much looks mixed race.

    We all know that looks, hair color etc are down to what genes are given to us, so if we have a white person, black person, asian person or any other person in our family that is not the same color or race as us, the chances are that our children could be born with the same genes they had.

    Also, its not unknown that somewhere along the line one of our children may resemble someone else in the family. My daughter with blonde hair and blue eyes resembles her aunt (her fathers sister). She would pass for being her daughter, not mine.

    There is no quesion what so ever that my children ARE my children. I am their mother, i gave birth to them so i know they’re mine.

  17. #17 marc
    December 5, 2007

    i think the baby’s nose looks rather african than european. so there seems to be at least a little bit of his black grandfather visible… but that’s just a wild guess. maybe it’s merely within normal variability.

  18. #18 Kimmie
    May 21, 2008

    Just wondering Marc why, do you assume that the grandfather is black instead of the grandmother? And, most babies have round noses because they do not have bridges in their noses. Most kids do not form a bridge (bone) in their nose until they are 3-6 years old. So, in general all small babies have tiny round noses. When the (bone) or bridge in their nose forms they will get temporary dark rings under their eyes, because their bone has to break through soft tissue to form. So, his nose is not a black nose it’s just a baby nose.

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