Father of Genetics
D. Pitman M.D.
"Pea hybrids form germinal and pollen cells that in their composition correspond in equal numbers to all the constant forms resulting from the combination of traits united through fertilization."
His beautifully designed experiments with pea plants were the first to focus on the numerical relationships among traits appearing in the progeny of hybrids. His interpretation for this phenomenon was that material and unchanging hereditary elements undergo segregation and independent assortment. These elements are then passed on unchanged (except in arrangement) to offspring thus yielding a very large, but finite number of possible variations.
Mendel often wondered how plants obtained atypical characteristics. On one of his frequent walks around the monastery, he found an atypical variety of an ornamental plant. He took it and planted it next to the typical variety. He grew their progeny side by side to see if there would be any approximation of the traits passed on to the next generation. This experiment was designed to support or to illustrate Lamarck's views concerning the influence of environment upon plants. He found that the plants' respective offspring retained the essential traits of the parents, and therefore were not influenced by the environment. This simple test gave birth to the idea of heredity.
Mendel was well aware that there were certain preconditions that had to
carefully established before commencing investigations into the
characteristics. The parental plants must be known to possess constant
differentiating characteristics. To
establish this condition, Mendel took an entire year to test "true
breeding" (non-hybrid) family lines, each having constant characteristics.
The experimental plants also needed to produce flowers that
would be easy
to protect against foreign pollen. The
special shape of the flower of the Leguminosae family, with
enclosed styles, drew his attention. On
trying several from this family, he finally selected the garden pea
sativum) as being most ideal for his needs.
also picked the common garden pea plant because it can be grown in
and its reproduction can be manipulated. As with many other
plants, pea plants have both male and female reproductive organs.
result, they can either self-pollinate themselves or cross-pollinate
plants. In his experiments, Mendel was able to selectively
purebred plants with particular traits and observe the outcome over
generations. This was the basis for his conclusions about the
observed seven pea plant traits that are easily recognized in one of
1. Flower color: purple or white
2. Flower position: axial or terminal
3. Stem length: long or short
4. Seed shape: round or wrinkled
5. Seed color: yellow or green
6. Pod shape: inflated or constricted
7. Pod color: green or yellow
Mendel's Law of Segregation
hypothesis essentially has four parts. The first part or "law" states
that, "Alternative versions of
genes account for variations in inherited characters." In a nutshell,
the concept of alleles. Alleles are different versions of genes that
same characteristic. For example,
each pea plant has two genes that control pea texture.
There are also two possible textures (smooth and wrinkled) and
different genes for texture.
The second law states that, "For each character trait (ie: height, color, texture etc.) an organism inherits two genes, one from each parent." This statement alludes to the fact that when somatic cells are produced from two gametes, one allele comes from the mother, one from the father. These alleles may be the same (true-breeding organisms), or different (hybrids).
third law, in relation to the second, declares that, "If the two
differ, then one, the dominant allele, is fully expressed in the
appearance; the other, the recessive allele, has no noticeable effect
fourth law states that, "The two genes for each character segregate
gamete production." This is
the last part of Mendel's generalization. This references meiosis when
chromosome count is changed from the diploid number to the haploid
genes are sorted into separate gametes, ensuring variation.
This sorting process depends on genetic "recombination."
During this time, genes mix and match in a random and yet very
way. Genes for each trait only
trade with genes of the same trait on the opposing strand of DNA so
that all the
traits are covered in the resulting offspring.
For example, color genes do not trade off with genes for texture.
Color genes only trade off with color genes from the opposing
sight as do texture genes and all other genes.
The result is that each gamete that is produced by the parent is
different as far as the traits that it codes for from every other
gamete that is
produced. For many creatures, this
available statistical variation is so huge that in all probability, no
identical offspring will ever be produced even given trillions of years
since a pea plant carries two genes, it can have both of its genes be
Both genes could be "smooth" genes or they could both be
"wrinkled" genes. If both genes
are the same, the resulting pea will of course be consistent.
However, what if the genes are different or "hybrid"?
One gene will then have "dominance" over the other "recessive"
gene. The dominant trait will then
be expressed. For example, if the
smooth gene (A) is the dominant gene and the wrinkle gene (a) is the
gene, a plant with the "Aa" genotype will produce smooth peas.
Only an "aa" plant will produce wrinkled peas.
For instance, the pea flowers are either purple or white.
