Comments on: Selection, but for what???

By: Neziha

Neziha — Sat, 01 Mar 2008 07:59:11 +0000

Head lice can only specialize in holding on to one type of hair cross-section (oval for Africans, round otherwise) For instance, in the states African American children generally don’t get head lice. In Africa Europeans don’t.

By: p-ter

p-ter — Sat, 01 Mar 2008 00:02:06 +0000

consider two alleles with a given s and a frequency f. one is recessive, the other dominant. the recessive one has been in the population much longer than the dominant one (on average). thus more time for recombination to break up the haplotype and less signal.

By: razib

razib — Fri, 29 Feb 2008 21:45:26 +0000

i’ll be looking into this case further, but it seems to me that the expectation is strong enough that at this boundary condition we can make inferences based on conditionals. but i could be wrong. perhaps one of the authors of the cited article will stumble onto this post and comment if i am misconstruing their point….

By: agnostic

agnostic — Fri, 29 Feb 2008 21:20:21 +0000

Yeah, I’m probably being too nitpicky about the general case, since you were only talking about a particular allele or two. Partly a reflex after reading about detecting selection every week, and partly to clarify to readers.

By: razib

razib — Fri, 29 Feb 2008 19:30:06 +0000

That paper assumes the allele has fixed. Even then, when selection is weak, degree of dominance doesn’t leave a different signature.
1) selection on OCA2-HERC2 isn’t weak, is it? so that’s an irrelevant point.
2) the derived OCA2 frequently is pretty high in the european hapmap sample, 0.8 – 0.9 range.
3) so you don’t think we can make any assessments of the conditional probabilities of such a long haplotype being selected for due to a trait with a low h?

By: agnostic

agnostic — Fri, 29 Feb 2008 19:24:39 +0000

I’m talking about finding a signature in the data people are using, where anything with minor allele freq greater than 5% gets looked at, and where the selection coefficients are mostly unknown.
That paper assumes the allele has fixed. Even then, when selection is weak, degree of dominance doesn’t leave a different signature.
Maybe there are ways of getting around these things, but it seems like the different labs would’ve already thought of and done something about this, since the results would be very interesting.

By: razib

razib — Fri, 29 Feb 2008 18:40:52 +0000

Probably due to their being the result of their being one of the first waves of migration out of Africa (back when all humans had curly hair) and then subsequent mixing with straight-haired european colonials a few thousand years later.
no. there are studies which check abo genetic (not many, but enough). the unmixed ones are unmixed.

By: razib

razib — Fri, 29 Feb 2008 18:33:07 +0000

I can understand that. The skin colour isn't stable, especially here in Northern Europe, where the amount of sunlight varies so much over the year. Even darkest Africans get paler during the winter. they use unexposed regions. Which ones? yes, melanesians and negritos. I don't think recessive vs. dominance leave different signatures in haplotype length. Directional Positive Selection on an Allele of Arbitrary Dominance: "As can be seen, both reach 0 faster for smaller h. For example, for these parameters, the means of these statistics 18 kb from the selected site are ∼0 when h = 0.1, but they are still negative 40 kb away for h = 0.9. This finding suggests that, all else being equal, it will be more difficult to detect a selective sweep if the beneficial allele was recessive."

By: Hayden

Hayden — Fri, 29 Feb 2008 17:50:43 +0000

Aboriginal Australians seem to range from straight to mild curl

By: agnostic

agnostic — Fri, 29 Feb 2008 16:22:04 +0000

I don’t think recessive vs. dominance leave different signatures in haplotype length. The prediction is that most of these sweeping alleles are dominant, as you point out, but no one knows whether they are or not. And since many labs have worked on this problem, they’d all have to be overlooking the signature you’re talking about.

By: Sven DiMilo

Sven DiMilo — Fri, 29 Feb 2008 13:07:37 +0000

When in doubt, invoke sexual selection!
Problem solved…just so!!

By: toto

toto — Fri, 29 Feb 2008 11:09:41 +0000

many out of africa groups have very curly hair. Which ones? Except for some "Arabs" (in a wide sense), pretty much everybody I can think of has flat hair. This includes "black" people living in hot climates such as Southern Indians. Aboriginal Australians seem to range from straight to mild curl. Ironically, the only exception I can think of are Melanesians - supposedly the most "derived" group on Earth... Who did I miss?

By: Lassi Hippeläinen

Lassi Hippeläinen — Fri, 29 Feb 2008 10:42:39 +0000

“I have been having a difficult time getting specific reflectance values from the researchers who reported the complexion trend; they simply binned their observations into 3 categories (fair, medium and olive).”
I can understand that. The skin colour isn’t stable, especially here in Northern Europe, where the amount of sunlight varies so much over the year. Even darkest Africans get paler during the winter.
Hair colour isn’t stable either. I was born a blonde, but by my teens my hair was medium brown, and now it is slowly turning to transparent. (Or maybe it is falling off – how does baldness relate to these things?)
The same goes for eyes. My sister is a blue-eyed blonde. Her two sons were born blue-eyed, but after a few months they had the brown eyes of their father.
Obviously there are many things going on. Maybe the genes only define a tendency of a colour, and there are some other genes that define how quickly the tendency gets activated.

By: razib

razib — Fri, 29 Feb 2008 09:43:14 +0000

many out of africa groups have very curly hair.

By: Hayden

Hayden — Fri, 29 Feb 2008 09:09:15 +0000

Interesting post. The simplest explanation I can think of is that the straight hair phenotype could just be a by-product of normal variation/drift mixed with the genetic bottleneck that happened as we ventured out of Africa. This bottleneck has been used to explain a bunch of different things, like the high frequency of deleterious mutations in Europeans, so surely we can just throw this one into the mix as well.