Monday, February 26, 2007

The Genetics of Eye Color

The genetics of blood type is a relatively simple case of one locus Mendelian genetics—albeit with three alleles segregating instead of the usual two (Genetics of ABO Blood Types).

Eye color is more complicated because there's more than one locus that contributes to the color of your eyes. In this posting I'll describe the basic genetics of eye color based on two different loci. This is a standard explanation of eye color but, as we'll see later on, it doesn't explain the whole story. Let's just think of it as a convenient way to introduce the concept of independent segregation at two loci. Variation in eye color is only significant in people of European descent.

At one locus (site=gene) there are two different alleles segregating: the B allele confers brown eye color and the recessive b allele gives rise to blue eye color. At the other locus (gene) there are also two alleles: G for green or hazel eyes and g for lighter colored eyes.

The B allele will always make brown eyes regardless of what allele is present at the other locus. In other words, B is dominant over G. In order to have true blue eyes your genotype must be bbgg. If you are homozygous for the B alleles, your eyes will be darker than if you are heterozygous and if you are homozygous for the G allele, in the absence of B, then your eyes will be darker (more hazel) that if you have one one G allele.

Here's the Punnett Square matrix for a cross between two parents who are heterozygous at both alleles. This covers all the possibilities. In two-factor crosses we need to distinguish between the alleles at each locus so I've inserted a backslash (/) between the two genes to make the distinction clear. The alleles at each locus are on separate chromosomes so they segregate independently.*


As with the ABO blood groups, the possibilities along the left-hand side and at the top represent the genotypes of sperm and eggs. Each of these gamete cells will carry a single copy of the Bb alleles on one chromosome and a single copy of the Gg alleles on another chromosome.

Since there are four possible genotypes at each locus, there are sixteen possible combinations of alleles at the two loci combined. All possibilities are equally probable. The tricky part is determining the phenotype (eye color) for each of the possibilities.

According to the standard explanation, the BBGG genotype will usually result in very dark brown eyes and the bbgg genotype will usually result in very blue-gray eyes. See the examples in the eye chart at the lower-right and upper-left respectively. The combination bbGG will give rise to very green/hazel eyes. The exact color can vary so that sometimes bbGG individuals may have brown eyes and sometimes their eyes may look quite blue. (Again, this is according to the simple two-factor model.)

The relationship between genotype and phenotype is called penetrance. If the genotype always predicts the exact phenotpye then the penetrance is high. In the case of eye color we see incomplete penetrance because eye color can vary considerably for a given genotype. There are two main causes of incomplete penetrance; genetic and environmental. Both of them are playing a role in eye color. There are other genes that influence the phenotype and the final color also depends on the environment. (Eye color can change during your lifetime.)

One of the most puzzling aspects of eye color genetics is accounting for the birth of brown-eyed children to blue-eyed parents. This is a real phenomenon and not just a case of mistaken fatherhood. Based on the simple two-factor model, we can guess that the parents in this case are probably bbGg with a shift toward the lighter side of a light hazel eye color. The child is bbGG where the presence of two G alleles will confer a brown eye color under some circumstances.

*If the two genes were on the same chromosome this assumption might be invalid because the two alleles on the same chromosome (e.g., B + g) would tend to segregate together. Linked genes don't obey Mendel's Laws and this is called linkage disequilibrium.

62 comments:

  1. All very well and good, but when do we get the answer to the ABO puzzler?

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  2. I really dislike leaving a comment that's only about a typo, but since you are a real stickler for detail....

    "...so I've inserted a backslash (/)..."

    should be: "I've inserted a slash"

    Thanks for these posts on simple topics. I'm learning/remembering a lot due to them.

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  3. Hmmm, so where do grey eyes fit in the picture? I'm guessing bbGg or something similar? "Blue", but with enough contribution at the G locus to skew it into something decidedly non-blue?

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  4. Linked genes don't obey Mendel's Laws and this is called linkage disequilibrium.

    well no, that's called linkage. linkage disequilibrium is a population level association of alleles (which may or may not be due to physical linkage between loci).

    also, you might be interested in this article
    http://www.journals.uchicago.edu/AJHG/journal/issues/v80n2/44159/brief/44159.abstract.html

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  5. p-ter, I'm going to cover the paper you refer to in a posting on the OCA2 gene. This article was originally intended to set up the one on the gene. Sorry for the delay but it's proving to be more complicated than I anticipated.

