David Elihu
7.021 Section E
ID#999779727
Changes to Paper
- made the corrections suggested by the grader (I would assume it was you )
- general corrections to methods section.
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 – it
has been a Herculean task to undertake because of the breadth of study and research required.
Previous experiments have led to inconclusive results due to faulty experimental design. The
results have not helped differentiate traits inherited by offspring of the same parents. Nor have
they isolated generations from one another. Most important, there is no statistical data on the
relationships between generations. We thus have proposed an alternative theory of inheritance
after choosing plants that are of different order and share little if any traits with each other.
Our study aims to discover the general laws concerning the formation and development
of hybrids, and form a predictable method 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 help elucidate the laws that
govern 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,
gray-brown, 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.
Results
First Generation
From this point forward, those traits that remain unchanged in the offspring of the hybrid
cross will be known as dominant. Those that become hidden in the process of a cross will be
known as recessive. Throughout the experiments, it was shown that whether the dominant trait
is contained within the seed plant or the pollen plant does not affect the outcome of the hybrid
crosses. The dominant characteristics in the differentiating crosses were determined to be as
follows:
1.
The round or roundish seed with or without shallow depressions.
2.
The yellow coloring of the seed abdomen.
3.
The gray, gray-brown, or leather brown color that lies in the seed coat, in
conjunction with red-violet blossoms and reddish spots in the leaf axils.
4.
the inflated form of the pod
5.
the green coloring of the unripe pod in conjunction with the stems, leaf-veins and
calyx all having the same color.
6.
The distribution of flowers along the stem.
7.
The longer length of the stem.
These results were all determinable within the first generation of crosses between the species.
In this first generation, the ratio of dominant to recessive recurrence of traits appeared in a 3:1
ratio. This was observable in virtually all of the first generation crosses without exception. The
recurrence of the 3:1 ratio can be witnessed in the table below:
Experiment
1. form of the
ripe seeds
2. color of the
seed
endosperm
3.
seed-coat
color
4. form of the
ripe pods
5. color of the
unripe pods
6.
flower
position
7. stem length
Total number
of hybrids
253
Total number
of seeds
obtained
7324
Number of
Dominant
Number of
Recessive
5474
1850
Ratio of
Dominant to
Recessive
2.96:1
258
8023
6022
2001
3.01:1
Unknown
929
705
224
3.15:1
Unknown
1181
882
299
2.95:1
Unknown
580
428
152
2.82:1
Unknown
858
651
207
3.14:1
unknown
1064
787
277
2.84:1
TOTAL
RATIO:
2.98:1
It should be noted that these results were obtained over a wide population survey. In
more specific cases, there were drastic fluctuations in individual small crosses, but once these
data were averaged out the above results were obtained.
It should also be noted that in
experiment 7, where experiments were done for the length of the stem, the shorter plants that
were grown were immediately transferred to areas where they were not close to taller-stemmed
plants, since the taller plants would just overgrow them in neighboring areas.
Second Generation
When the plants with the recessive forms of the traits were crossed with each other, the
second generation results proved to be the same as the first generation results. The offspring
remained constantly recessive in the particular trait that was being followed. The same could not
be said for the dominant forms that were self-fertilized.
In the dominant crossing of experiment 1, of a total of 565 plants that came from the
round seeds of the first generation, 193 had round seeds only and were the same as the parental
generation. However, 372 plants gave both round and wrinkled seeds, in the proportion of 3:1.
The overall ratio was thus 1.93:1 for hybrids versus constants.
In experiment 2, the test of the color of the seed endosperm, a total of 519 plants were
grown. 166 yielded only the dominant yellow albumen, while 353 yielded both yellow and
green in the proportion of 3:1. The overall ratio for experiment 2 was 2.13:1.
Experiments 3-7 all had a population of 100 plants.
In experiment 3, 36 plants had only gray-brown seed coats, while 64 had some graybrown and some white. The ratio for experiment 3 was 1.78:1.
In experiment 4, 29 plants had only the dominant simply inflated pods, while 71 had a
mix of inflated and constricted. The ratio for experiment 4 was 2.45:1.
In experiment 5, 40 plants had green pods while 60 had both green and yellow pods. The
ratio for experiment 5 was 1.5:1. On a second trial, 35 had green pods while 65 had both green
and yellow pods. This ratio was 1.86:1.
In experiment 6, 33 plants had only axial flowers while 67 had some axial and some
terminal flowers. The ratio for experiment 6 was 2.03:1.
In experiment 7, 28 plants had long stems, while 72 had a mix of long and short stems.
The ratio for experiment 7 was 2.57:1.
This let us conclude that of the plants that showed the dominant trait in the first
generation, two-thirds were of the hybrid character, yielding a mix of traits in the second
generation; the other one-third kept the dominant trait in successive generations. Going back to
the original ratio of 3:1 in dominant to recessive show of traits, this ratio can be further refined
into a 2:1:1, which includes 2 hybrid dominant, 1 dominant, and 1 recessive in their traits.
Subsequent Generations
When further crosses were carried through, the results from above were confirmed.
Experiments 1 and 2 were carried through 6 generations, experiments 3 and 7 were carried
through 5 generations, and experiments 4, 5, and 6 were carried through 4 generations. The
offspring of the hybrids consistently segregated into the ratio of 2:1:1 determined above.
The Offspring of Hybrids in Which Several Differentiating Characters are Associated
When parental plants having more than one differentiating characteristic were crossed,
the offspring tended to approach the appearance of the parental plants which had the greater
number of dominant characters. When plants that differed in the form of seed and in the color of
the albumen were crossed, the following results were obtained. 556 seeds total were conceived
by 15 plants. Of the 556, 315 were round and yellow, 101 were wrinkled and yellow, 108 were
round and green, and 32 were wrinkled and green.
The round and yellow seeds were cross-fertilized again, and 38 had round yellow seeds,
65 had round yellow and green seeds, 60 had round yellow and wrinkled yellow seeds, and 138
had round yellow and green, wrinkled yellow and green seeds.
Of the wrinkled yellow seeds 96 plants resulted, of which 28 had only wrinkled yellow
seeds and 68 had wrinkled yellow and green seeds. Of the round and green seeds 102 plants
fruited, where 35 had only round green seeds and 67 had round and wrinkled green seeds. The
wrinkled green seeds fruited 30 plants which bore seeds all of the same character as their parents.
From these results, it was seen that 33 times there was a group seen that has constant
traits over time and does not vary in continuing generations. 65 times it was seen that there was
a group that had one hybrid with one dominant trait that continued throughout the generations.
138 times the plants were hybrid in both characters, and behaved just like the parents in their
own offspring. This resulted in an overall ratio of 1:2:4.
When similar experiments were carried out using a larger number of differentiating
characteristics, similar results were obtained. It was seen that the offspring of the hybrids which
have in between them several differentiating characteristics exhibit the terms of a series of
combinations, in which the offspring for each pair of differentiating traits are united.
Through these experiments, it was seen that the crossing between these plants was
essentially a crossing between a pollen and egg cell.
Conclusions