Spread of recessive genes

[wobblycogs]

Hi,

This is a question I've wondered about on and off for years so I hope someone can help me

As I understand it the gene for blue eyes either is or acts very much like a recessive gene so if two parents are carriers then 25% of their children will have blue eyes and 50% of the children will be carriers of the blue gene. Everything else being equal in seems that the gene for blue eyes will slowly spread through out the whole population as it doesn't cause carriers any problems with reproduction and in fact that seems to be what is happening.

Assuming the above is broadly correct I don't understand how very rare conditions like trimethylaminuria (http://en.wikipedia.org/wiki/Trimethylaminuria) which apparently also only affect a single gene, are recessive, and cause no reproductive problems (other than the obvious one) don't spread throughout the whole population? There must be something that is limiting the spread of the gene causing the condition otherwise huge numbers of us would be wandering around smelling if fish!

Many thanks to anyone that can point me in the direction of a solution.

[aptitude]

Let the recessive allele be a.

So you said that when a heterozygote or a homozygous recessive is crossed with another heterozygote or a homozygous recessive, it can produce aa. However, don't forget that a homozygous dominant or a heterozygote with another homozygous dominant or a heterozygote can produce AA. Because of Hardy-Weinberg equilibrium, the gene frequencies don't change, and the phenotype frequencies remain constant.

[wobblycogs]

Thanks for that. Just had a read about the Hardy-Weinberg formula and now I see why rare genetic mutations such as the one that causes trimethylaminuria don't spread. In fact if anything I would expect them to slowly die out as there is a mild selective pressure against them.

That raises a different question though. According to Wikipedia the mutation that produces blue eyes appeared around 6000 to 10000 years ago. In order for that allele to spread it would have to confer some benefit to the host but as far as I can see it doesn't make much, if any, difference. How come we have ended up with countries like Estonia where 99% of the population have blue eyes? Also, while 10000 years is quite a long time in human terms in evolutionary terms it's not that many generations yet the gene has spread throughout Europe. That makes me think it must provide quite a benefit to the carrier.

The best advantage I can come up with for blue eyes is that in snowy surroundings having less pigment seems to be an beneficial for both hunting and hiding. I don't feel that explains the spread though as there are plenty of people with blue eyes that live in warmer climates where snow is rare.

[aptitude]

I don't think that blue eyes confers any benefit to the individual.

The reason why Estonians would have blue eyes is due to the founder effect. In the past, a blue-eyed human, or a group of closely related humans with blue eyes, have probably wandered to present-day Estonia, and through successive generations of breeding with other individuals in the same area, the blue-eyed allele has become the prevalent allele in Estonia.

The reason for why the blue-eyed allele is more common than you would think today is probably due to inbreeding. Individuals with blue eyes are probably somehow related, and since in the past mating has occurred between related individuals (for example, within Estonians), this causes inbreeding. Inbreeding increases the percent of individuals that are homozygous recessive, and decreases the percent of heterozygotes (this is also why inbreeding increases the incidence of homozygous recessive diseases).

Hope that helps.

[Darby]

There was an article last year connecting the blue-eyes allele to the lower melanin in Northern-adapted human populations, so there may be some selection on it.

[zombiesagan]

As Darby alluded to, I was going to remind you that just because a trait is present doesn't necessarily mean that it has some direct evolutionary advantage; it could be that blue eyes are linked to another evolutionarily advantageous gene. Aptitude also mentioned the founder effect, which is another common cause. It could also just be due to random selection (genetic drift), although I don't think that applies in this instance. I could be wrong though. The point is that a trait doesn't have to offer direct advantages to occur in a population.