Encyclopedia of Evolution.pdf - Online Reading Center

Mendelian genetics
an American geneticist, noticed that chromosomes behaved like
Mendelian elements during sexual cell division. Nearly 20 years
after Mendel’s death, the world discovered him.
Further Reading
Henig, Robin Marantz. The Monk in the Garden: The Lost and
Found Genius of Gregor Mendel, the Father of Genetics. New
York: Mariner, 2001.
Mendelian genetics Mendelian genetics is the study of the
inheritance pattern of traits from one generation to the next,
as first explained by Gregor Mendel (see Mendel, Gregor).
Mendelian genetics is the foundation of modern genetics and,
as such, one of the foundations of evolutionary science.
Inheritance Is Not Lamarckian
Even in prehistoric times, people understood that offspring
resembled their parents. Not only did plants and animals
reproduce “after their own kind,” to use the phrase from the
biblical book of Genesis, but the offspring resembled the parents
more closely than they resembled other members of the
same species. Although ancient people did not have a modern
concept of species, they recognized a similar concept; anthropological
studies have shown that tribal words for plants and
animals closely align with modern species definitions. Before
recorded history, ancient people had applied their rudimentary
understanding of inheritance patterns well enough to
produce all of the most important crop plant and livestock
animal species, from wheat and rice to cattle and sheep, upon
which the world food supply still depends (see agriculture,
evolution of).
Previous to Mendel’s work, scientific and popular opinions
about inheritance patterns had not been systematically
studied and were subject to hearsay and confusion. Most
importantly, many ancient people believed that environmental
conditions could induce changes in organisms (this
is correct) that can be passed on to future generations (this
is incorrect). This is called the inheritance of acquired characteristics.
The biblical character Jacob placed striped tree
boughs near the water troughs where goats mated, and the
goats produced striped offspring. The writer of that Genesis
account, and all contemporary readers, assumed that striped
offspring resulted because the goats looked at the striped
boughs. (Despite the dependence of creationism upon biblical
literalism, creationists have not embraced the inheritance
of acquired characters as their model for genetics.) In reality
the striped pattern of the goats probably resulted from crossbreeding
between light and dark goats.
Belief in the inheritance of acquired characteristics persisted
well into the age of modern science. This belief is often
named after the French biologist Jean-Baptiste de Lamarck,
although his beliefs were the same as those of most of his
contemporaries about inheritance patterns (see Lamarckism).
Mendel’s work demolished the inheritance of acquired
characteristics. One particularly tragic example of belief in the
wrong theory of inheritance was that of Trofim Lysenko, who
espoused Lamarckism in the Soviet Union decades after it had
been discredited by all competent scientists (see Lysenkoism).
Inheritance Is Particulate, Not Blending
Most people, from ancient times up into 19th-century science,
also believed in blending inheritance. First, according to this
view, the offspring were intermediate between the parents for
each trait. Second, the conditions of the two parents could
not be retrieved. It is called blending inheritance because it is
like blending two colors of paint.
Mendel blazed a new trail away from these beliefs and
toward an understanding that inheritance is particulate, that
is, the traits are passed on as units rather than blending. First,
Mendel was able to control the crossbreeding of his experimental
organisms. The garden peas that he used in his experiments
had flowers that did not open fully; therefore he could
transfer pollen from one flower to another with a little brush,
without having the pollen blown randomly about by the wind.
He could prevent the flowers from pollinating themselves by
removing the immature male parts from the flowers which
he intended to pollinate. Second, he was very systematic and
organized. He defined the traits that he was studying—flower
color, seed color, seed surface texture—and he labeled the
flowers and seeds so that he could be certain which seeds came
from which flower, and which plant of the next generation had
come from which seed of the previous generation. Rather than
making generalized observations, he counted the numbers of
plants that displayed the characteristics he was studying.
Mendel began by identifying true breeds. For example,
some pea plants produced purple flowers generation after generation;
others consistently produced white flowers. Once he
had identified these true-breeding plants, he crossbred them.
Blending inheritance theory would lead to the expectation
that the resulting flower color should be halfway between
purple and white. Instead, all of the resulting seeds grew into
plants that had purple flowers. Conventional wisdom would
have interpreted this to mean that the white color was recessive
and had been lost in the dominant purple color like a
drop of white paint in a can of purple. But Mendel carried
the experiment on into another generation, crossbreeding the
purple-flowered plants that had resulted from mixed parentage.
The white flower trait reappeared in the third generation.
The white trait had not been lost, or blended into the purple
trait, but had retained its individuality. Traits, Mendel discovered,
are particulate. Mendel was not the first investigator to
notice this phenomenon, but he used it as the basis of a theory
of inheritance rather than as just an interesting observation.
The white trait had reappeared in only one-fourth of
the third-generation offspring. This is where Mendel’s mathematical
acumen came into play. Mendel proposed that each
individual plant had two copies of each characteristic. Today
geneticists would say that chromosomes, and therefore genes,
come in pairs (see DNA [raw material of evolution]), and
that these individuals are therefore diploid. In the true-breeding
plants, both copies were the same; today geneticists say
that they were homozygous. One of the original parents was
homozygous for purple flowers, the other homozygous for
white flowers. White and purple are two alleles of the same
gene. The true-breeding plants could be denoted AA (purebreeding
purple) and aa (pure-breeding white). The hybrid
plants had one copy from each parent; today geneticists say
that they were heterozygous (denoted Aa). The purple trait