    You're right about the difference between linkage and linkage disequilibrium. I used the more fancy term because it often comes up in our discussions about population genetics, evolution, and sex.

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  6. Sigh, I guess I'm going to remain puzzled. Both of my parents have distinctly blue eyes, my Mom's blue-gray, Dad blue with a tiny amount of gold toward the middle. Both of my older brothers have very blue eyes. Me? My eyes are hazel. A slate-blue background with a definite netting of mid-brown over the entire iris, usually appearing green or light brown. I'm definitely a product of both parents (I resemble Dad strongly), so where the heck did my eye-color come from?

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  7. well Perpetual Beginner, you may find the answer to your question in this site http://www.genetree.com/about/eye-color-chart.asp

    I have a question: every one in my family has brown eyes, but some of us are lighter than others, like my uncle's eyes are almost hazel and mine are lighter than my some of my siblings, I will marry someone with very light blue eyes so will all my children have brown eyes or is there a chance for some green or hazel?

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  8. Thank you for this information. My husband and I both have hazel eyes. His are almost entirely green and mine are almost entirely blue. We have one daughter with grey-blue eyes (one pediatrician asked me if her eyes were violet one day) and my youngest daughter has deep brown eyes like the color of whiskey with no signs of hazel whatsoever.

    My husband and I are both redheads too. I've never seen a brown-eyed red-head. It's pretty sweet.

    Anyway, my Mother-in-law has made a few wisecracks over the years regarding my five-year-old's true paternity. It's hard to argue with someone when you both have the basic knowledge of the Punnett square. We were all brought up with the knowledge that brown always dominates blue.

    Thanks again,

    Tammy

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  9. OK I have 1 Blue and 1 Brown eye and my husband has Blue eyes. My mother has Green eyes my father Brown, Mother in law Brown Father in law Brown. First of all how did my husband get Blue eyes and what color would our child's eyes be. And how did I get 1 Blue and 1 Brown, and they are very distinct very noticeable. Help!
    Tracy

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    1. Tracy, you absorbed cells from a twin. Actually you are your own twin, a human chimera.

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  10. I have a question:
    My father has deep blue-green colored eyes, they almost look teal. My mother has very deep blue eyes. My brother has the same as my mom's, so how did I get rainbow eyes? My eyes are brown, green with a little yellow, blue, and a gray ring around the outside.

    Also, my fiance has light blue eyes. What color eyes will my children have?

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  12. To Anonymous...

    If a person has two different eye colors this is called heterochromia. This is considered abnormal and may be pathological. If a person has always had two different colored eyes, then this is called congenital heterochromia and is cause by a difference in the early stages of development of the iris. This is rare, but nothing to have medical concern over. If the heterochromia is acquired after time then there is reason to believe that something has happened within the body to create changes in melanin synthesis or degradation. This can be caused by diseases in the eye or by certain types of medicines.
    from: http://www.sewanee.edu/chem/Chem&Art/Detail_Pages/ColorProjects_2003/Guttery/index.htm

    I bet your eyes are amazing!

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  13. My mum's got brown eyes and my dad's got blue eyes. I have blue eyes. I know my dad's my dad because I get my blue eyes from him, obviously. So what happened to me? I guess I'm just a freak of nature!

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  14. my mom has green-blue eyes and my dads eyes are brown, but my left eye is darker blue than my moms eyes and my right eye is brown on one side and blue or green (depending on the day) on the other side. how is that explained?

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  15. I am African American mix. I have brown eyes, but depending on the light they have a true violet what I call afterglow. Many people have noted this in my eyes. Where does this particular eye color spring from and what is the science behind it.

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  16. Excellent. I've always found Grey a questioning iris color, may you should approach the topic and blood type for that matter, it would be a fantastic sbject!

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  17. Most all of my siblings posses the color light blue, like my dad. My 2others have been blessed with my mom's eye color which is medium Green, Me? I have light Bluish/Green with a dark Blue ring around the Iris. Is it true eye color has to do with descent? Sorry, but this question has been bugging me for a while because my dad is from Irish descent also and it's likely that Blue is big beside Green in Ireland. True or False?

