How Mendel succeeded in formulating in his laws of inheritance, while the previous workers failed
Mendel
established principles that describe transmission of genes from parents to
offspring. When Mendel began his studies, he knew nothing about chromosomes and
role of meiosis in inheritance later investigation established parallel
behaviour between chromosomes and Mendel’s unit of inheritance during meiosis.
Mendel in 1856 performed his first hybridization experiment. Mendel was born in
1822 and died in 1884.
Mendel selected
garden pea plant (Pisum sativum) because it showed several sharply contrasting characters
that were without intermediate forms and were relatively unaffected by
environmental factors. Also the flowers of garden pea are self pollinating in
nature and so the crosses can be arranged according to the choice of breeder.
Mendel’s
characters are:
(1) Length of
Stem: tall or short
(2) Position of
flowers: axial or terminal
(3) Colour of
unripe pods: green or yellow
(4) Form of ripe
pods inflated or constricted
(5) For of ripe
seeds: round or wrinkled
(6) Colour of
cotyledons: Yellow or green
(7) Colour of seed
coat: Grey, brown or white.
The significance
of Mendel’s work lies in his ability to formulate a scientific problem. The aim
of his experiments was to study the numbers and kinds of offspring produced by
hybrid individuals and to determine from the observation whether any statical relationships
existed among these offsprings.
The pea plants
that Mendel used in his experiments were grown from seeds obtained from plants that
had ‘bred true’ when self fertilized for at least two generations before the
experiment. Such plants are called true breeding. When self pollinated and self
fertilized the offspring always resemble the parents for the given trait.
Mendel performed corsses in which only one pair of contrasting alleles or
alternative traits is being followed. Each such cross is known as monohybrid
cross.
Using results of
monohybrid crosses for various contrasting traits and to explain low unit
factors could account for the results of monohybrid corsses.
The results
Mendel’s eight years breeding experiments were read before the natural history.
Society of Brunn in 1865 and in the following year these were published in the
transactions of the society. But his work remained un noticed until 1990 when three
distinguished botanists Hugo De Vries in Holland, Tschermak in Austria and
Corrents in Germany discovered its significance. Since then Mendel’s work has
formed the bosis of study of genetics. Mendel dies in 1884 before he could see
his work accepted and appreciated. From the results of his experiments on
carefully selected crossings, Mendel formulated certain laws to explain the
interitance of characters as follows:
(1) LAW OF UNIT CHARACTERS:
This means that
all characters of the plant are units by themselves being independent of one
another so far as their unheritance is concerned. There are certain factors or
determiners (now called genes) of unit characters, which control the
expressions of these characters during the development of the plants.
(2) LAW OF DONUNANCE:
The characters
are controlled by factors or genes. These occur in pairs (in chromosomes) and
are responsible for tell ness and dwarfness separately. Tallness is dominant
character while dwarfness is recessive character. The contrasting pairs of
characters are allelomorphs. Thus tallness and dwarfness are allelomorphs.
Mendel gave two
laws (1) Law of Segregation of gametes (2) Law of independent assortment.
(1) FIRST LAW OF SEGREGATION
The factors for
contrasting characters remain associated in pairs in the somatic cells of each
plant throughout its whole life. Later when spores are formed as a result of
reduction division, the factors located in homologous chromosomes become
separated out, and each of the four gamete will have only one factor (tallness
or dwarfness) of the pair but not both i.e. gamete becomes pure for a
particular character. This law is also called law of purity of gametes.
Mendel made
experiment on Pea plants and found the ratio 3:1 in this law of segregation.
(2) SECOND LAW OF INDEPENDENT ASSORTMENT:
For dihybrid
cross Mendel found that two pairs of contrasting characters are taken into
idea. Mendel selected tall plant with red flower and a dwarf one with white
flowers. Factors for tallness or dwarfness and red flowers or white are
independently inherited and may be considered to be located in separate
chromosome pairs when seeds were crossed the result became 9:3:3:1.
Mendel succeeded
in formulating in his Laws of inheritance by doing experiments on plants in his
garden and got good results & formed two laws. Later Mendel was considered
as the father of Genetics. Later other scientists got base of Mendel’s work and
enhanced in further work of Genetics.
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