Evolution, 4th Edition

158 CHAPTER 6

and environmental effects on the phenotype are more difficult to control, much

larger sample sizes are needed with GWAS.

Another approach that is used to find human QTL takes advantage of the dis-

tinctive traits seen in some populations. The strategy is to scan the genome for

loci that have large allele frequency differences among populations with divergent

phenotypes. This strategy has discovered QTL that underlie adaptive evolution of

traits that include disease resistance, body height, and tolerance to high elevation

and cold (FIGURE 6.28).

QTL mapping identifies regions of chromosomes that can range in size from a

few hundred to many hundreds of thousands of DNA bases. A single QTL often

spans several genetic loci. Other research strategies are needed to find which DNA

base or bases within a QTL are responsible for the phenotypic variation. The cause

is sometimes discovered to be just a single nucleotide. In other cases, several loci or

a chromosomal rearrangement are responsible (see Figure 6.23).

The genetics of quantitative traits

One of the most basic questions we can ask about the genetics of quantitative

traits is how many loci contribute to their phenotypic variation. The answer is

important for several reasons. Population genetics theory tells us that if many loci

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Marine

diet

Lactose

tolerance

Malaria

resistance

Cholera

resistance

Arsenic-rich

environment

Cold

climate

Elevation Light skin

pigmentation

Short

stature

FIGURE 6.28 QTL involved in adaptive evolution of several traits in humans have been

identified by correlating large differences in allele frequencies among populations

with distinct phenotypes. (After [51].)

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