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  18. This is awesome! I plan to look way back, or maybe just to my parents, I'm pretty lazy

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  19. I am black not mixed. my eyes are hazel almost green and my brothers are grey. How is that explained.

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  20. I have one light grey eye and one crimson, I was wondering why because nearly evryone in my family has blue eyes. How did i get such different one when they were blue when i was younger? Could it have something to deal with the fact that i was blind as a baby?

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  21. That´s funny. I used the first figure in your blogpost to demonstrate quantitative genetics to students. I numbered the eyes 1-9 from blue to brown (That is not entirely accurate, but it works fine for this purpose). Then I made the students score their own eye color and that of their parents. By regressing the students´ score unto the midparent value you obtain the narrow sense heritability. About two-thirds of the variation in eye colour turned out to be heritable. Not bad.
    This example is especially nice, because you can also debunk the misconception that it is a single gene trait. One of my great teaching moments :-)

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  22. My mum has dark brown eyes, my dad has light blue eyes, mine are green. no one else in my family have green eyes.

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  23. Very interesting that this subject arouses so much interest after a couple of years past its posting date. What a shame there are sooo many blog sites on the web that I'll probably never find this one again!

    I am one of those with blue eye genes as far back as Adam and Eve, as is my wife's history, but my Dad has black hair, my half brother has black hair, his son has black hair, and everyone else descended from my Dad has blonde or light brown hair. Doesn't quite match the dominance theory.

    Of even further interest is that my grandfather on my mother's side had a distinct blond streak in his light brown hair which I have inherited and one of my sons has some evidence of the same, but less defined and more widely distributed.

    So going to the one dark and one blue eye poster, note that this happens with hair as well, as my grandfather, myself and one of my sons, are all both brown-haired and blond!

    What I haven't inherited is my Grandfather's baldness which occurred in early thirties and I have a mop of hair in my early 40s which even the Bank of England can't afford to keep short!

    Hence there are many anomalies which even the best scientists of today can't quite get to grips with - probably because there are simply too many variables and relationships between them for us to process yet.

    Doug

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  24. I have a 5 month old with gray/green eyes. Im african american female. My husband is as well. My eyes are dark brown his eyes are light brown. Both our parents and grandparents have light to dark brown eyes. Can someone explain?

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  25. I have brown eyes and my wife has hazel eyes. Our 3 month old son has blue eyes and they don't look like they'll be changing. How is this possible? (no smart alec comments please) There are lots of blue eyed people in her family, but not either of her parents. There are only 2 blue eyed people I know of in my family and they are 2nd and 3rd cousins.

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  26. to the person with the blue eye i'm just a student i think that the blue eyed trait is resesive to you or your wife.


    (HI MR.B if u read this)

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  27. My Father had very blue eyes and my Mother pale grey /green eyes. Three brothers and sisters are all blue eyed and I have dark brown eyes. How would my brown eyes derived from such a mix. My Father would have been (guessing) bb/gg and my Mother may have been bb/GG. Where did my dark brown eyes come from? Any suggestions - scientific possibilities that is. GF

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  28. hi sandwalk...i love reading your articles..im a biology teacher and im very interested of your articles...

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  29. i do love genetics..i wish i could meet you...

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  30. I don't really like the Punnet Square, dihybrid cross that you made. Since eye color is incomplete dominance, there are 16 main variations of eye color. There are also other things that affect what color eyes you have that cause things like a different color spouting from the center. Eye color is a tricky concept to deal with, so I would be more cautious to what you teach people

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  31. in my biology class I learned what is the way to find out the color of eyes of a child .. I love this theme, it's very odd to realize the magic of life!

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  32. i have dark brown eye which seem almost black, i that just rear or i it abnormal?

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  33. i have dark brown eyes which seem almost black, is that just rear or is it abnormal? (sorry for the earlier pot my keyboard topped working)

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  34. i have semi light brown eyes and my husband has blue.. both his parents have blue and my parents have light brown and brownish green eyes. what will mine and my husbands kids have?

