Mendelian Genetics

[RandFan]

Hi Folks,

I am having a debate in the politics forum and I need some help.

Based on some article I read that could have easily been incorrect I stated that a group of white people from Scandinavia who moved to sub-Saharan Africa without the benefits of civilization could spawn dark colored offspring with in 6 generations. I can't much find much to substantiate my claim of 6 generations.

In any event.

According to Mendelian Genetics: A person with the alleles AABBCC is very dark skinned, a person with aabbcc alleles is very light skinned, and a person with AaBbCc (or any combo) has an intermediate skin color.

Questions:

Based on Polygenic Inheritance It seems to stand to reason that persons with intermediate skin color could be bred with other persons of intermediate skin color and their offspring bred in such a way as to spawn dark skinned or very light skinned people. Right?

It would also stand to reason that a person with alleles AABBCC or aabbcc could not produce offspring with opposite alleles based simply on heritable variation because there is no variation. Correct?

Is it likely or is it possible that peoples of Scandinavia have sufficient variation to change significantly their skin color by breeding within their own group exclusively?

 

[American]

I believe they are 3 groups of genes, not 3 genes.

Here is the basic pathway for the biosynthesis of melanin, which I pulled directly out of my arse:

In summary: when you can't answer directly, provide confusing useless information, and act like the question and the asker are both crazy.

 

[American]

Thinking about your example- how long have the British been in South Africa? At least 4 or 5 generations, no??? So you'd think they should be turning black by now? Or would they have to revert to spears and face a life of being eaten by lions to speed up selection pressure and turn them all black, all based on the general fact that blacks can cope better with the sun than whites.

 

[RandFan]

Thinking about your example- how long have the British been in South Africa? At least 4 or 5 generations, no??? So you'd think they should be turning black by now? Or would they have to revert to spears and face a life of being eaten by lions to speed up selection pressure and turn them all black, all based on the general fact that blacks can cope better with the sun than whites.

You are correct about selection pressures. The British have homes, cars and clothes to protect them from the elements which kind of negates natural selection.

Also, there must be significant offspring as I understand it. 2 or 3 children are not sufficent numbers to significantly affect natural selection.

 

[Dancing David]

Great question!

But it presupposes that there is a determinism in determining skin color, there would have to be a good reason darker skin color was selected for for there to be a shift in six generations, given an age of breeding of 15 that would be only 90 years, or 120 given an age of breeding of 20.

Not really a very long time for a subtle effect like melenoma to change skin color. Most people with light skin would still survive long enough to breed. Unlike the flu melanoma is a slow killer which allows people to breed.

But take 1,000 years and 50 generations and there might be a small effect pushing the genome to select out light skinned individuals. But even more than melanoma would be possible benefits to survival from the darker skin.

However there are plenty of things that effect survival Other Than Skin Color.

On the 'could a population of white folks eventualy produce black children', the answer is suprisingly yes, first off the general population of Scandanavia is bound to have a large amount of genetic variation, unlike an isolated and inbred population. There are people who are much like myself, white folks who can become very dark upon exposure to the sun (german, english, scotish ancestry). There are other folks who are bound to have very pale skin. But given the way recessive alleles work it is going to be very complex with skin color, there are like nine alleles for eye color. So the chance that a light skinned person actualy carries the dark potential is very high.
And if there is sufficient survival presure then there might be a shift to darker colored skin.

And the number of offspring certainly does effect the selection process, but only if there is known survival benefit.

The Khe San of south africa are 'light skinned' for Africans.

 

[Shane Costello]

Here's a good paper on the subject.

Originally posted by Randfan:
Is it likely or is it possible that peoples of Scandinavia have sufficient variation to change significantly their skin color by breeding within their own group exclusively?

I'm fairly certain that the answer would be no. The Scandanavians have been breeding fairly exclusively within their own group for millennia, since that part of Europe remained fairly unaffected by the countless invasions and population movements that have occurred throughout the continent's history, and as far as I know there hasn't been a noticeable darkening. Jokes about kissing cousins aside, but has there been a noticeable darkening of skin colour in parts of Appalachia?

The paper mentions that skin colour is a complex genetic trait. While there may be x number of alleles directly controlling pigmentation, other genes would be important, such as those encoding regulatory proteins. Alleles of these genes would down- or up- regulate expression of the pigmentation genes. This is known as an epigenetic effect, and the textbook example I've seen cited is that of chocolate coloured Labradors, where expression of the allele or alleles causing black coat colour are down regulated to some degree, leading to an intermediate between fawn coloured and dark coloured dogs.

The hypotheses are endless, since the genetics of pigmentation is complex, not to mention the fact that measurement of the phenotype is problematic. Say we did move a group of Swedes or Danes to the Serengeti, how could we judge how "black" they and their descendants were becoming over time?