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How can brown-eyed parents have blue-eyed children?

  1. Mar 6, 2013 #1
    I am learning about genes and heritage currently and I am a bit baffled about something.

    In my book it states that in a plant's gene, if S is for smooth leaves and s for rough leaves, then we get these combinations:

    Code (Text):

    Parent plant | Child plant | Dominant alleles
    S + s       ->     Ss     ->     S
    s + S       ->     sS     ->     S
    S + S       ->     SS     ->     S
    s + s       ->     ss     ->     s
    So the only way to get a plant with smooth leaves is if both parents have the recessive gene s.

    But what happens when it comes to eye color? If we say that for brown eyes we have B and for blue eyes we have b then how can we get blue eyed children from brown eyed parents? For example:

    Code (Text):

    Parent      |    Child     |  Dominant alleles
    B + b      ->     Bb      ->     B
    b + B      ->     bB      ->     B
    B + B      ->     BB      ->     B
    b + b      ->     bb      ->     b
    So the only way for a kid to get blue eyes is the two parents have blue eyes, according to my table. In any other case the kid has brown eyes.

    This ofcourse is wrong as I know families where the parents have brown eyes and some of their children have blue eyes. So where did I do wrong?
  2. jcsd
  3. Mar 6, 2013 #2


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    Each parent has two alleles, with the brown eyed B dominant, not just one as in your table. So the way brown-eyed parents have a blue-eyed child is as follows:

    Mother - Bb Father Bb Possible Children BB, Bb, bB, bb

    The bb combination will have blue eyes. Can blue-eyed parents have a brown-eyed child?
  4. Mar 6, 2013 #3


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    Note that eye colour isn't just determined by one gene. In simple models it can be represented by two genes: one governing brown/blue and one green/blue with dominance being a hierarchy of brown > green > blue but in reality it's more complicated than that. There are a wide variety of genes involved in the formation of the eye that dictate eye colour.
  5. Mar 6, 2013 #4
  6. Mar 6, 2013 #5


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  7. Mar 6, 2013 #6
    Fascinating indeed! I also often think of that couple by the Black Sea 6000 years ago when they first looked in to their newborn baby boys blue eyes and what they would have thought of his and their 1.2 billion direct descendants? Depending were in the world you are they seem to be getting rarer. But here in Ireland were there has been little immigration for over 2000 years blue eyes make up 96% of the population!
  8. Mar 6, 2013 #7


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    What is surprising in the newborn having blue eyes? They all do, eye color changes much later.
  9. Mar 6, 2013 #8


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  10. Mar 6, 2013 #9
    All of them!!! Most Jewish people of Kazakh/Ashkenazi decent such as myself are born with dark brown or even black eyes. Not to mention most African races south of the Sahara. Newborn brown eyes also occur among native Americans/Asians/Hispanics. And yes evenwhite Europeans occasionally!!
  11. Mar 6, 2013 #10
    It is good to know Monique, there are so many superstitions and old wives tales when it comes to eye colour!
  12. Mar 6, 2013 #11
    It is because is a Mendelian trait; if both parents have the b gene, even if they are brown eyed they may have blue-eyed children, as it is recessive. As far as I remember, the percentages are: 25% brown eyed children, 50% Bb, and 25% bb, that is, blue eyed children.
  13. Mar 7, 2013 #12
    Ok, but if each parent has 2 alleles, then we have a total of 4. How are only two alleles chosen from those 4 for the child? Is it a random choice?
  14. Mar 7, 2013 #13


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    One allele is on each chromosome and each parent gives their child one chromosome. It's a bit more complicated than that but is this what you're asking?
  15. Apr 15, 2015 #14
    Ok. I have a question though it appears it's a couple years after your discussion. lol.

    my wife thinks she might be adopted but her parents aren't talking to her right now and have motive to possibly lie. Here is our situation.
    Me/Dad: Brown eyes. Mom has brown eyes, dad, his mother and her mother have blue.
    Her/Mom(my wife): Brown eyes. Six siblings have brown eyes. Mom and Dad and their respective moms and dad ALL have brown eyes. Here cousins ALL have brown eyes.
    We have two kids, a boy and girl. Both with blue eyes. Is this possible if she's really genetically from people with brown eyed parents and grandparents?
  16. Apr 15, 2015 #15


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    Two brown eyed people can have babies with blue eyes.


    My ex fiancee is Sicilian, his parents had dark brown hair and eyes, he has sky blue eyes and had blond hair until his teens when it turned browner. He believes he may have Norman ancestry. Sounds reasonable, he's also very pale.
    Last edited: Apr 15, 2015
  17. Apr 15, 2015 #16
    So the gene leading to blue eyes can be 'recessive' for several generations, not just one or two?
  18. Apr 16, 2015 #17
    The gene(and it isn't just one gene, and it isn't just genes) doesn't know or care about how many generation, or the colour of the parents. If it is there, it is there.
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