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GENETICS

1.2    THE BRITH OF GENETICS  

 There can be little doubt that the first human on earth pondered the observation that children resembled their parents more  than other member of population, unfortunately, however, we havee no record of their ideas as to why this occurred. The Greek philosophers, Hippocrates and Aristotle obviously taught extensively about this fact and developed theories to explain resemblances among relatives.

            Gregor mendel (1822-1884) is appropriately called the father of genetics. His precedent –setting experiments with garden peas pisum sativum  published in 1866, were conducted in the limited space of a monastery garden while he was also employed as a substitute school teacher. The conclusion he drew from his elegant investigations constitute the foundation of today’s science of genetics. mendels was not the first to perform hybridization experiment, but he was one of the first to consider the result in terms of single traits. Sagret in 1826 had studied the inheritance of contracting traits. Other of Mendel’s predecessors had considered whole organisms which incorporate a nebulous  complex of traits, thus they could observed only that similarities and differences occurred among parents and progeny, and so missed the significance of individual differences. Employing the scientific method, Mendel designed the necessary experiments, counted and classified the peas resulting from his crosses, compared the proportion with mathematical models, and formulated a hypothesis for these differences. In 1900, mendel’s paper was discovered simultaneously by three botanists; Hugo devries in Holland, known for his mutation theory and studies on the evening primrose and maize, Carl correns in Germany, who investigated maize, peas, and beans, and Eric von tschermak seysenegg in Austria, who worked with  several plants including garden peas. Each of these investigators obtained evidence for mendel’s principle from his own independent studies. They all found Mendel ‘s report while searching the literature for related work and cited it in their own publications. william bateson, an English man, gave this developing science the name ‘genetics‘ in 1905. he coined the term from a greek word meaning “ to generate”.

            In addition to naming the science, Bateson actively promoted Mendel’s  view of paired genes. he used the word ’’allelomorph”, subsequently shortened to “allele”, to identity members of pairs that control different alternative traits. Also during the early 1900s, a French man, Lucien cuenot, showed that genes controlled fur colour in the mouse; an American, W.E castle, related genes to sex and to fur color  and pattern in mammals, and a Dane W.L Johannsen , studied the influence of heredity and environment in plants. Johannson began using the word “gene” from the last syllable of Darwin’s term ‘’pangene”   the gene concept, however, had been implicit in mendel’s visualization of a physical element or factor (Anlgae ) that acts as the foundation for development of a trait. These men and their peers were able to build on the basic principles of cytology, which were established between 1865 (when mendel’s work was completed) and 1900 (when it was discovered ).

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2.1       HISTORY OF GENETICS

            Prior to 1900, different theories were proposed to explain inheritance of character in living organism. The earliest, perhaps was the theory of spontaneous emergence of organism from organic matter. Then there was the per formation theory which regarded organism as present in miniature form in one of the gametes, and required only proper nutrition to mature. There was also the theory of epigenesis which attributed the emergence of structures that were not originally present in the individual to some unexplainable force.

            Later on in the nineteen the century, the father of modern evolutionary theory, charles  Darwin, proposed a blending theory (pangenesis )whereby offspring arose simply from a ‘mixing’ of the different characters in their parents. Lamarck’s theory, which followed, proposed that acquired characters were inherited. For example an individual who has one limb amputed as a result produce some offspring with only one limb.

            However, as more advances were made regarding  cell  structure, cell division and behavior of chromosomes, more plausible theories were propounded. One of such theory was Weisman’s ‘germ plasm‘ theory that organisms produces two types of tissues, namely, somatoplasm and germ plasm. The germ plasm transmits hereditary materials from generation to generation. A firm basis for particulate inheritance was provided by the finding of an Austrian monk Gregor Johann Mendel. From his experiments with the garden pea  (pisum sativum), Mendel demonstrated that the appearance of different characters followed specific laws. His findings were published in 1865, but they were however not widely recognized until 1900 when they were independently rediscovered by Hugo Dc vries , CG correns E .tschermak.

1.2         PRINCIPLE  AND THEORIES

Some theories have been developed to back up some of the scientific / experimental findings by contributors. Such theories are;

  1.                       i.        Charles Darwin’s theory –thus, know as the father of modern evolutionary theory, he proposed a blending (pangenesis) whereby offspring arose simply from a ‘mixing‘  of the different characters in their parents.
  1.                     ii.        Lamarck’s theory – He proposed that acquired character were inherited
  2.                    iii.        Hardly –Weinberg law-       It states that gene, and hence, genotype frequencies in a large random – mating population that is not subjected to selection, migration and mutation, remains constant every generation.
  3.                    iv.        Mendel’s experiments with the garden pea gave rise to two fundamental laws of genetics universally known as mendel’s  law. These are the law of segregation and law of independent assortment.

