Encyclopedia of Evolution.pdf - Online Reading Center

was dominant and hid the white trait in the flowers of heterozygous
plants.
When plants produce pollen and ovules, the alleles are
separated. This also occurs when animals produce eggs and
sperm. Each sex cell carries just one allele and is therefore
haploid. This separation is caused by the process of meiosis.
Normal duplication of the cell nucleus, called mitosis,
results in two nuclei that are identical to the original
nucleus. Meiosis, in contrast, produces nuclei that have only
half as many chromosomes as the original nucleus. Some
pollen and ovules produced by Mendel’s heterozygous peas
carried one copy of A, others carried one copy of a. Sex
cells fuse together into a zygote, a process called fertilization.
When Mendel crossed two heterozygous plants, they
received one allele from each parent. There were four possible
outcomes (see figure).
• two purple-flower alleles, one from each parent
• two white-flower alleles, one from each parent
• a white allele from the first parent and a purple allele from
the second parent
• a purple allele from the first and a white allele from the
second parent
Only one of these four outcomes would produce plants
with white flowers. This would explain why only one-fourth
of the plants had white flowers: the famous Mendelian ratio
of 3:1 dominant trait to recessive trait in the third generation.
Mendel also found a 3:1 ratio when he paid attention to
seed color (yellow was dominant over green) or seed texture
(smooth was dominant over wrinkled). Mendel did not know
what was causing this to happen, but he had cracked the code
of inheritance patterns.
Mendel’s results drew little attention. One reason is
that Mendel hardly had time for a complete research program.
Once he was promoted to a leadership position, Mendel
had no time at all for scientific studies. Another reason
is that the Mendelian ratio did not show up in every trait.
When other researchers tried similar experiments, they did
not observe the expected 3:1 ratio. Mendel himself worked
on hawkweeds after finishing his garden pea studies; this
time, he failed to find interpretable results. He died thinking
that he had found an interesting, but not very important,
pattern of inheritance.
A 3:1 Mendelian ratio is not found in the inheritance
patterns of all traits for several reasons. Among them are:
• Some traits do in fact show what looks like a blending
inheritance pattern. In some plant species, purebred
red flowers crossed with purebred white flowers produce
seeds that grow into pink-flowered plants. The red trait is
codominant with the white trait. The red trait dominates
over the white trait; white is simply the absence of red. The
red color genes are present in only half their previous number
in the heterozygotes; because there is not enough red
pigment to produce a pure redness, a pink color results.
This is not blending inheritance, because the red and white
alleles are still separate. In later generations, red and white
flowers can emerge again among the offspring, whereas
pink paint will never be either red or white again. This will
Mendelian genetics
This diagram shows what happened with Mendel’s experiment with the
colors of pea flowers. Pure-breeding purple-flowered peas (parental P
generation) had only allele A; pure-breeding white-flowered peas had
only allele a. The hybrids of the F generation were all Aa. When the
hybrids were crossed, they produced gametes, which came together in
three possible combinations: AA (purple flowers), Aa (purple flowers),
and aa (white flowers).
not be observed unless the investigator continues the experiment
past the first generation.
• Most traits are influenced by more than one gene. Each of
the genes may have a 3:1 ratio, but the joint result of all
of the genes will not. Most of these polygenic traits show
a whole range of outcomes. Human height, for example,
shows a whole range of values, from a few short people
to a lot of medium-sized people to a few tall people. In
humans, there are relatively few traits that have a simple
Mendelian inheritance pattern. These include blue vs.
brown eyes; attached vs. unattached earlobes; presence vs.
absence of a widow’s-peak hairline; presence vs. absence
of the hitchhiker’s thumb; the ability to taste the bitter
chemical phenylthiocarbamide; and a considerable list of
metabolic and other genetic diseases. Most human traits,
including other traits related to eye color, ear structure,
hair patterns, and thumbs, not to mention traits like intelligence
that are so complex that they cannot even be defined,
result from more than one gene.
• Almost all traits are influenced by the environment, especially
the experiences of the embryo during its development
(see adaptation). The characteristics of organisms result
from both genes and environment. The pattern of gene
expression is determined by the environment at many different
levels. This occurs in addition to the direct effects of