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Extension: Genetic Distance Values

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Genetic Distance Values activity

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Genetic Distance Values

TEACHER'S GUIDE

This optional extension for advanced students shows how Dr. Epps' genetic distance data and FST values are translated to the maps in the datasets.

How do you measure the breeding levels between these three populations, A, B, C?

Scientists measure the level of breeding between populations by using a mathematical equation to compare the frequency of genes in each population to each other in pairwise comparisons. In the example above, they would compare the frequency of genes in:

  1. Population A to Population B
  2. Population A to Population C
  3. Population B to Population C. 

The value that they calculate is called the genetic distance between populations.  The greater the genetic distance between populations, the less breeding there is between them and the more isolated they are from one another. The lower the genetic distance between populations, the more breeding there is between them and the less isolated they are from one another. This genetic distance value is a number that is also known as the fixation index or FST value. It is a measure of the difference in the allele frequency between two populations. 

FST Values

The fixation index can range from 0 to 1, where 0 means complete sharing of genetic material and 1 means no sharing. For values equal to 1(meaning no sharing), scientists say that the populations are fixed. If populations are referred to as fixed, it means that they do not share any alleles with one another, i.e. the do not breed with one another; they are completely isolated from one another. Practically speaking FST values are never as high as 1 because these values are only utilized to measure breeding between populations of the same species. Different populations of the same species by definition would show some evidence of interbreeding, even if that breeding may no longer occur today.

As a point of reference for the bighorn sheep, FST values for mammals generally range from 0 to 0.25, with most values being close to 0.1. Values on the high end of this range (close to 0.2) indicate some isolation between populations, and most likely mean that the populations are not currently breeding with one another. Values on the low end of this range (i.e. close to 0) indicate that the populations are sharing their genetic material through high levels of breeding.

Genetic Distance for Populations A, B and C

Doing a quick scan of the above diagram, you can see that populations A and B, share many more of the same genes than they do with population C, meaning that the genetic distance or FST value for populations A and B is low and the FST value for population C to population A or B is high.  The low FST value for the relationship between population A and B means that they show high levels of breeding and the high FST value for population C and population A or B means that they show low levels of breeding with one another. 

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