Download The Famous Pea Experiment Mendel`s results depended on a lot of

The Famous Pea Experiment
Mendel’s results depended on a lot of peas—and a lot of patience
Prior to Mendel, naturalists and scientists tried to understand heredity by
crossbreeding different varieties of plants or animals. Nevertheless, Mendel was the
first to conduct broad, thorough, systematic, and sufficiently rigorous experiments to
discern any universal laws governing inheritance.
Mendel began by identifying seven pairs of contrasting traits found among garden
peas:
Seed color (yellow or green)
Seed shape (smooth or wrinkled)
Pod color (yellow or green)
Pod shape (inflated or pinched)
Flower color (purple or white)
Flower position (axial or terminal)
Stem height (tall or short)
For two years, the scientist grew different varieties of peas to make sure that their
offspring were always the same. Then be began breeding different varieties together
to make hybrids. He brushed the pollen off yellow pea plants and put it on green pea
plants, and did the same for plants with each of the seven pairs of traits. He then grew
generation after generation of hybrids and tracked the inheritance of the traits.
Mendel noticed that some traits disappeared in the first generation of hybrids. He
called these traits “recessive.” He called those that did appear “dominant.” In later
generations the recessive traits reappeared—and in a mathematically predictable
pattern. For example, later generations of plants had one green pea for every three
yellow peas. The same ratio appeared for all seven pairs of traits.
Mendel grew an estimated 28,000 pea plants over eight years. In 1864 he published the
results of his experiment.Mendel picked common garden pea plants for the focus of his
research because they can be grown easily in large numbers and their reproduction can be
manipulated. Pea plants have both male and female reproductive organs. As a result, they
can either self-pollinate themselves or cross-pollinate with another plant. In his experiments,
Mendel was able to selectively cross-pollinate purebred plants with particular traits and
observe the outcome over many generations. This was the basis for his conclusions about the
nature of genetic inheritance.
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What Mendel Discovered
Mendel’s mathematical mind allowed him to “see” hidden patterns of heredity.
Earlier scientists had noticed the disappearance and reappearance of traits in hybrid
plants. What Mendel did differently was count. And count. And count. Mendel used
mathematics to draw conclusions about what was happening deep inside the cell.
Mendel concluded that every trait must be controlled by two “elements” (what we now
call genes) that are present in every pea plant. As part of sexual reproduction, these
elements separate and only one is passed down to the offspring. Whether a plant has
green or yellow peas depends on the combination of elements that it received from its
parents. If a plant receives two dominant elements, its trait will be dominant (i.e. yellow
peas). If it receives two recessive elements, its trait will be recessive (i.e. green peas).
If it receives one of each element, the dominant trait will mask the recessive trait. This
is what caused all the green peas to disappear in Mendel’s first generation of plants.
Mendel went on to cross plants that differed in more than one trait—round-yellow peas
with wrinkled-green ones, or tall, violet-blossomed plants with short, white-blossomed
ones. As in his initial experiments, the traits appeared in predictable ratios. This told
Mendel that the elements governing traits were not linked, but passed separately to
the offspring.
Mendel’s results were published in a local scientific journal in 1866, but other scientists
did not understand the importance of his work for several decades.
Meet Modern Mendels
Mendel’s scientific “heirs” use modern techniques to broaden our
understanding of heredity.
While Mendel would probably be amazed by the technology used by his modern
counterparts, he would find the questions they are investigating familiar. Scientists
today use DNA to explore evolution, conservation and other aspects of the natural
world.
For example:
Mendelian genetics helps fill the
branches on the Darwinian “Tree
of Life.”
The “Early Bird” project, led by
Field Museum scientist Dr. Shannon
Hackett aims to chart genetic links
among major groups of birds.
DNA can reveal things about an
animal’s behavior that
binoculars can’t.
Field Museum scientist Dr. Kevin
Feldheim samples DNA from lemon
sharks in the Bahamas for a
University of Illinois-Field Museum
study of reproductive patterns—
information that is crucial to protect
these threatened creatures.
DNA proves that Neanderthals
and modern humans are not
even distant relatives.
In 1997 Svante Pääbo and a team
of German and American scientists
were the first to extract DNA from
Neanderthals. Their research
indicates that it has been more
than 600,000 years since
Neanderthal and Homo sapiens
shared a common ancestor.
Today’s plant geneticists study
“gene flow.”
Sonal Singhal, a student at
Washington University in St. Louis,
studies the exchange of genes
between domesticated rice and its
wild ancestor. “Gene flow” can
produce weedy, invasive hybrids
that interfere with cultivation and
reduce crop yield.