Yesterday I was making some calculations, and I started to wonder the average number of alleles per gene the human species have today. The best question would be how many mutations per base or how many alleles in an average 1000 base gene, as genes vary in size.

What made me wonder this? Sometime ago I saw a 1200 base carcinogenic gene that had 31 mutations documented. For me this is actually different from 31 alleles as someone could have more than one mutation, resulting in 2^31 = 2 billion possible alleles. And we also have alleles that were not documented. As the chance of a specific gene suffer mutation from all the genes we have is low, I would say that more than 1 mutation has a very low chance. And 50-100 alleles would be a good guess. Let's take 60. That would give 1/20 mutations per base in our genome (e.g. considering all the 7 billion people living today, we would expect to find 1 mutation in each 20 bases).

But we know the mutation rate in germinative cells is 2.5 10^-8 per base per generation. Considering the human specie began 1 million years ago, and a 20 year generation period, we would expect 1/800 mutations per base in our genome.

So, where is the mistake or the wrong assumption? I thought about 2: The first one is that we considered the mutation rates for germinative cells, and the gene I considered was a somatic carcinogenic gene that could and possibly had a higher mutation rate than germinative cells. That leads me to the previous question, what is the right number of mutations per base we have today considering all the human population (and the mutations we are born with) , or the average number of allele per gene? The second is that in the second calculation we considered a model in which all the humans came from 1 and only 1 ancestor. Something like 1 pre-human gave birth to all the human that ever existed. This is because we did not consider that the first ancestors already had different genes one another (i.e. mutations). If we considered this, it is not right to calculate the number of mutations since human species started as there were mutations even before.

Can someone help me to solve this paradox?

  • $\begingroup$ I just changed specie to species. Species comes from the Latin and is both singular and plural. There is no current English word, specie. $\endgroup$ – David Sep 8 '19 at 16:16

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