This is what a friend of mine said: When there\'s human offspring, it will look

Question

This is what a friend of mine said:

When there's human offspring, it will look a rough 'medium' between the two parents, with it sharing features from the two. well if the offspring of one set of parents has sex with the offspring of another set, the outcome becomes a 'medium' of those two. So the range from what we shall call 'completely average looks' (a hypothetical state of the average of everyone in the world, which is acceptable as we're looking at a process of millions of years) becomes less and less the further into the future we go. Eventually, there is almost no range between people and 'completely average looks'.

In summary: take the hypothetical average of everyone in the world, and because the offspring of two people is a slight mixture of those two people, they are a tiny bit closer to the hypothetical average of everyone in the world, which means that eventually people will be getting closer and closer to this average, so will one day look the same. So he's suggesting that slowly people will become more and more similar.

My argument is that, although this baby may be that tiny bit closer to the overall average, they're new look has effectively altered the average, meaning that they aren't actually getting closer to the average.

I'd like an opinion and flaw from some experts, such as yourselves, so please...

Explanation / Answer

Quantitative genetics is one reason why this doesn't happen. For example height is affected by many genetic loci, not just 1 gene, and we can for the sake of the following illustration call them Locus A through Locus J - 10 Loci.

Now this is purely illustrative and hypothetical - in actuality there are some differences between this example and the real world, but nevertheless I shall plough on. Imagine everyone carries a basic set of genes which makes them 150cm tall. Loci A-J each have two alleles, either T or t, with one inherited from each parent. Each T allele adds 2.5cm to the basic height per copy (each parent has two copies, one goes to the offspring) and t has no effect. The parents are both heterozygous at each loci, that is they have one T and one t at each pair of loci. Therefore both parents are 175cm tall.

When these parents mate the offspring inherit one copy of each parent at random. As such the most likely single outcome is that the offspring will inherit 10 T's and 10 t's - roughly five of each from each parent. However, purely by chance they could receive 10 T alleles from each parent, giving them a height of 150+(20*2.5) = 200cm. Likewise, a sibling could also inherit only t alleles thus giving them a height of 150+(0*2.5)= 150cm. Thus variation can actually increase from one generation to the next.

There are other reasons why we do not regress to the mean as well, but I am writing a quick answer over breakfast, but I will be in the office soon where I can get some more thorough explanations from text books. E.g. (Dis)assortative mating, Mutation, Recombination, Environmental variance, Selection. However, with the increase in global movement it has become easier for dominant alleles to spread which is why the number of blue eyed people in the US is decreasing.

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