Intermediate colors do not appear in the offspring of these
observation that there are inheritable traits that do not show up in
intermediate forms was critically important because the leading theory
biology at the time was that inherited traits blend from generation to
generation (Charles Darwin and most other cutting-edge scientists in
century accepted this "blending theory.").
Of course there are exceptions to this general rule.
Some genes are now known to be "incompletely dominant." In
this situation, the "dominant" gene has incomplete expression
resulting phenotype causing a "mixed" phenotype.
For example, some plants have "incomplete dominant" color genes
as white and red flower genes. A
hybrid of this type of plant will produce pink flowers.
Other genes are known to be "co-dominant" were both alleles are
equally expressed in the phenotype. An
example of co-dominant alleles is human blood typing.
If a person has both "A" and "B" genes, they will have an
"AB" blood type. Some traits
are inherited through the combination of many genes acting together to
certain effect. This type of
inheritance is called "polygenetic." Examples
of polygenetic inheritance are human height, skin color, and body form.
In all of these cases however, the genes (alleles) themselves
unchanged. They are transmitted
from parent to offspring through a process of random genetic
can be calculated statistically. For
example, the odds of a dominant trait being expressed over a recessive
a two-gene allelic system where both parents are hybrids are 3:1.
If only one parent is a hybrid and the other parent has both
alleles, then 100% of the offspring will express the dominant trait.
If one parent has both recessive alleles and the other parent is
hybrid, then the offspring will have a phenotypic ratio of 1:1.
Law of Independent Assortment
most important principle of Mendel's Law of Independent Assortment is
emergence of one trait will not affect the emergence of another. For
pea plant's inheritance of the ability to produce purple flowers
white ones does not make it more likely that it would also inherit the
to produce yellow peas in contrast to green ones. Mendel's
allowed other scientists to simplify the emergence of traits to
probability (While mixing one trait always resulted in a 3:1 ratio
dominant and recessive phenotypes, his experiments with two traits
was so successful largely thanks to his careful and nonpassionate use
scientific method. Also, his choice of peas as a subject for his
quite fortunate. Peas have a
relatively simple genetic structure and Mendel could always be in
control of the
plants' breeding. When Mendel wanted to cross-pollinate a pea plant he
only to remove the immature stamens of the plant. In this way he was
of each plants' parents. Mendel made certain to start his experiments
true breeding plants. He also only measured absolute characteristics
color, shape, and texture of the offspring. His data was expressed
and subjected to statistical analysis. This method of data reporting
large sampling size he used gave credibility to his data. He also had
foresight to look through several successive generations of his pea
record their variations. Without his careful attention to procedure and
Mendel's work could not have had the same impact that is has made on
cross-pollinating plants that either produce yellow or green peas
Mendel found that the first offspring generation (f1) always has yellow
However, the following generation (f2) consistently has a 3:1
yellow to green.
3:1 ratio occurs in later generations as well. Mendel realized
is the key to understanding the basic mechanisms of inheritance.
is important to realize that in this experiment, the parent plants were
for pea color. That is to say, they each had two identical forms
of the gene for this trait--2 yellows or 2 greens. The plants in
generation were all heterozygous. In other words, they
inherited two different alleles--one from each parent plant. It
clearer when we look at the actual genetic makeup, or genotype,
pea plants instead of only the phenotype, or observable
that each of the f1 generation plants (shown above) inherited a Y
allele from one parent and a G
allele from the
other. When the f1 plants breed, each has an equal chance of
either Y or G alleles to each
all of the seven pea plant traits that Mendel examined, one form
over the other. Which is to say, it masked the presence of the
allele. For example, when the genotype for pea color is YG
(heterozygous), the phenotype is yellow. However, the dominant
allele does not alter the recessive green one in any way.
alleles can be passed on to the next generation unchanged.
observations from these experiments can be summarized in two principles:
The Principle of Segregation
The Principle of Independent
came to four important conclusions from these experimental results:
inheritance of each trait is determined by "unitsÂ" or "factors"Â (now
called genes) that are passed on to descendents unchanged.
individual inherits one such unit from each parent for each trait.
trait may not show up in an individual but can still be passed on to
genes for each trait segregate themselves during gamete production.
