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Wednesday, October 17, 2018

biology:History of Genetics

History of Genetics
Genetics is primarily and originally a science dealing with heredity i.e. the
transmission of characteristics from parents to offspring. From such
considerations, laws are derived concerning the relationships. In addition,
genetics also involves a study of the factors, which show the relationship
between parents and offspring and which also account for the many
characteristics which organisms possess. You are familiar with the
observations that “Like begets like”, that children tend to resemble their
parents as well as their siblings (or sibs i.e. their brothers and sisters), but
they also tend to vary or look different from one another in many ways.
Genetics is the science, which tries to account for similarities and variations
between related individuals. The science studies the transmission of
hereditary factors from parents to offspring. Put differently, it is a study of
biological “communication” between generations using the hereditary
factors. Another facet of the science is the study of the expression or effect
of the factors during development.
If one were to put the above “descriptive definition” of Genetics in a capsule
form, Bateson, who coined the term Genetics in 1906 aptly defines it as
follows:
Genetics is the science dealing with heredity and variation, seeking to
discover laws governing similarities and differences in individuals related to
descent. The factors which are transmitted were called “Genes” by
Johannsen in 1909.
As mentioned above, Genetics provide explanations to the phenomenon of
heredity and variation. It is therefore, not surprising that the beginning of
genetics dated back to the centuries before Christ. Around 400 BC
Hippocrates theorized that small representative elements of all parts of the
parental body are concentrated in the semen. It is these elements, which
provide the building blocks for the corresponding parts of the embryo.
According to this theory characteristics acquired by parents can be
transmitted to offspring.
Aristotle (384-322 BCE), one century later disproved the theory postulated
by Hippocrates (about 470 BC-about 410 BC), pointing out the facts that
crippled and mutilated parents do not always produce abnormal offspring.
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Aristotle, in turn advanced the theory that the father’s semen provides the
plans according to which the amorphous blood of the mother is to be shaped
into the offspring. Put differently the semen supplied the FORM while the
mother’s blood supplied the SUBSTANCES. It is important at this point to
note that Aristotle recognized that biological inheritance consists of a
transmission of information for embryonic development, and not simply a
transmission of samples of body parts. The fact that the information in the
seminal fluid could not be seen, it was regarded as a mystical influence.
Early in the 17th century, Harvey called this influence the AURA
SEMINALIS.
In the 17th and 18th centuries, new theories of inheritance were propounded,
following the discoveries of the egg and the sperm. One theory was the
PREFORMATION THEORY, which depending on the school of thought,
stated that either the egg or the sperm contains the entire organism in a
miniaturized but perfect form. In the case of men, the theory postulated a
miniature human being, called a homunculus, present in the sperm. This
theory was postulated by Jan Swammerdam (1637-1680). Not too
surprisingly there were scientists who claimed that they saw homunculi in
spermatozoa. They even drew diagrams to illustrate what they saw. One
person who made an elaborate drawing of homunculus was Nicolass
Hartsoeker (1656-1725). The major drawback with the pre-formation theory
is the fact that is implies that one homunculus contained another, which in
turn contained yet another ad infinitum.
Another theory of development was the THEORY OF EPIGENESIS. In the
18th century, Christian Wolff (1679-1754) discovers that adult structures in
plants and animals arise from embryonic tissues, which do not resemble the
corresponding adult structures. In other words, there is no pre-formation. But
Wolff thought that mysterious vital forces were responsible for what he
thought was a de novo origin of adult parts. Wolff’s view modified in the 19th
century by Karl Ernst Von Baer (1792-1876) who stated that adult parts arise
as a result of a gradual transformation or differentiation of embryonic tissues
into increasingly specialized tissues. Although the modified epigenetic theory
is correct. It did not account for the form in which the materials to be
transformed existed in the original embryonic cell, zygote.
Early in the 19th century, Pierre-Louis Maupertuis (1698-1759) postulated
that minute particles from each part of the body of the parents are united in
sexual reproduction such that during development particles from the male
dominate in some cases; in other cases those form the female parent
dominate. In one important aspect, this theory recognized the fact that an
offspring receives two of each type of particle, one from each parent, but
exhibits only one. However, by suggesting that the body parts contribute
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particles, this theory leads to the theory of evolution advanced by Jean-
Baptiste Lamarck (1744–1829). According to Lamarck’s interpretation
characteristics such as well-developed muscles acquired by parents in the
course of their life can be transmitted to their offspring. This idea was
formalized by Charles Darwin (1809-1822) as the “Provisional Hypothesis of
Pangenesis.” According to Darwin, exact miniature replicas, called
gemmules, of the body parts and organs are carried in the blood stream, to be
assembled in the gametes. In the zygote, the gemmules from both sexes
come together and are parceled out to form the appropriate structures during
development. Since a gemmule is an exact replica of a parental part it means
that acquired characteristics should be inherited by the offspring. If that were
so it would be easy to understand evolution. Recall that the theory of
pangenesis is essentially the same theory advanced by Hippocrates in the 5th
century B. C. and disproved by Aristotle.
The theory of pangenesis lends itself readily to testing, and it was tested by