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  35. Wow! What causes eye color to change? My eyes changed all the time from blue to hazel to green and back again until I was about 10 or 12. Since then they have been green, with no more changes.

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  36. A very informative post.My teacher asked me to research on this and i found your post really easy to understand.Thanks :)
    Nikhila,India

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  37. I also remember learning in genetics at university the lady who went to see if her children were compatible to donate her their kidney. It was found her DNA did not match her children whatsoever. Further analysis found the DNA from her donated eggs to be different from her normal cells. While in the womb she had absorbed her non-identical twin and ended up inheriting the DNA in certain areas including egg production. This would be a situation when both parents could be recessive while the child could have a dominant phenotype.

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  38. This is a very good model if two genes are involved and it's important to remember you won't really know your and your partners actual inheritance just by looking at your eye colour.
    I actually think for eyes, there is going to be several genes involved and this is my theory on eye colour based on what I have learnt at university and observation:
    While the baby is developing cells receive information to determine their role. As many people have a different colour on the inner and outer part of the iris there may be two different genes determining the level of expression depending on the location. At this point, a concentration gradient would be determined but the actual colour will not be seen until later.
    It is not until the baby is born and the pigment is transcribed from the DNA the colour will actually be seen. Now we know there are blue eyes, brown eyes and green eyes and a multitude of colours that seem to fall in between. Brown is the original and most dominant pigment. Blue eyes originated from a genetic mutation so no pigment is expressed and eye colour remains the same blue as a new born. This makes brown eyes dominant because if one parent does not have the information to produce the pigment (blue eyes) the child will get the information from the other parent. As not as much pigment is produced, the child will have paler brown or hazel eyes.
    There are also green eyes, which may be a separate gene altogether, or I would predict it to be a different version of the blue eye mutation. This would mean pigment is still produced but it is a slightly different form so the colour green is reflected instead of the dark brown. In this case it’s important to remember you can only inherit one gene from each parent, so if you have green from Dad and blue from Mum you won’t have brown but then a very dark green and dark blue together may appear dark enough to be brown.
    Now we do see people with bright blue eyes, a lot darker or brighter than a new born baby which is possibly due to a separate gene. The presence of an enhancer gene would affect the amount of pigment produced, a positioning gene may affect how the pigments align with each other or a protective type of gene may affect how the light is reflected as it hits the eye. How this gene works may be inherited and may be affected by the environment, such as a response to UV light.
    As we age different genes in each cell are switched on and off depending on where you are at in the life cycle such as the genes determining the eye development in a foetus may never be switched on again in later life. It is different genes to switch on causing the eye colour you see. For some people this may also have an affect on the pigments at different stages as will environmental effects such as UV light.
    If, for argument sake, we say there are 6 genes involved in eye colour, the model would be a lot larger. This gives 64 possible combinations from each parent. A 64 by 64 model means inheritance is out of 4,096. This would explain why there are so many variations and while two dark eyed parents are more likely to have a dark eyed child, two light eyed parents may still have a darker eyed child.
    This is just my theory and I welcome any feedback.

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  40. I'm doing a project on eye color and this really helped me out. I thank you a lot, reading this will help me on my test coming up as well. --JC

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  41. Does this mean everybody contains some sort of G or g allele?

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  42. I am still confused. My mom has green eyes and my dad has hazel eyes. I have brown eyes. Where did I get this trait from? My dad's hazel eyes are more brown than green by the way. In fact, some people may even argue that they are brown!

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  43. Oops. Sorry, my mom told me that my dad has brown eyes but some people seem to think they are hazel.

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  44. My husband is Japanese with brown eyes (obviously and I am mostly German with blue eyes. Our daughter has blue eyes. If brown is dominant, how is blue expressed in my daughter?

