Encyclopedia of Evolution.pdf - Online Reading Center

0 Mendelian genetics
the environment on the organism. The range of measurable
organism characteristics under a range of different conditions
is called the norm of reaction. Height in humans is
influenced not only by several genes but also by nutrition
during fetal development and childhood.
Most traits that show a Mendelian pattern are simple
examples of mutation: For example, blue eyes result from a
mutation that inactivates the gene that creates the brown pigment
in the iris of the eyes.
Ratios other than 3:1 can also result for sex-linked traits.
Men have one X and one Y chromosome, while women have
two X chromosomes. The genes on the Y chromosome do
not match up with those on the X chromosome. The X chromosome
of the man, therefore, may express all of its genes,
regardless of whether they are dominant or recessive, because
there is no second allele to hide them. Many recessive alleles on
the X chromosome, like most other recessive alleles in humans,
are associated with genetic deficiencies. Some are serious,
such as hemophilia (the inability to stop bleeding), while others
are less serious, such as the inability to visually distinguish
red and green. These genetic disorders are much more common
in males, because whenever the defective allele is present
there will never be a dominant one to mask its effects. Women,
however, would need to receive two recessive alleles, one from
each parent, in order to have the disorder. For hemophilia, this
was unlikely in the past, as the father would have had to be
a hemophiliac, and hemophiliacs seldom survived to sexual
maturity. Injections of blood proteins now allow hemophiliacs
to survive and reproduce. Many other metabolic mutations
on the X chromosome remain deadly despite the advances of
modern medicine, and it is mostly men that they affect.
What advantage might an organism obtain by being diploid?
The diploid condition of a cell is like a truck with double
tires. If one tire blows out, the other will still support the
truck. Similarly, if one allele is defective, the other can still
produce the protein and, at least partially, compensate for it.
Once geneticists George Beadle and Edward Tatum
had discovered that there was a one-to-one correspondence
between genes and proteins (or protein components), biologists
could understand that Mendelian inheritance patterns
worked perfectly well on the DNA level. Many traits resulted
from the complex interactions of proteins, but the proteins
often express themselves by a Mendelian 3:1 ratio. This has
been confirmed by numerous studies of proteins, most of them
associated with metabolic deficiencies. Mendel was vindicated,
on the molecular level. Recently, geneticists have begun
to realize that a single gene can result in several to many different
proteins, as the expression of a single gene is altered in
different ways. This new science of epigenetics reveals an even
greater diversity of raw material for evolution.
Mendelian genetics made modern evolutionary theory
possible. Among Darwin’s many critics was the engineer
Henry Charles Fleeming Jenkin, who wrote that natural
selection could never cause a rare, favorable trait to become
common. All good traits, when they first start, must be rare.
But a rare good trait will disappear into a mediocre population
like a drop of white paint in a bucket of red (actually Jenkin
used a different, and racist, analogy). (Jenkin was apparently
referring to rare, large mutations, rather than minor genetic
variation.) Having no Mendelian alternative to the blending
theory, Darwin was at a loss for an answer. It was largely in
response to this challenge that he contrived his pangenesis theory,
which was basically Lamarckism. Because genes are particulate,
natural selection can cause a rare and good allele to
become common in a population, and do so very quickly.
Mendel’s work, forgotten during his lifetime, was rediscovered
independently by three scientists in 1900: Erich von
Tschermak von Seysenegg, Hugo DeVries, and Carl Correns
(see DeVries, Hugo). Strangely enough, when Mendelian
inheritance patterns were first rediscovered, they were
used as a criticism rather than a support of natural selection.
This is because these researchers, and most of their contemporaries,
focused only on big mutations that would cause
major changes in plants and animals, even the sudden origin
of new species, rather than small mutations that are the fuel
for gradual changes in plant and animal species. The unification
of Mendelian genetics and Darwinian natural selection,
called the modern synthesis, had to wait until the 1930s
and the work of Theodosius Dobzhansky working with fruit
flies, Ernst Mayr working with birds, George Gaylord Simpson
working with fossils, and G. Ledyard Stebbins working
with plants (see Dobzhansky, Theodosius; Mayr, Ernst;
Simpson, George Gaylord; Stebbins, G. Ledyard).
Linkage and Epistasis Affect Inheritance
Mendel’s calculations assumed that each trait had independent
assortment. This means that the alleles of one trait could
go to any of the offspring, unaffected by the alleles of any
other trait. Consider the example of flower color and seed
color in Mendel’s peas. If these traits assort independently,
plants with purple flowers could produce either green or yellow
peas, and plants with white flowers could produce either
green or yellow peas.
Studies of inheritance patterns, shortly after the rediscovery
of Mendelian genetics in the early 20th century, revealed
that many traits did not assort independently. The traits
were linked, thus forming linkage groups. If pea plants with
white flowers always had green seeds and plants with purple
flowers always had yellow seeds, this would indicate a linkage
between flower and seed color. (This is a hypothetical
example, using real traits.) Further studies indicated that the
number of linkage groups was similar to the number of chromosomes.
Therefore, traits that are linked were those traits
that are encoded on the same chromosome. Chromosomes
are, roughly speaking, linkage groups. In the hypothetical pea
plant example, the purple-flower allele would be on the same
chromosome as the yellow-seed allele, and the white-flower
allele on the same chromosome as the green-seed allele.
Linkage groups are not permanent. During meiosis, pairs
of chromosomes are not only separated from one another, but
chromosomes can exchange segments, a process called crossing
over. Whenever a portion of one chromosome breaks off
and changes places with the corresponding part of the other
chromosome, new linkage groups are formed. The purpleflower
allele would no longer be on the same chromosome
as the yellow-seed allele but become linked to the green-seed