August Weismann (1834–1914), toward the end of the 19th century. He cut
off the tails of mice for 22 generations, yet the offspring of such mince
continued to show tails of normal length in every generation. The experiment
can be represented schematically as follows:
Generation I: Cut off tails of the mice and mate them.
Generation II: Offspring with tails; repeat operation
Generation III:Offspring with tails; repeat operation
Generation IV: Offspring with tails; repeat operation
: :
: :
Generation XXI: Offspring with tails; repeat operation
Generation XXII: Offspring with tails.
The result therefore showed that it cannot be true that acquired characteristics
can be inherited.
In spite of this proof there are people who still accept the inheritance of
acquired characteristics. Perhaps the most prominent adherent in recent times
was the Russian, Trofim Lysenko (1898–1976). He coerced many Russian
geneticists to accept the theory, because he wielded political power.
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To replace the theory of pangenesis August Weismann (1834-1914) proposed
the GERMPLASM THEORY in 1885. According to this theory, multicellular
organisms are made up of two types of tissues, viz the somatoplasm and the
germplasm. The somatoplasm is made up of tissues which are essential for
the functioning of the organism, but they do not determine what is
transmitted to the offspring. In other words, changes in the somatic tissues
are not transmitted. The tail of a mouse is a type of somatic tissue. On the
other hand the germplasm is a tissue whose sole function is the formation of
gametes. Since the gametes give rise to the offspring, changes in the
germplasm may lead to changes in the offspring. Notice, however, that the
theory does not indicate what the germplasm transmits.
Many biologists including Josef Gottlieb Kolreuter (1733-1806) compared
the similarities and differences between plant hybrids and their parents. A
hybrid is an offspring from two different parental types. Kolreuter found that
although hybrids from two parental stocks are usually similar, such hybrids if
fertile usually produce offspring which show considerable diversity. The
results of such hybridization studies were recorded simply as qualitative
observations.
Kolreuter and many others after him did not record the ratios in which the
original parental characters occurred among the progeny. As we shall see
later, it is therefore not surprising that the early hybridizers did not discover
any underlying principles of inheritance. Thus, even though they made many
important observations, the hybridizers pre-date the origin of genetics.
In many ways Genetics is a precise and somewhat mathematical science
dealing with specific offspring ratios which are predictable on the basis of the
known genetic constitutions of the parents. In the reverse process, the genetic
constitution of the different types of offspring they produced.
Gregor Johann Mendel (1822-1884), an Austrian monk, is regarded as the
father of Genetics. It is generally agreed that Mendel’s success can be
attributed to the fact that he was lucky in choosing the garden pea, Pisum
sativum, for his studies. This plant, although, normally self-pollinating can be
easily cross-pollinated. Mendel was also successful because he studied the
inheritance of single contrasting characters (i.e. smooth versus wrinked),
unlike his predecessors who studied several characters simultaneously.
Equally important was the fact that Mendel counted and carefully recorded
the numbers of each type of offspring from each of his crosses.
Mendel published his results in 1866 after he had reported them at a Natural
Science meeting in 1865. He clearly stated the laws of inheritance which can
be derived from his results. The law constitute the foundation stones of
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Genetics. In spite of the fundamental nature of Mendel’s discoveries and the
clarity with which he stated his results and conclusions, his papers had no
immediate impact on the scientific world. However, one Russian botanist,
Ivan Ivanovich Schmalhausen (1884-1963) stressed the importance of
Mendel’s findings soon after they were published. Mendel’s discovery did
not have an immediate effect because the related information required for
understanding his deductions were not available at the time. Thus it may be
said that Mendel was “ahead of his time”.
After publication of Mendel’s results other relevant information about
development were provided by various workers. In 1875, Oscar Hertwig
(1849-1922) and later, Hermann Fol, and Eduard Strasburger described the
process of fertiliztion including the fusion of the egg and the sperm nuclei.
Between 1880 and 1885, Fleming, van Beneden and Strasburger described
chromosomes and their division in mitosis as well as their constancy in
number. Later Hertwig and Strasburger developed the theory that the nucleus
contains hereditary materials. These discoveries were reflected in
Weismann’s theory of the Germplasm. Weismann postulated that in the
process of gametogenesis, i.e. the formation of gametes there must be a
reduction in half of the number of chromosomes. If that were not so, there
would be a doubling of the chromosome number at each fertilization.
However, as mentioned earlier the chromosome number is constant from
generation to generation. The postulate by Weismann of reduction in
chromosome number was later observed by Boveri and other investigators.
The process involved is meiosis.
Three investigators unaware of Mendel’s work and results independently
carried out similar plant breeding experiments. During the process of writing
their findings for publication, they each came across Mendel’s paper and they
referred to it in their rediscovery of the Mendelian laws of inheritance.
Although the three people, Correns, Hugo de Vries and Tschermak are
generally regarded as the rediscoverers, some scientists (Stern & Sherwood,
1966) do not think that Tschermak’s work on its own could have yielded the
laws of inheritance. Hence, there should be only two rediscoverers.
Although the laws of inheritance were first demonstrated with plants, Bateson
in 1902 showed that the laws apply equally to animals.
From this brief history of Genetics one would hope that you would derive and
appreciate the tortuous steps leading to the establishment of various laws in
science

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