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 be
carefully established before commencing investigations into the inheritance of
characteristics. The parental plants must be known to possess constant and
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 their
enclosed styles, drew his attention. On
trying several from this family, he finally selected the garden pea plant (Pisum
sativum) as being most ideal for his needs.
also picked the common garden pea plant because it can be grown in large numbers
and its reproduction can be manipulated. As with many other flowering
plants, pea plants have both male and female reproductive organs. As a
result, they can either self-pollinate themselves or cross-pollinate with other
plants. 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
observed seven pea plant traits that are easily recognized in one of two forms:
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. Seen color: yellow or green
6. Pod shape: inflated or constricted
7. Pod color: yellow or green
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, this is
the concept of alleles. Alleles are different versions of genes that impart the
same characteristic. For example,
each pea plant has two genes that control pea texture.
There are also two possible textures (smooth and wrinkled) and thus two
different genes for texture.
third law, in relation to the second, declares that, “If the two alleles
differ, then one, the dominant allele, is fully expressed in the organism's
appearance; the other, the recessive allele, has no noticeable effect on the
fourth law states that, “The two genes for each character segregate during
gamete production.” This is
the last part of Mendel's generalization. This references meiosis when the
chromosome count is changed from the diploid number to the haploid number. The
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 specific
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 allelic
sight as do texture genes and all other genes.
The result is that each gamete that is produced by the parent is uniquely
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 two
identical offspring will ever be produced even given trillions of years of time.
since a pea plant carries two genes, it can have both of its genes be the same.
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 recessive
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 in
biology at the time was that inherited traits blend from generation to
generation (Charles Darwin and most other cutting-edge scientists in the 19th
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 in the
resulting phenotype causing a “mixed” phenotype.
For example, some plants have “incomplete dominant” color genes such
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 produce a
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 remain
unchanged. They are transmitted
from parent to offspring through a process of random genetic recombination that
can be calculated statistically. For
example, the odds of a dominant trait being expressed over a recessive trait in
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 dominant
alleles, then 100% of the offspring will express the dominant trait.
If one parent has both recessive alleles and the other parent is a
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 that the
emergence of one trait will not affect the emergence of another. For example, a
pea plant's inheritance of the ability to produce purple flowers instead of
white ones does not make it more likely that it would also inherit the ability
to produce yellow peas in contrast to green ones. Mendel's findings
allowed other scientists to simplify the emergence of traits to mathematical
probability (While mixing one trait always resulted in a 3:1 ratio between
dominant and recessive phenotypes, his experiments with two traits showed
was so successful largely thanks to his careful and nonpassionate use of the
scientific method. Also, his choice of peas as a subject for his experiments was
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 needed
only to remove the immature stamens of the plant. In this way he was always sure
of each plants' parents. Mendel made certain to start his experiments only with
true breeding plants. He also only measured absolute characteristics such as
color, shape, and texture of the offspring. His data was expressed numerically
and subjected to statistical analysis. This method of data reporting and the
large sampling size he used gave credibility to his data. He also had the
foresight to look through several successive generations of his pea plants and
record their variations. Without his careful attention to procedure and detail,
Mendel's work could not have had the same impact that is has made on the world
cross-pollinating plants that either produce yellow or green peas exclusively,
Mendel found that the first offspring generation (f1) always has yellow peas.
However, the following generation (f2) consistently has a 3:1 ratio of
yellow to green.
3:1 ratio occurs in later generations as well. Mendel realized that this
is the key to understanding the basic mechanisms of inheritance.
is important to realize that in this experiment, the parent plants were homozygous
for pea color. That is to say, they each had two identical forms (or alleles)
of the gene for this trait--2 yellows or 2 greens. The plants in the f1
generation were all heterozygous. In other words, they each had
inherited two different alleles--one from each parent plant. It becomes
clearer when we look at the actual genetic makeup, or genotype, of the
pea plants instead of only the phenotype, or observable physical
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 passing on
either Y or G alleles to each offspring.
all of the seven pea plant traits that Mendel examined, one form appeared dominant
over the other. Which is to say, it masked the presence of the other
allele. For example, when the genotype for pea color is YG
(heterozygous), the phenotype is yellow. However, the dominant yellow
allele does not alter the recessive green one in any way. Both
alleles can be passed on to the next generation unchanged.
observations from these experiments can be summarized in two principles:
Principle of Segregation
Principle of Independent Assortment
came to four important conclusions from these experimental results:
inheritance of each trait is determined by “units” or “factors” (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 the next
genes for each trait segregate themselves during gamete production.
ideas on heredity and evolution were diametrically opposed to those of Darwin
and his followers (although neither Mendel nor Darwin knew of the other’s
work).2 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 acquired
characters (mutations) as well as the concept of continuous evolution. The laws
discovered by him were understood to be the laws of constant elements for a
great but finite variation, not only for cultured varieties but also for species
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 Gärtner'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 someone
should succeed in inventing a universally applicable, static definition of
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 space was
left in the next decades for the acceptance of the true scientific laws of
heredity discovered by Mendel. Further
work in genetics was continued mainly by Darwin's critics. In agreement with de
Vries, Tschermak-Seysenegg, Johannsen, Nilsson, et al., Bateson stated:
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 regarding
variation and the inheritance of acquired characters are, in fact, the main
reasons for the neglect of Mendel's work, which (in clear opposition to Darwin)
pointed to an entirely different understanding of the questions involved.1
131: 245-253, 1992.
Callender, L. A., Gregor Mendel: An opponent of descent with
modification. History of Science
26: 41-75. 1988.
Mendel, Gregor. Experiments in Plant Hybridization. 1865.
4. Bateson, W. Mendel's Principles of Heredity. Cambridge: Cambridge University Press, 1909.
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