A question on genetic variation.

[Ratamahatta]

Hello. I am reading a book called "The Journey of Man: A Genetic Odyssey" and there is one part that I need some help understanding. It's about Lewontin research and I will just quote from the book:

"[...]In the analysis, Lewontin used as his model the new science of biogeography because he thought this was analogous to what he was doing with humans - looking for geographic subdivision in order to define race. In fact, unsure of how to define a "race" objectively, he divided humans largely along geographical lines - Caucasian (western Eurasia), Black African (sub-Saharan Africa), Mongoloids (east Asia), South Asian Aborigines (southern India), Amerinds (Americas), Aceanians and Australian Aborigines.

The surprising result he obtained was that the majority of the genetic differences in humans were found within population - around 85 per cent of the total. A further 7 per cent served to differentiate populations within a "race", such as the Greeks from Swedes. Only 8 per cent were found to differentiate between human race. A startling conclusion - and clear evidence that the subspecies classification should be scraped.[...]

[...]Lewontin likes to give the example that if a nuclear war were to happen, and only Kikuyu of Kenya (or the Tamils, or the Balinese...) survived, then that group would still have 85 per cent of the genetic variation found in the species as a whole." [...]

Now this is very confusing. Can anyone explain to me how genetic difference is measured and what it means? What do all those percentages mean? What does it mean to have 85 per cent genetic difference? Also: you always hear that Humans and Chimps share 96% of genes while in this quote it says that "A further 7 per cent served to differentiate populations within a "race", such as the Greeks from Swedes." Does it mean that Swedes and Greeks share 93% of their genes? It can't be.

You can tell that I don't know anything about genetics, but I find the topic very interesting. Please explain to me what it all means. Some book recommendations will also be welcomed. Thanks!

[CaptainPanic]

For a start, we (humans) share 99% of all our genes with Chimps.

So, in this whole story, we are talking about a percentage of the percentage that is actually different among humans. Swedes and Greeks share waaaaaaay more than 93% of their genes. After all, they all have 2 legs, 2 arms, a nose, 2 eyes, etc.

[jp255]

I'd imagine that the book would contain a definition of what the authors mean by genetic difference and what they include and don't include. By genetic difference, they could mean a number of things eg single nucleotide polymorphism, copy number variation etc. I am not sure which ones they are including or excluding in this extract.

 

I think you are confused about sharing genes and genetic difference. They are not the same thing. We essentially share the same genes (most likely over 99.9%, not sure on the exact figure), but we will have differring alleles for some of those genes. For example, we both have the gene p53, but we might not have the same allele (version) of it. If we have different versions then that is a genetic difference. Genetic differences occur outside of genes also.

 

Of those genes that we share, there are genetic differences of many kinds (SNPs, CNV, deletions, insertions etc.). The percentages in that extract refer to the percentage of all genetic differences (That the authors know of/are considering. The authors should have mentioned/defined this somewhere) across all of the shared genes.

 

So, 85% of all genetic differences across all of the genes can be found in a population (don't know which they are referring to). 7% of all genetic differences across all of the genes can be used to differentiate between two populations within a "race". The remaining 8% can be used to differentiate between "races".

 

I hope this makes the passage easier to understand.

 

 

 

A startling conclusion - and clear evidence that the subspecies classification should be scraped.[...]

 

I really don't see how the authors came to that conclusion at all. Shame you stopped the quote there but I would not take those findings and conclude from them that the subspecies classification should be slain. The extract initally starts off with the term "race", which is fine as they did crudely define it. Making the jump from data concerning "race" to the conclusion of slaying the classification termed subspecies is not ok. It is also not justified.

Edited January 23, 2013 by jp255