Essential Cell Biology 5th edition

Generating Genetic Variation
303
percentage of population
that is lactose tolerant
100%
90–99%
80–89%
70–79%
60–69%
50–59%
40–49%
30–39%
20–29%
10–19%
0–9%
no data
Native Americans
Indigenous Australians
G C
C T
regulatory DNA sequence
lactase gene
Figure 9–6 The widespread ability of adult humans to digest milk followed the domestication of cattle.
Approximately 10,000 years ago, humans in northern Europe and central Africa began to raise cattle. The
subsequent availability of cow’s milk—particularly during periods of starvation—gave a selective advantage to those
humans able to digest lactose as adults. Two independent point mutations that allow the expression of lactase in
adults arose in human populations—one in northern Europe and another in central Africa. These mutations have
since spread through different ECB5 regions n9.100-9.06 of the world.
By repeated rounds of this process of gene duplication and divergence
over many millions of years, one gene can give rise to a whole family of
genes, each with a specialized function, within a single genome. Analysis
of genome sequences reveals many examples of such gene families: in
Bacillus subtilis, for example, nearly half of the genes have one or more
obvious relatives elsewhere in the genome. And in vertebrates, the globin
family of genes, which encode oxygen-carrying proteins, clearly arose
from a single primordial gene, as we see shortly. But how does gene
duplication occur in the first place?
Many gene duplications are believed to be generated by homologous
recombination. As discussed in Chapter 6, homologous recombination
provides an important mechanism for mending a broken double helix;
it allows an intact chromosome to be used as a template to repair a
damaged sequence on its homolog. But as we discuss in Chapter 19,
homologous recombination can also catalyze crossovers in which two
SPECIES A
embryonic stage 1
transcription
regulator turns
on gene 1
gene 1 gene 2 gene 3
embryonic stage 2
regulatory DNA sequences
PRODUCT OF GENE 1
TURNS ON GENE 3
transcription
regulator
gene 1 gene 2 gene 3
SPECIES B
embryonic stage 1
gene 1 gene 2 gene 3
embryonic stage 2
PRODUCT OF GENE 1
TURNS ON GENE 2
gene 1 gene 2 gene 3
Figure 9−7 Changes in regulatory
DNA sequences can have dramatic
consequences for the development of
an organism. In this hypothetical example,
the genomes of two closely related species
A and B contain the same three genes
(1, 2, and 3) and encode the same two
transcription regulators (red oval, brown
triangle). However, the regulatory DNA
sequences controlling expression of genes
2 and 3 are different in the two species.
Although both express gene 1 during
embryonic stage 1, the differences in their
regulatory DNA sequences cause them
to express different genes in stage 2. In
principle, a collection of such regulatory
changes can have profound effects on an
organism’s developmental program—and,
ultimately, on the appearance of the adult.