LAW OF SEGREGATION = It states that hereditary characters are controlled by pair of factors’ now called genes, which separate during gamete formation such that only one member of the pair is transmitted by a particular gamete.

Law of independent assortment=It states that members of one allelic pair segregate independently of other pairs and combine randomly to produce different gametes.

1.3           GENETICS AND HEALTH

The growth and development of a ‘’normal,” “ healthy’’ adult human from a single cell, the zygote or ovum after fertilization by a sperm, is one of the most spectacular phenomena in biology. This complex process requires the coordinated action of thousands of genes. Each of the thousand of genes involved in the growth and differentiation of the multitude of different cell types in the adult organism must be expressed at the proper time and place in the developmental pathway. One fact is very clear. The genes in the human genome are highly co-adapted, and a malfunction in the expression of even a single gene can disrupt the entire developmental process. If the gene  involved  encodes an essential function, its malfunction will be lethal. In many other cases involving important but not absolutely essential (for survivial )  genes, such a malfunction will produce an unhealthy” or” abnormal “ phenotype. Often a defect in a single gene will lead to a whole series of phenotypic abnormalities collectively referred to as a ‘’syndrome’’ one well- known example is congenital hyperuricemia or lesch –Nyhan syndrome .this syndrome is caused by a defect in single gene. New born with this syndrome appear normal but produce excess amounts of uric acid in their urine. By about 10 to 12 months of age, mutilation (teeth,  grinding, lip biting , and so on ) usually ensues. Death often results from severe neurological and renal damage during adolescence.

            The loss of activity of this single gene  product upsets the normal metabolic balance in the developing infant and leads to the entire array of phenotypic abnormalities. however genetics impacts on health and medical practices at many other levels are far too numerous to describe. One of the greatest successes of modern medicine has been the development of antibiotics to combat disease caused by pathogenic bacteria.

            About hundreds of human diseases are known to be caused by defects in  individual genes. The question is how can these be prevented or treated ? it is hoped that somatic cell gene therapy will provide an effective means  of treatment for some of these diseases in the not too- distant future. All genes have the same basic chemical composition, and all are undoubtedly capable of undergoing mutation to non functional states. Clearly, then, the number of inherited disease in humans will be large, probably on the order of 10,000. With this in mind, one of society’s goal must be to keep mutation rates and the genetic load” ( The accumulation of deleterious mutant genes) in the human genes pool at a tolerable level. To do so, we must minimize the pollination of our biosphere with substances that will increase the frequency of mutations, most notably radioactive agents and mutagenic chemicals.

1.5         GENETICS AND FOOD PRODUCTION  

The contributions of genetics to increased food production is without question, one of the premier success stories of science in the twentieth country. Moreover, the pinnacle of these achievements was clearly the development of hybrid corn. Impressive increases in yield also have been achieved for most of the other important food crops. These dramatic increase in yields of crop plants, collectively call the ‘’green revolution’’, have played a major role in our present ability to feed an over populated world if economic and political systems that would permit adequate world wide distribution of the food produced were in place.

            Although we will never know for certain exactly when human carried out the first “genetic selection” experiments, this probably occurred during the period from 10,000 to 7000(AD) year ago. Fossil records indicates that almost all our present food crops  were domesticated during this early Neolithic period coincident with the development of stone tools. Initially, successful selection was done with no knowledge of the genetic basis of responses that occurred. The largest, more vigorous individuals or those with desired characteristics were simple chosen as parents for subsequent generations. This general approach is still a mainstay of modern plant and animal breeding. However, knowledge about the genetic variability in the population for the trait of interest now permits plants and animal breeding to fine-tune these selection  experiments and to predict the changes that will be realized in response to specific selection strategies.

            The important role of genetics in the achievement of the green revolution is documented by a comparison of the striking increase in agricultural production. Today scientists through the rest of the  world, are performing sophisticated plant and animal breeding and selection experiment that are designed on the basis of detailed information about the genetic control of traits such as yield growth rates in domesticated plants and animal. In addition, scientists are now using  genetic engineering techniques to design crop plants with desired gene such as insect and disease resistance for example, genes from fungi, other microorganisms ,or wild plant species that provide these organisms with resistance to insect pest can now be isolated, tailored for new host organisms by recombinant DNA procedure in vitro- for example, by adding new regulatory elements so that they will be expressed in the proper tissues in the new hosts and introduced into agronomically important domestic plants species by gene transfer technology.

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