Weird Question...?

[Gankfest]

I have a friend who asked me a weird question about science the other day. I'll try to explain this the best I can, as I know little about Genetics.

 

Claim 1: People develop genetic disorders like Hunting's disease because two halves of one gene. Example, two people have sex and both have half the copy of Hunting's, but don't suffer from Hunting's; the chances of the baby having Hunting's is more likely to occur.

 

Claim 2: Having incest sex can lead to a higher risk of genetic disorders; due to the engaging parties having almost the same genetics.

 

Question: If Claim 1 is true, then is it possible as the population rises; the risk of genetic disorders are greater as more people are spreading around genetics by having sexual relations. Creating a Claim 2 scenario effect, but on a larger scale. Kinda like you spread out genetics so far; that it ends up spreading so many copies around, and everyone has a copy of Hunting's. Increasing the risk of Hunting's for everyone.

 

This might sound ridiculous, and I have no idea how I would even look something like that up.I said I would ask somewhere so...

 

[Ringer]

It's a bit more complicated than how this makes it seem. First is that Huntington's disease is defined by probability for some types of mutation of the gene, while some mutations are virtually guaranteed. Also, incestual relationships can risk higher genetic disorders, but that to is based on probabilities and the amount of relatedness (everyone is a bit related). But as the population rises the amount of interrelatedness will decline because there will be more genetic variation to choose from (on average), even though that assumes non-isolation between sub-populations.

 

As for increasing risk of genetic diseases, one of the big causes of that is better medicine and less social stigma (this is a good thing, don't want it to seem like I think we shouldn't help people) so people with genetic disorders are reaching breeding age and having children. But a larger population allows for a better 'cushion' when recessive genetic disorders are present.

[chadn737]

In a large population with random mating (i.e. not incestuous pairings, etc) the frequency of a disease causing allele will not increase unless there is some other factor at work. This is a basic premise of population genetics, part of Hardy-Weinberg Equilibrium. There are other factors that play into this, but lets not worry about those for the moment. Now as the population increases, the total number of individuals with Huntington's will increase, but as a percentage of the population, this will remain relatively stable. That's because as the population increases, you increase the number of non-Huntington's causing alleles as well as the number of alleles that cause it.

 

Now the percentage of the population carrying Huntington's causing alleles can shift if you have non-random mating or there is natural selection at work (we will ignore the effects of migration and mutation as that makes it far more complicated). Humans have a fairly large population, so its less likely that genetic drift will randomly drive up the prevalence of Huntington's, especially as this is such a horrible disease. Now we don't have totally random mating, but there are strong taboos against incestuous relationships in the Western world at least and for all intents and purposes, random mating is a relatively safe assumption in this context.

 

So no, its really rather complicated and it is not likely that Huntington's will increase as a percentage of the population, even as the total number of individuals with the disease increases.

Edited February 16, 2014 by chadn737

[Gankfest]

Ya, that actually makes sense. I forgot about genetic mutation, but it has been so long since I've read into genetics.

 

Thanx!