MadSci Network: Genetics

Re: Have humans enough genes to account for 6 billion individuals?

Date: Wed Feb 2 14:18:10 2000
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology, Roche Molecular Systems
Area of science: Genetics
ID: 946859927.Ge

Well, the short empirical answer to your question is is that there have to be, since so much of the variation we observe between individuals seems to be heritable. However, lets take a longer look at the issues that you raise.

Most of the physical variation between individuals falls into a few categories, most notably; skin, hair and eye color; hair texture; body segment length; and facial characteristics. While these might seem like extremely dramatic differences, it is beginning to look like alot of these characters result from allelic differences at a single locus.

For example, much of the variation in skin and hair color seen in our species seems to be due to to allelic variation at the melanocortin stimulating hormone receptor gene (aka MC1R). Alleles of this receptor result in different ammounts of melanin ( eumelanin and phaeomelanin) production in melanocytes, the cells that give us our characteristic colors.

However, these characteristics are primarily superficial, and account for a very small degree of the genetic diversity of our species. So far, it looks like about 25% of 70,000 to 100,000 genes in the human genome have allelic variants. That means that there are 20,000 or so genes that vary in some way between members of our species.

In addition, the size of our species, in terms of the numbers of individuals, really does not have much to do with the degree of genetic variation within the species. This might seem counterintuitive, but most ( 85% on average) of the genetic variation within the human species can be found in ANY human population. That means that even though people in a particular population (Papua New Guinea Highlanders, or Germans, or Siberian Tuvans, for example) LOOK very much alike, you can find 85% of the allelic diversity found within our entire species in that single population. So, any given population holds most of the genetic potential of our entire species within its own members.

If you look at two populations on the same continent and estimate the degree to which they differ from one another -- these are the genetic differences that distinguish those populations -- you find that only 6% of our species' genetic diversity can be accounted for in this manner. So, the characteristic regional differences between populations only account for 6% of the total genetic diversity of our species.

Finally, if you look at the genetic differences between entire continents, the number is somewhere on the order of 10%. So, even though it seems like populations on different continents are extremely different from each other based on external physical characteristics, they are mostly (90%) the same on the genetic level.

This story becomes even more interesting when you look at the genetic diversity of Chimpanzees and Bonobos, our nearest relatives. The number of these animals is small compared to ours, but their genetic diversity is much larger than that of our species. In fact, the entire 6 billion member human species has a level of genetic diversity which is on the order of a large chimpanzee population; genetically, our species looks like a subset of the chimpanzee species.

Anyway, the overall answer to your question is that there are more than enough genes, alleles, and variant loci in our genome to account for the variation seen between individuals, and in our species taken as a whole.

Some References

Skin color and MC1R Alleles

Valverde P, Healy E, Jackson I, Rees JL, Thody AJ. (1995) Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans. Nature Genetics. 11 : 328-30.

Rana BK, Hewett-Emmett D, Jin L, Chang BH, Sambuughin N, Lin M, Watkins S, Bamshad M, Jorde LB, Ramsay M, Jenkins T, Li WH. (1999) High polymorphism at the human melanocortin 1 receptor locus. Genetics. 151 :1547-57.

Estimates of Human Diversity

Barbujani G, Magagni A, Minch E, Cavakki- Sforza LL. (1997) An apportionment of human DNA diversity. Proceedings of the National Academy of Science USA. 94 : 4516-4519

Chimpanzee versus Human Diversity

Kaessmann H, Wiebe V, Paabo S. (1999) Extensive nuclear DNA sequence diversity among chimpanzees. Science. 286:1159-62.

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