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  45. Sorry this is making me be anonymous - I previously posted as katspj, anyway...
    Some ethnic group dark eye colours are very interesting because the colour is expressed from birth. While this model eye colour is based a baby being born with blue eyes and if the dominant brown eye gene is present it is expressed sometime during infancy.
    In some ethnic groups, usually when the colour is so dark almost black, the eye colour is expressed from birth. This gene is active long before the brown eye genes referred to in this model.
    In this case, sometimes the pale eye gene may prove to be more dominent. When the babies eyes are being formed in the womb, the gene is only be active on either the chromosome inherited from the mother or that inherited from the father, not both. This can occur when genes include a "bar body". This is like a little enzyme the goes and binds to one chromosome so it is not active. In this case a gene won't be active until it's bar body is bound. Whether the pale eyes or the dark eyes are dominant can vary from one couple to the next as it is dependent on how efficiently one genes "bar body" works against the opposing gene.
    I hope this makes sense :-)

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  46. Hi, My question to you is it possible to have the same mother and father if my two siblings have different coloured eyes to me... eg. my oldest sister has blue eyes, my middle sister has brown eyes and I have green eyes?

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    1. As shown by the punned square, in that eg you could have 16 siblings with lots of different eye colous

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  47. Very possible. Even in the basic model if only one set of genes determined eye colour you could get a brown a green and a blue in each of the offspring.

    Lets say Mum had brown eyes (B) and carried the gene for blue eyes(b) and Dad had green eyes (G) carrying the gene for blue eyes (b). So Mum has genes Bb will give each child either the brown eye gene or the blue eye gene and Dad has gene Gb so he will give each child either the green eye gene or the blue eye gene.

    Children will inherit the following combinations
    Bb - Brown eyes
    Gb - Green eyes
    bb - Blue eyes
    BG - this is an interesting one likely to be Brown, dominant over green or a very dark green.

    As I have said in a previous post; there are more genes than this involved so it is a lot more complicated so there are a lot more variations but this does give an example of how one gene can cause 4 variations.

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  48. I would really like an answer on the mystery behind my eye color. I am of 100% Irish heritage and have eyes that can only be described as yellow. There are hints of green around the edges, but the center of the iris is almost completely golden and extremely light-colored. My eyes are photosensitive and I have to wear glasses outdoors at all times. I am married to a british man with hazel eyes. Both of our children have sky-blue eyes. My mom had brown eyes and my father had blue. Is there a way I can send a picture of my eyes to you? I had one doctor suggest it might be some sort form of albinism...I didn't really buy that, considering I have brown hair.

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    1. I can't possibly answer your question without a lot of information about the genes (alleles) you carry and those of your biological parents. Neither can anyone else.

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    2. My mother has dark brown eyes and brown hair. My father has light brown hair and sky blue eyes. My maternal grandmother had blue eyes, as did my paternal grandmother. I believe my maternal grandfather had brown eyes and my paternal grandfather had bright blue. Almost everyone in my family has brown hair, spare my maternal grandmother, who was a redhead. My daughter is a redhead and I am worried that she has inhereted my eye trait, though it is too early to tell (she is 6 months). My son has sky blue eyes and brown hair.

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  49. From katspj

    To Melanie Dodd
    Albinism is when a pigment is not expressed and it is possible this occurred only in your eyes. Maybe you inherited a pale brown gene from your Mum but the pigment gene from your Dad was suppressed. Remember brown is dominant so your Mum will also carry a gene for another colour but you won't know what this colour is and this is possibly the gene you inherited. It may be still possible the eye genes inherited from your Mum and Dads still gave you the yellow colour without the albinism trait.
    Although lacking pigment genes is also likely to cause your eyes to be more sensitive.
    Your children can also inherit the blue eye gene from you without inheriting the cause for albinism.
    As Laurence A. Moran said, a lot more information is required to an exact answer but I hope you feel satisfied with this as a feasible explanation.

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  50. Dear Sir, myself and my husband are both Indians and I have dark brown and my husband has light brown eyes. Our parents have similar, although my mother has honey coloured eye balls. our first child has dark brown eyes and the second one has grey/blue eyes. she is one year old now and the color did not change. Should we be worried if there is some defect? if so what are they. What can we do to test at home to see if everything is normal?
    Please help.
    Thanks in advance
    Prathusha.

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  51. I have distinctly Green/Hazel eyes. My mother has dark brown eyes (almost black in color, they are so DARK brown!). My father had the most beautiful, clear, steel blue eyes that I have ever seen in my life. So, is it at ALL possible that my Green/Hazel eyes came from my parents? According to your chart, it is not even possible.

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  52. as an spanic brown eyed man aby chance to have blue eyed children with red haired woman?

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