Encyclopedia of Evolution.pdf - Online Reading Center

allele, if crossing over occurred in the portion of the chromosome
that is between their respective locations. The breakup
of linked traits is called recombination.
Several factors affect linkage between two traits:
• Distance. Crossing over will disrupt the linkage of two
traits more often if they are farther apart from one another
on the chromosome. Two traits that are very close together
may remain linked for a very long time, while two traits
that are far apart will be separated in a few generations.
This general pattern has allowed geneticists to construct
linkage maps, which specify the relative locations of genes
on each chromosome.
• Time. Traits distant from one another on a chromosome
are separated after a few generations, while it takes many
generations to separate traits that are closely linked. If
genes for two traits are relatively close together on a chromosome,
the origin of the trait may be recent, whereas if
the traits are not linked, their origin may have occurred
further into the past. The same reasoning can be applied to
a trait and noncoding DNA which is close to it (see markers).
Linkage disequilibrium can therefore be used as a
measure of the evolutionary age of the trait.
• Recombination hotspots. Some parts of a chromosome
(hotspots) experience crossing over much more frequently
than others.
Linkage can obscure the study of natural selection. Consider
again the linkage between flower color and seed color in
peas. Suppose that natural selection acted upon flower color,
causing white flowers to become more common, and purple
flowers less common, in the pea plant population. The greenseed
color trait would be dragged along with the white flower
color by linkage. An investigator would therefore also discover
that green seeds are becoming more common, and yellow
seeds less common, in the population. The investigator might
assume, erroneously, that natural selection favors green seeds.
Early 20th-century mathematical studies of Mendelian
genetics, such as those of evolutionary biologist R. A.
Fisher (see Fisher, R. A.), assumed that different genes had
an additive effect: Each contributed separately and equally to
the characteristics of an organism. Geneticist Sewall Wright
(see Wright, Sewall) introduced the concept of epistasis.
Epistatic genes affect one another’s expression. If a scientist
knows the separate effects of two epistatic genes, that scientist
cannot predict the combined effect of those genes. Numerous
examples of epistasis have been found. One recent study
example found that mutations that had drastically bad effects
on one species of fruit fly, Drosophila melanogaster, were the
very same mutations that were found in normal, healthy fruit
flies of another species, D. pseudoobscura. The researchers
explained that epistatic genes allowed D. pseudoobscura to
compensate for the negative effect of the mutation. They also
found that about 10 percent of the mutations that are lethal
in D. melanogaster are present in wild flies and mosquitoes.
Similarly, about 10 percent of mutations that are lethal in
humans are present in some other mammal wild type. Epistasis
has been found in different strains of HIV (see AIDS,
evolution of).
Mendelian Genetics Affects Humans
Now that the Mendelian basis of inheritance is known, it is
common for people to think of genes as determining the fates
of individuals. Modern biologists consider this view, called
genetic determinism, to be incorrect, especially for characteristics
such as intelligence which are strongly influenced by the
environment and actions of the individual. Even some genetic
disorders can often be treated by environmental therapy and
are therefore not genetically determined. Children with PKU
(phenylketonuria) may die if they eat food that contains significant
amounts of the amino acid phenylalanine; they cannot
metabolize this amino acid, and it instead becomes a toxic
side product that poisons the brain. The solution is simple: The
children should not eat food that has phenylalanine. Hemophilia
results from the inability to make a certain blood clotting
protein. Hemophiliacs can now inject the clotting protein into
their blood, entirely compensating for their missing protein.
Now hemophiliac men not only survive but can even have children,
with the result that the number of hemophiliac women
is now increasing. Some other mutations, such as fragile X
syndrome (in which the X chromosome breaks) or the allele
that causes cystic fibrosis, are much harder to overcome. The
mucus buildup that accompanies cystic fibrosis causes respiratory
infections. This is treated by laboriously removing and
attempting to reduce the mucus buildup and continually battling
the infections. The Human Genome Project has, as one
of its main objectives, the identification of all human genes,
therefore of all genetic disorders, so that the defective or missing
protein can be identified and perhaps replaced.
Mendelian genetics has therefore allowed scientists to
understand the process of evolution and has in many cases
given them the tools to bring genetic disorders and defects
under control, thus benefiting the lives of many individuals.
Further Reading
Bonhoeffer, Sebastian, et al. “Evidence for positive epistasis in HIV-
1.” Science 306 (2004): 1,547–1,550. Summarized by Michalakis,
Yannis, and Dennis Roze, Science 306 (2004): 1,492–1,493.
Gould, Stephen Jay. “Fleeming Jenkin revisited.” Chap. 23 in Bully
for Brontosaurus. New York: Norton, 1991.
Kulathinal, Rob J., Brian R. Bettencourt, and Daniel L. Hartl. “Compensated
deleterious mutations in insect genomes.” Science 306
(2004): 1,553–1,554.
Mesolithic See technology.
Mesozoic era
Mesozoic era The Mesozoic era (the era of “middle life”)
is the second era of the Phanerozoic Eon, or period of visible
multicellular life, which followed the Precambrian time in
Earth history (see geological time scale). The Mesozoic
era began with the mass extinction event that occurred at
the end of the Permian period of the Paleozoic era (see
mass extinctions; Permian extinction). The Mesozoic era
consists of three geological periods, the Triassic period, the
Jurassic period, and the Cretaceous period.
The single great continent of Pangaea broke apart during
the Mesozoic era, to form two groups of continents, one of
which would later become North America and Eurasia, the