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Description : This revolution in scientific knowledge and expertise has led to the realization that genetics is an area of major importance in almost every medical discipline. Learn about GREGOR MENDEL AND THE LAWS OF INHERITANCE.

Tags : Genetic History, genetics,LAWS OF INHERITANCE, homozygous, heterozygous, Punnetts square

Published on : Dec 25, 2020 | Views : 722 | Downloads : 1

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By Dr. Sawsan Shurrab 1
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The history of genetics in relation to medicine is one of breathtaking discovery from which patients and families already benefit hugely but in today's world success will be measured by continuing progress in the treatment and prevention of disease. GREGOR MENDEL AND THE LAWS OF INHERITANCE Early Beginnings
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Developments in genetics during the twentieth century have been truly spectacular. In 1900 Mendel's principles were awaiting rediscovery,chromosomes were barely visible, and the science of molecular genetics did not exist.
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By contrast, at this time over 14000 single-gene disorders or traits have been identified, chromosomes can be analyzed to a very high level of sophistication, and the draft sequence of the entire human genome has been reported.
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This revolution in scientific knowledge and expertise has led to the realization that genetics is an area of major importance in almost every medical discipline. Recent discoveries impinge not just on rare and esoteric 'small-print' diseases, but also on many of the common acquired disorders of adult life, such as cardiovascular disease, psychiatric illness and cancer.
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Consequently genetics is now widely accepted as being at the forefront of medical science and has become an important and integral component of the undergraduate medical curriculum.
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In order to put the exciting development and growth of genetic science into context we start with a brief overview of some of the most notable milestones in the history of medical genetics.
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Early Greek philosophers and physicians such as Aristotle and Hippocrates concluded, with typical masculine modesty, that important human characteristics were determined by semen, utilizing menstrual blood as a culture medium and the uterus as an incubator. Semen was thought to be produced by the whole body; hence baldheaded fathers would beget baldheaded sons.
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These ideas prevailed until the seventeenth century, when Dutch scientists such as Leeuwenhoek and de Graaf recognized the existence of sperm and ova, thus explaining how the female could also transmit characteristics to her offspring.
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1865
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In his breeding experiments Mendel studied contrasting characters in the garden pea, using for each experiment varieties that differed in only one characteristic. For example, he noted that when strains that were bred for a feature such as tallness were crossed with plants bred to be short, all the offspring in the first filial or F1 generation were tall. If plants in this F1 generation were interbred, then this led to both tall and short plants in a ratio of three to one .
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Mendel's proposal was that the plant characteristics being studied were each controlled by a pair of factors, one of which was inherited from each parent. The pure-bred plants, with two identical genes, used in the initial cross would now be referred to as homozygous
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The hybrid F1 plants, each of which has one gene for tallness and one for shortness, would be referred to as heterozygous. The genes responsible for these contrasting characteristics are referred to as allelomorphs, or alleles .
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An alternative method for determining genotypes in offspring involves the construction of what is known as a Punnett's square .
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Punnett's square
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On the basis of Mendel's plant experiments, three main principles were established. These are known as the laws of uniformity, segregation and independent assortment.
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The law of uniformity refers to the fact that when two homozygotes with different alleles are crossed, all the offspring in the F1 generation are identical and heterozygous. In other words, the characteristics do not blend as had been believed previously, and can reappear in later generations. THE LAW OF UNIFORMITY :
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The law of segregation refers to the observation that each individual possesses two genes for a particular characteristic, only one of which can be transmitted at any one time. Rare exceptions to this rule can occur when two allelic genes fail to separate because of chromosome non-disjunction at the first meiotic division . THE LAW OF SEGREGATION :
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Refers to the fact that members of different gene pairs segregate to offspring independently of one another. In reality, this is not always true, as genes that are close together on the same chromosome tend to be inherited together, i.e. they are linked. THE LAW OF INDEPENDENT ASSORTMENT:
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There are a number of other ways by which the laws of Mendelian inheritance are breached but, overall, they remain foundational to our understanding of the science.
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As interest in Mendelian inheritance grew, there was much speculation as to how it actually occurred. At that time it was also known that each cell contains a nucleus within which there are several thread-like structures known as chromosomes, so called because of their affinity for certain stains chroma = color, soma = body. THE CHROMOSOMAL BASIS OF INHERITANCE
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In 1903, Walter Sutton, an American medical student, and Theodor Boveri, a German biologist, independently proposed that chromosomes carry the hereditary factors, or genes. These suggestions were prompted by the realization that the behaviour of chromosomes at cell division could provide an explanation for how genes could segregate.
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When the connection between Mendelian inheritance and chromosomes was first made, it was thought that the normal chromosome number in humans was 48; the correct number of 46 was not established until 1956, 3 years after the correct structure of DNA was proposed.
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Within a few years it was shown that some disorders in humans could be caused by loss or gain of a whole chromosome as well as an abnormality in a single gene.
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Whilst James Watson and Francis Crick are justifiably credited with discovering the structure of DNA in 1953, they were only attracted to working on it because its key role as the genetic material was established in the 1940. Formerly many believed that hereditary characteristics were transmitted by proteins. DNA AS THE BASIS OF INHERITANCE
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The genius of Watson and Crick, at Cambridge, was to hit on a structure for DNA that would explain the very fact of biological reproduction, and their elegant double helix has stood the test of time. Crucial to their discovery was the X-ray crystallography work of Maurice Wilkins and Rosalind Franklin at King's College, London.
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The relationship between the sequence of bases in DNA and the sequence of amino acids in protein, i.e. the genetic code, was unravelled in some very elegant biochemical experiments in the 1960s. Thus it became possible to predict the base change in DNA that led to the amino acid change in the protein.
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However, direct confirmation of that prediction had to wait until DNA sequencing methods became available after the advent of recombinant DNA techniques. Interestingly, however, the first genetic trait to be characterized at the molecular level had already been identified in 1957 by laborious sequencing of the purified proteins. This was sickle-cell anemia, in which the mutation affects the amino-acid sequence of the blood protein hemoglobin.
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Before returning to historical developments in human genetics, it is worth a brief diversion to consider the merits of this unlikely creature that has proved to be of great value in genetic research. The fruit fly, Drosophila possesses several distinct advantages for the study of genetics. THE FRUIT FLY
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These are as follows : It can be bred easily in a laboratory. It reproduces rapidly and prolifically at a rate of 20-25 generations per annum. It has a number of easily recognized characteristics, such as curly wings and a yellow body, which show Mendelian inheritance.
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Drosophila melanogaster, the species most frequently studied, has only four pairs of chromosomes, each of which has a distinct appearance so that they can be identified easily. The chromosomes in the salivary glands of Drosophila larvae are among the largest known in nature, being at least 100 times bigger than those in other body cells.
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In view of these unique properties, fruit flies were used extensively in early breeding experiments. Today their study is still proving of great value in fields such as developmental biology, where knowledge of gene homology throughout the animal kingdom has enabled scientists to identify families of genes that are very important in human embryogenesis .
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