While Mendel knew of Darwin's work (though Darwin was evidently not aware of Mendel's work), Mendel's ideas on heredity and evolution were fundamentally opposed, in certain key ways, to those of Darwin. 2,5
"In a letter to William Bateson written in 1902 by Mendel's nephew, Ferdinand Schindler, stated, "He [Mendel] read with great interest Darwin's work in German translation, and admired his genius, though he did not agree with all of the principles of this immortal natural philosopher" (Orel, 1996, p. 188). Bateson (1913, p. 329) wrote, "With the views of Darwin which at that time were coming into prominence Mendel did not find himself in full agreement."5
Now, this isn't to say that there isn't a great deal of controversy in this regard. Arguably most past and present authors and scientists view or viewed Mendel as a supporter of Darwinism. By contrast, Olby (1979, 1985) studied the historical context of evolutionary thought during Mendel's day and determined that Darwin's "'views on the role of hybridization in evolution were very far removed from Mendel's'".5
"The extreme disagreement among scholars about Mendel's view of Darwin's writings is probably because Mendel wrote very little about Darwin, and thus most claims are suppositions about what Mendel must have thought about Darwin. In his surviving writings, Mendel's overtly referred to Darwin only four times, all in 1870, four years after the publication of "Versuche" One reference is in Mendel's (1870) Hieracium paper and three are in his eighth and ninth letters to Nageli (Stern and Sherwood, 1966). All four references are brief and reveal neither strong support of nor opposition to Darwin's theories." 5
However, in Mendel's copy of Origins, he did make occasional marks and margin notations. Mendel marked one passage where Darwin discusses the uniformity of hybrids in the F1 generation and the variability of their F2 offspring. Darwin's explanation for this was that there was some alteration in the reproductive system, some mutational effect. This explanation differs substantially from Mendel's explanation of independent assortment of independent traits or alleles. Also, Mendel directly contradicted Darwin's claim in Origin that changing conditions of life were the cause of variation in domesticated species. 5
In short, Darwin believed
in the inheritance of acquired characters.
This led him to his famous theory of continuous evolution.
Mendel, in contrast, rejected both the idea of inheritance of
characters (mutations) as well as the concept of continuous evolution.
discovered by him were understood to be the laws of constant elements
great but finite variation, not only for cultured varieties but also
in the wild.3 In
his short treatise, Experiments in Plant Hybridization, Mendel
incessantly speaks of "constant characters", "constant
offspring", "constant combinations", "constant forms",
"constant law", "a constant species" etc. (in such
combinations the adjective "constant" occurs 67 times in his original
paper). He was convinced that the laws of heredity he had discovered
corroborated GÃƒÂ¤rtner's conclusion "that species are fixed with limits
beyond which they cannot change". And
as Dobzhansky aptly put it, "It is...not a paradox to say that if
should succeed in inventing a universally applicable, static definition
species, he would cast serious doubts on the validity of the theory of
the Darwinians won the battle for the minds in the 19th century, no
left in the next decades for the acceptance of the true scientific laws
heredity discovered by Mendel. Further
work in genetics was continued mainly by Darwin's critics. In agreement
Vries, Tschermak-Seysenegg, Johannsen, Nilsson, et al., Bateson stated:
"With the triumph of the evolutionary idea, curiosity as to the significance of specific differences was satisfied. The Origin was published in 1859. During the following decade, while the new views were on trial, the experimental breeders continued their work, but before 1870 the field was practically abandoned. In all that concerns the species the next thirty years are marked by the apathy characteristic of an age of faith. Evolution became the exercising-ground of essayists. The number indeed of naturalists increased tenfold, but their activities were directed elsewhere. Darwin's achievement so far exceeded anything that was thought possible before, that what should have been hailed as a long-expected beginning was taken for the completed work. I well remember receiving from one of the most earnest of my seniors the friendly warning that it was waste of time to study variation, for 'Darwin had swept the field.'" 4
general acceptance of Darwin's theory of evolution and his ideas
variation and the inheritance of acquired characters are, in fact, the
reasons for the neglect of Mendel's work, which (in clear opposition to
pointed to an entirely different understanding of the questions
131: 245-253, 1992.
Callender, L. A., Gregor
Mendel: An opponent of descent with
modification. History of
26: 41-75. 1988.
Mendel, Gregor. Experiments
in Plant Hybridization. 1865.
Bateson, W. Mendel's Principles of Heredity. Cambridge: Cambridge University Press, 1909.
Daniel J. Fairbanks and Bryce Rytting, Mendelian Controversies: A Botanical and Historical Review, Invited Special Paper, American Journal of Botany 88(5): 737; 752. 2001.
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