The contribution of
Mendel to Genetics is called Mendelism. It includes all concepts brought out by
Mendel through his original research on plant hybridization. Mendelian genetic
concepts are basic to modern genetics. Therefore, Mendel is called as Father
Geneticist, Gregor Johann Mendel unraveled the mystery of heredity. He
was born on 22nd July 1822 in Heinzendorf Silesia (now Hyncice, Czechoslovakia
) , Austria. After school education, later he studied botany, physics and
mathematics at the University of Vienna.He then entered a monastery of
St.Thomas at Brunn in Austria and continued his interest in plant
hybridization.In 1849 Mendel got a temporary position in a school as a teacher
and he performed a series of elegant experiments with pea plants in his garden.
In 1856, he started his historic studies on pea plants. 1856 to 1863 was the
period of Mendel’s hybridization experiments on pea plants. Mendel discovered
the principles of heredity by studying the inheritance of seven pairs of
contrasting traits of pea plant in his garden. Mendel crossed and catalogued
24,034 plants through many generations. His paper entitled “Experiments on
Plant Hybrids” was presented and published in The Proceedings of the Brunn
Society of Natural History in 1866. Mendel was the first systematic researcher
in the field of genetics.
Mendel was successful
He applied mathematics and statistical methods to biology and laws
of probability to his breeding experiments.
He followed scientific methods and kept accurate and detailed
records that include quantitative data of the outcome of his crosses.
His experiments were carefully planned and he used large samples.
The pairs of contrasting characters which were controlled by
factor (genes)were present on separate chromosomes.
The parents selected by Mendel were pure breed lines and the
purity was tested by self crossing the progeny for many generations.
System – The Garden pea.
He chose pea plant
It is an annual plant and has clear contrasting characters that
are controlled by a single gene separately.
Self-fertilization occurred under normal conditions in garden pea
plants. Mendel used both self-fertilization and cross-fertilization.
The flowers are large hence emasculation and pollination are very
easy for hybridization.
Mendel’s theory of
inheritance, known as the Particulate theory, establishes the existence of
minute particles or hereditary units or factors, which are now called as genes.
He performed artificial pollination or cross pollination experiments with
several true-breeding lines of pea plants. A true breeding lines (Pure-breeding
strains) means it has undergone continuous self pollination having stable trait
inheritance from parent to offspring. Matings within pure breeding lines
produce offsprings having specific parental traits that are constant in
inheritance and expression for many generations. Pure line breed refers to
homozygosity only. Fusion of male and female gametes produced by the same
individual i.e pollen and egg are
Self pollination takes place in Mendel’s peas. The experimenter can remove the anthers (Emasculation) before fertilization and transfer the pollen from another variety of pea to the stigma of flowers where the anthers are removed. This results in cross-fertilization, which leads to the creation of hybrid varieties with different traits. Mendel’s work on the study of the pattern of inheritance and the principles or laws formulated, now constitute the Mendelian Genetics.
The First Model Organism
in Genetics – Garden Peas
Can you identify
Mendel’s gene for regulating white colour in peas? Let us find the
molecular answer to understand the gene function. Now the genetic
mystery of Mendel’s white flowers is solved.
It is quite fascinating to trace the Mendel’s
genes. In 2010, the gene responsible for regulating flower colour in peas
were identified by an international team of researchers. It was called Pea
Gene A which encodes a protein that functions as a transcription
factor which is responsible for the production of anthocyanin pigment.
So the flowers are purple. Pea plants with white flowers do not have
anthocyanin, even though they have the gene that encodes the enzyme involved in
normal copies of gene A into the cells of the petals of white flowers by the
gene gun method. When Gene A entered in a small percentage of cells of white
flowers it is expressed in those particular cells, accumulated anthocyanin
pigments and became purple.
In white flowers the
gene A sequence showed a single-nucleotide change that makes the transcription
factor inactive. So the mutant form of gene A do not accumulate anthocyanin and
hence they are white.
Mendel worked at the
rules of inheritance and arrived at the correct mechanism before any knowledge
of cellular mechanism, DNA, genes, chromosomes became available. Mendel
insights and meticulous work into the mechanism of inheritance played an
important role which led to the development of improved crop varieties and a
revolution in crop hybridization.
Mendel died in 1884. In
1900 the work of Mendel’s experiments were rediscovered by three biologists, Hugo
de Vries of Holland, Carl Correns of Germany and Erich von
Tschermak of Austria.
Mendel noticed two
different expressions of a trait – Example: Tall and dwarf. Traits are
expressed in different ways due to the fact that a gene can exist in alternate
forms (versions) for the same trait is called alleles.
If an individual has two
identical alleles of a gene, it is called as homozygous(TT). An
individual with two different alleles is called heterozygous(Tt).
Mendels non-true breeding plants are heterozygous, called as hybrids.
When the gene has two
alleles the dominant allele is symbolized with capital letter and the recessive
with small letter. When both alleles are recessive the individual is called homozygous
recessive (tt) dwarf pea plants. An individual with two dominant
alleles is called homozygous dominant (TT) tall pea plants. One
dominant allele and one recessive allele (Tt) denotes non-true breeding
tall pea plants heterozygous tall.
Mendel proposed two
rules based on his observations on monohybrid cross, today these rules are
called laws of inheritance The first law is The Law of Dominance and the second
law is The Law of Segregation. These scientific laws play an important role in
the history of evolution.
The Law of Dominance: The characters are controlled
by discrete units called factors which occur in pairs. In a dissimilar pair of
factors one member of the pair is dominant and the other is recessive. This law
gives an explanation to the monohybrid cross (a) the expression of only one of
the parental characters in F1 generation and (b) the expression of
both in the F2 generation. It also explains the proportion of 3:1
obtained at the F2
The Law of Segregation
(Law of Purity of gametes): Alleles do not show any blending, both characters are
seen as such in the F2 generation although one of the characters is
not seen in the F1 generation. During the formation of gametes, the
factors or alleles of a pair separate and segregate from each other such that
each gamete receives only one of the two factors. A homozygous parent produces
similar gametes and a heterozygous parent produces two kinds of gametes each
having one allele with equal proportion. Gametes are never hybrid.