Quote: (11-14-2012 07:28 AM)Thomas the Rhymer Wrote:
A lot of people are shouting that blondness, blue-eyedness, and paleness are genetically recessive.
Where's the evidence for this?
Here,
here,
here and
here.
You're correct to note that eye-color is a
more complicated subject than the simple "
Brown dominant, blue recessive" narrative (this allows for one to express colors that lie between these two on the color spectrum). There are a number of other genes that can influence the shade of the expressed color, and cause folks with purely recessive traits (blue eyes) to produce brown eyed children or to cause the expression of other eye colors (grey, green, hazel, etc).
The fact that lighter eyes/features are generally recessive, however, still remains, even if the larger story is more complicated. Their expression requires the presence of said "lighter" genes in both parents, which allows for the genetic interaction/combination to take place and pass them on to the child. If one parent completely lacks that genotype (read: genes code only for brown/dark brown eyes), you will not get light features
barring a mutation.
In short, lighter features require a genetic presence on both ends of the parental spectrum in order to be expressed. Darker features need only be present on one side. If even one end lacks the ancestral/genetic presence of lighter features, you will not see those lighter features expressed in the offspring. This is why they're recessive.
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As far as I remember, eye colour is a bizarre mish-mash of genes that does not follow mendellian inheritance. In my own family, my father has green eyes, my mom has blue eyes, I have grey eyes and my brother has brown eyes. The eye colour of a baby born to parents with mixed eye colours can be anything.
You are correct, assuming that the baby's parents do in fact possess mixed eye color to begin with or have inherited the genes to code for said colors from ancestors.
If one of the parents does not possess this mixed eye color and does not have it in his/her ancestral history, you will always get brown eyes, even if his/her partner has the genes for lighter eye color. This is why said brown eyes are considered a "dominant" trait-in the absence of a heterogenous genetic picture in
both parents, they will always take the lead (again, barring mutation).
Your own phenotype (grey eyes) is only possible because you had two parents who possessed the genes necessary to code for lighter eye color. Had one of your parents lacked this history (i.e. say your father came from a population/genepool with no lighter features in its genetic history and no recent admixture from fairer populaces to throw them in), you would be brown eyed, though you would probably carry the traits for lighter features (inherited from mom) and have the ability to pass them to your own offspring.
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So this myth that whiteness is recessive needs to stop.
Check out this article on mixed race twins with pictures:
http://www.thesun.co.uk/sol/homepage/new...twins.html
The one twin came out black and the other twin came out white, how do you explain that as a recessive vs dominant thing?
This isn't hard to explain. Both parents possess recessive traits, and these traits combined in such a way as to give each child a different phenotype. The lighter features in two of those children are only possible because the Black father in question likely carries some of the code for lighter features in his own genetic history (read: he had fairer caucasian ancestors whose genes have been passed down the line to him). For lighter features to be expressed, children must inherit a combination that is only possible if the "light" codes exist in the genetic history of
both parents.
That was the case here. It isn't uncommon for mixed couples like this (or any couple with partners who posses some genetic biodiversity) to produce several children who have different shades/ Many black diasporan families (including my own) have seen this. The only unique thing here is that each twin inherited a very different combination, and that this occurred twice.
If the father here had lacked this diverse genetic history (as individuals from within some African and Asian populations without recent European admixture tend to do), then what you see there likely would not have happened. The children would
all be much darker, because those features (brown eyes, dark hair, dark skin, etc) are dominant and only need to be present in one parent in order to take a phenotypical lead. Their lighter counterparts cannot express themselves without being present on
both sides of the parental equation.