David Elihu
7.021, Section E(W9-11)
ID#999779727
Paper by Gregor Mendel, laws of segregation and independent assortment
Introduction
There are currently two prevailing theories of inheritance. One theory, fathered by
Aristotle, predicts that one parent is the majority contributor to the offspring’s inherited features.
The other theory contends that the traits of the parents become mixed and are lost in the mixed
traits of the offspring. This theory is called blended inheritance. About 15 years ago, Abbot
Napp proposed many questions about inheritance that were never answered, such as how traits
are inherited and whether or not chance is a determining factor in passing along characteristics.
To this day no successful attempt has been made to explain the laws of inheritance,
because to do so would be a Herculean task because of the breadth of study.
Previous
experiments have led to inconclusive results due to error in experimental design. They have not
offered results that help differentiate differences in traits between offspring of the same parents.
They also have not isolated generations from each other and do not provide statistical data on the
relationships between generations. We thus have proposed an alternative method of study by
choosing plants that are of different order and share little if any traits with each other.
Our study aims to discover a general law concerning the formation and development of
hybrids, and form a predictable pattern of artificial fertilization in order to obtain new variations
in offspring. It is hoped that this study, which spanned eight years, will provide a better picture
of the relationship between parent and offspring and lead to further study in the field of
inheritance.
Studies previously conducted have demonstrated that if two plants that share virtually all
characteristics but a few are crossed, they will produce hybrids that share the common
characteristics but vary in the differentiating characters. The object of this experiment was to
determine the law by which these differentiating traits appear in successive generations of
offspring.
Methods
Plants were chosen that had distinguishing traits and fertility throughout subsequent
generations, and whose natural pollination could be prevented. The genus Pisum was chosen to
be the experiment subject through careful comparison to other types of plants, and in particular,
Pisum Sativum, the garden pea, was used in most of the experiments. Other types of pea plants
that were chosen and were used less frequently were P. quadratum, P. saccharatum, and P.
umbellatum. 34 varieties of pea plant were obtained in total. It should be noted that the species
denotations in these cases have a high probability of being immaterial since one can not always
distinguish between separate species or mere varieties of the same species.
Plants from the genus Pisum created a favorable situation because the reproductive
organs are closely packed inside the keel and the anthers burst directly inside the bug. Through
this process, the stigma is covered with pollen before the flower even opens, thus preventing
pollination from an insect. The peas were also chosen as the experimental model because of
their short growth time, which would help speed up the retrieval of our results.
The peas chosen were selected to show differences in length and color of the stem; size
and shape of the leaves; in the appearance of the flowers and pods; in the length of the flower
stalk; in the form and size of the seeds and the color of their coats; and in the color of the
endosperm.
During the experiment, there were seven different characteristics of the peas that were
selected for the experiments. They are listed in table 1a below:
Character
1. form of the ripe seeds
Description
Round (possibly partially), with occasional
shallow depressions on the surface. Otherwise
they are irregularly angular and considerably
wrinkled.
2. color of the seed endosperm
Pale yellow, bright yellow and orange, or a less
intense green tint. Since coats are transparent
can examine color
3. seed-coat color
White (correlate with white flowers); gray, graybrown, leather-brown, with or without violet
spotting. If without violet spotting, the color of
the standards is violet, the wings are purple, and
the stem in the axils of the leaves has a reddish
tint. When boiled in water, the gray seed-coats
become dark brown.
4. form of the ripe pods
Their shape is either dome shaped (convex) or
depressed (concave).
5. color of the unripe pods
Light to dark green, or intense yellow.
The
stalks, leaf-veins and calyx help determine the
color.
6. flower position
Axial, distributed along the main stem, or
terminal, where they gather at the top of the
stem. In the terminal case the upper part of the
stem is wider.
7. stem length
Vary considerably in length.
To be able to
differentiate crossing occurred among plants
with widely varying stem lengths (long X short
stem).
Using the characteristics above, crosses were conducted and studied among plants with a
single dissimilar trait, although the plants might also have other different traits that were not
visible. When there was more than plant that had the possibility of being cross-fertilized, the
more visibly-healthier plant with the stronger expression of the particular trait was chosen. In
addition, if a plant was to be used for the crossing for two different traits, it would be the pollenplant in one cross and the seed-bearer in the other.
The cross-fertilizations were conducted as follows:
Experiment (similar to trait number)
Fertilization Data
1st experiment
60 fertilizations on 15 plants
2nd experiment
58 fertilizations on 10 plants
3rd experiment
35 fertilizations on 10 plants
4th experiment
40 fertilizations on 10 plants
5th experiment
23 fertilizations on 5 plants
6th experiment
34 fertilizations on 10 plants
7th experiment
37 fertilizations on 10 plants
The plants were grown in a natural environment.
Garden beds were used, and
occasionally the plants were grown in pots for the controls. The plants were held upright using
sticks, tree branches and string.
After crossing plants which differed in one character, plants were crossed that differed in
2 and 3 characteristics. In the first cross, the parental plants differed in the form of seed and in
the color of the endosperm. In the second cross, which was between plants with three dissimilar
traits, the form of the seed, the color of the endosperm, and the color of the seed-coats were
cross-fertilized. After these two experiments, further crosses, though smaller in number, of the
same types of plants were done in order to retrieve a hybrid plant.
In order to learn more about the passing down of traits in organisms with dissimilar
characteristics, a plant with round form and a yellow endosperm was crossed in four different
instances with a form-wrinkled plant with a green endosperm. Further analysis of the passing
down of traits for hybrids was done in several other experiments where variety in traits was
available. Parents with different types of traits were crossed in order to determine the rate at
which different traits are passed down to an offspring.
Controls seemed to be appropriate to make sure insects in the natural outdoor
environment did not affect the offspring of the pollinations. The beetle Bruchus pisi can pose a
threat to the experimental purity as its females are known to lay the eggs in the flower, thus
opening the keel. Another possible impurity that might have been introduced might have been
partial exposure of the fertilizing organs of a weak and dying plant, causing fertilization when it
is not wanted. To prevent these potential problems, for each experiment, equivalent pot plants
were grown in a greenhouse to serve as a control.
Artificial fertilization, a relatively intricate process, allowed control of the reproductive
conditions of the peas. In the process, the bud of a pea was opened before development was
complete, and the keel was removed. The next step was to take out each stamen very carefully
using forceps, making sure it wasn’t punctured. Using pollen from the plant we wanted to
fertilize, the stigma was dusted over on the seed-bearing plant to cross-fertilize the two plants.
NOTE: THE METHODS ON THE SECTION WITH EXPERIMENTS WITH HYBRIDS OF
SPECIES OF PHASEOLUS WAS OMITTED ON PURPOSE. SINCE THIS EXPERIMENT
WAS JUST BEGINNING AND DOES NOT PLAY A FUNDAMENTAL ROLE IN THE
CONCLUSIONS OF MENDEL’S WORK, IT SHULD BE ONLY BRIEFLY MENTIONED
MOST PROBABLY IN THE CONCLUSION THAN ACTUALLY BEING MENTIONED IN
THE METHODS AND RESULTS SECTIONS.