Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

Monthly Update sign up
Mailing List signup
Search
Follow The Annenberg Learner on LinkedIn Follow The Annenberg Learner on Facebook Follow Annenberg Learner on Twitter
Rediscovering Biology Logo
Home
Online TextbookCase StudiesExpertsArchiveGlossarySearch
Online Textbook
Back to Unit Page
Unit Chapters
Genomics
Proteins & Proteomics
Evolution & Phylogenetics
Microbial Diversity
Emerging Infectious Diseases
HIV & AIDS
Genetics of Development
Cell Biology & Cancer
Human Evolution
Introduction
New Fossils
What Does DNA Tell Us About Our Position Among the Apes?
Variation Within and Among Human Populations
Out of Africa?
Neaderthals in Our Gene Pool?
Human Genetic Variation and Disease
Malaria, Sickle Cell Anemia, and Balancing Selection
Resistance to HIV
The Genetics of Asthma, a Complex Disease
Our History, Our Future
Neurobiology
Biology of Sex & Gender
Biodiversity
Genetically Modified Organisms
Variation Within and Among Human Populations

At the DNA level, humans are both very similar to and very different from one another. On average, pairs of individual humans share 99.9 percent DNA sequence identity. Due to the sheer size of our genomes, however, we possess numerous differences from one another. The human genome consists of just over three billion nucleotides; that 0.1 percent of difference represents three million variants between the average pair. The vast majority of these variants have no functional significance. However, even if one in a thousand did, that would still mean that we would each differ at thousands of functionally important sites.

How does this variation compare with that of other species? Humans actually have less genetic variation than do their closest relatives. For instance, the average difference between two randomly selected chimpanzees is roughly four times greater than between two humans. This is, at first glance, surprising. Based on population genetic theory, levels of genetic variation within species should correlate positively with population size. This predicted correlation comes about because the strength of random genetic drift - which results in the loss of genetic variation - increases at lower population sizes. Yet, the human population numbers in the billions and the population sizes of chimpanzees and gorillas is fewer than a hundred thousand.

What could explain that discrepancy? The strength of genetic drift is dependent not on the current census population size but on the historical population sizes. The relatively low levels of genetic variation in humans can be explained by a severe, but short-lasting, population bottleneck, where the population of our species was likely reduced to a few thousand. It could also be explained by a more moderate, sustained bottleneck. During this bottleneck, the population was possibly in the tens to hundreds of thousands for a more considerable time. Alternately, natural selection could also either increase or decrease the extent of variation in one of the species. Yet, because it is unlikely that natural selection would act in the same way on multiple regions of the genome, the difference in the extent of genetic variation between humans and chimpanzees is more likely a consequence of historical demography.

How is this variation partitioned according to known racial groups? During the 1970s the then state-of-the-art technique of electrophoresis of protein variants showed that around eighty to ninety percent of human genetic variation was within ethnic populations, five to ten percent was among ethnic populations within the major racial groups, and only about five to ten percent was among the major racial groups. In other words, "if everybody on earth became extinct except for the Kikiyu of East Africa, about eighty-five percent of all human variation would still be present in the reconstituted species"2. More recent analyses of DNA sequence data strongly confirm the results of earlier protein electrophoresis studies. In both the protein electrophoresis and the DNA sequence studies, the differences between racial groups are generally ones of frequencies and not kind. The situation of "fixed differences" - in which all individuals in one group have variant A and all individuals of another group have variant B - is extremely rare in humans. Instead, groups vary by having different frequencies of genetic variants. There are cases of "private alleles," however, where genetic variants are found in low to intermediate frequencies in some populations, but are virtually absent from others.

Back Next


  Home  |  Catalog  |  About Us  |  Search  |  Contact Us

| Follow The Annenberg Learner on Facebook
 

  © Annenberg Foundation 2013. All rights reserved.
Privacy Policy