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Why is skin color analog while eye color is digital?
February 15, 2010 1:05 PM   Subscribe

Parents with different skin colors end up with kids who have an intermediate skin color. Parents with two different eye colors end up with kids who have one color or the other. Why is that? Which traits are passed on as a mixture and which traits are passed on as a choice of one or the other?
posted by HappyEngineer to Science & Nature (8 answers total) 11 users marked this as a favorite
 
Parents with two different eye colors end up with kids who have one color or the other

It's actually a lot more complicated than this. My mother has green eyes. My father has hazel eyes. I have dark blue/gray eyes - definitely not "one color or the other".

Your question is really the first step into the wonderfully complex field of hereditary genetics.

Our very initial understanding of genetics came from an old-time monk named Mendel. This seems like a nicely basic-but-thorough primer on Mendelian genetics. Once you know the basic terminology, it will be easier to digest Moran's short blog post on the genetics of eye color. It also explains the limitations of Mendelian genetics, and why they don't really work for explaining skin color OR eye color inheritance.
Eye color is more complicated because there's more than one locus that contributes to the color of your eyes... This is a standard explanation of eye color but, as we'll see later on, it doesn't explain the whole story...
Most heritable traits (including skin color) are very complex, and controlled by multiple genes that are inherited from either the mother or the father. Add environmental agents on top of that and the picture gets very muddles.
posted by muddgirl at 1:18 PM on February 15, 2010 [4 favorites]


Part of what is going on here is the difference between a (seemingly) Mendelian inheritable trait, and a trait that is controlled by far more than one gene in its inheritance.

Human eye color was for a very long time believed to be a Mendelian inheritable trait, and its rules do very closely follow the rules of a trait governed by a single gene. In actuality, it is governed by a series of single-nucleotide polymorphisms. However, there do seem to be a very small number of these SNPs that govern human eye color. In fact, using just six, you can usually predict eye color between blue and brown with 90% accuracy. Thus, on the surface, human eye color is a relatively simple inheritable trait.

Human skin color, on the other hand, is by far more complicated in terms of genetic heritability. For one, there are two types of melanin produced in the human body - pheomelanin (red) and eumelanin (very dark brown). Both amount and type are determined by four to six genes which operate under incomplete dominance. However, the production mechanisms for these two chemicals are governed by a different set of genes, and on top of that, gender plays some role in which types express themselves more or less. Different areas of our bodies also produce more or less of these chemicals, and various mutations in these mechanisms also exist.

Case in point, variation of human skin color is not always a mix, either. Take for example the many documented cases of twins of different skin color.
posted by strixus at 1:20 PM on February 15, 2010 [4 favorites]


This is what's going on in my family.

Dad: Pakistani/Indian
Skin color: Beige-ish brown, skin color resembles Gandhi's tone; yellow undertones
Eyes: Medium-dark brown (noticeable difference from pupil, but still very brown)

Mother: Italian
Skin color: very fair, pale, with pink undertones
Eyes: blue

Me: Hybrid Vigor
Skin: Quite pasty. While I'm definitely very fair, I notice that I've definitely inherited yellow undertones from my dad
Eyes: hazel (olive green and golden brown)

My 3 siblings have some different combinations. One has blue eyes, the rest of us have hazel. One has more pinkish undertones, the rest of us are pasty but yellow undertones.

I could go further with my sisters' kids and husbands (they married two half Turkish/half Egyptian brothers, so there's some interesting stuff going on there when looking at inherited traits).

I'm not a geneticist and don't know the specifics, but I think people are rarely whole anythings. My Indian paternal grandfather had grayish eyes. My Italian maternal grandfather had strawberry-blond hair. As far as I know, both were "from" their remote regions for many many many generations. Genes are weird. Being mixed produces a lot of funky results that are way more complicated than "one or the other" or as a mixture.

For fun, here's what Russell Peters has to say on the topic of mixing.
posted by raztaj at 1:31 PM on February 15, 2010


I have read that if both parents have blue eyes the children always have blue eyes?
posted by SatansCabanaboy at 1:39 PM on February 15, 2010


I have read that if both parents have blue eyes the children always have blue eyes?

Nope. I have noticeably green eyes. Mom and Dad both have blue eyes.

"The once-held view that blue eye color is a simple recessive trait has been shown to be wrong. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur." via
posted by misha at 1:58 PM on February 15, 2010


An enjoyable book on the complexity of some traits is Cats are Not Peas, which goes from nice straightforward Mendelian genetics (peas) to some more complex cases (calico cats).
posted by hattifattener at 2:13 PM on February 15, 2010 [1 favorite]


"The once-held view that blue eye color is a simple recessive trait has been shown to be wrong. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur."

Ooh I hope so, I'd love my kids to have blue eyes like my fiance's parents. He got hazel eyes and mine are plain old brown (as does my entire line).

Off the top of my head, things that are inheritable directly through your mother or father:

- Rolling your tongue
- detached ear lobes
- cleft chin
- color blindness (might be wrong about this one)

Please note that most of my knowledge has come from crime shows.
posted by like_neon at 5:05 AM on February 16, 2010


I have hazel eyes (the brown green type) and my husband has hazel eyes (the blue-green type.) Two of our children have blue eyes.
posted by St. Alia of the Bunnies at 6:37 AM on February 16, 2010


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