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A Brief History Of Genetics
In the 1500s and 1600s people began the classification of plants and animals into different species which led to the theory that species were created at the beginning of time and would never change. One theory in the 1600s was that of a Dutch scientist named Antoni van Leeuwenhoek who thought that life was created spontaneously from decaying matter when he observed flies appearing out of refuse. Later in the 1600s discoveries of the existence of the female egg and male sperm were made. It was believed that the sperm contained a tiny completely formed human being, or homunculus. This theory was called preformationism. Dutchman Regnier de Graaf suggested in 1672 that the sperm and egg united in some way and that both were responsible for appearance and character. This theory was not accepted for many years. 
William Harvey's experiments in the 1600s proved that blood circulated around the body and that the unborn embryo in deer grew larger and more complex as the pregnancy progressed. However the results of his experiments were not accepted until the 1700s. In the 1700s German scientist Caspar Wolff disproved preformationism. He carried out experiments which showed that plants and animals grew from material which looked nothing like the final plant or animal.
Pangenesis was disproved by the experiments of German scientist August Weismann who thought that complex creatures had two sorts of tissue; somatoplasm which is needed to make the creature work and changes to which could not be inherited, and germplasm, changes to which could be inherited. During the 1800s German zoologist Theodor Boveri and a scientist called Henking showed that when cells divided, a process called mitosis, both resulting cells had an exact copy of the number and type of chromosomes in their nucleus.
For example, according to Lamarck, if feeding giraffes are unable to reach the branches of a tree then they will develop longer necks, whereas according to Darwin the giraffes with shorter necks would die out and only those with necks long enough would survive and pass the characteristic to their offspring.
Darwin was never able to show natural selection occurring and died believing all
the issues were unsolved, however in 1865 Czechoslovakian Gregor Mendel published
the results of his experiments with plant hybridization and his law is the
foundation of modern genetics. Mendel's Law is accepted as being one of the most
important biological discoveries of all time.
In 1869 Sir Francis Galton (who happened to be Charles Darwin's cousin) suggested that if identical twins inherit identical characteristics then any differences between them must be due to environmental differences. It was he who is responsible for the idea that a species can be improved by selective breeding, which became known as eugenics. He also discovered that fingerprints are unique. Swiss biochemist Friedrich Miescher researched into the chemical structure of cells in the late 1800s. In 1902 American biologist Walter Sutton was the first to suggest that chromosomes contained the necessary factors for heredity in his publication Chromosome Theory of Inheritance.
Following this, British biologist William Bateson discovered the principle of linkage which states that several factors are always found together on every chromosome which is made up of groups of these factors which 'are linked together and cannot behave independently'. In 1909 these hereditary factors were named genes by Danish botanist Wilhelm Johannsen. In 1904 American Thomas Hunt Morgan began using Bateson's work as the basis for his research and with a group of students developed the idea of a chromosome being made up of genes arranged in a long line, linked together in groups. Deoxyribonucleic acid (DNA) was established as genetic material in 1944 by American biologists Oswald T. Avery, Colin Macleod and Maclyn McCarty at the Rockefeller Institute in New York. They were able to show that DNA fragments from one variety of bacteria could enter the cells of another variety and change its heredity. In 1952 DNA was finally confirmed as the genetic material by American biologists Alfred Hershey and Martha Chase. Back to Top Basic GeneticsAll living things start life as a single fertilised cell called a zygote. The single-celled zygote splits into two and begins to multiply to make the building blocks of plants or animals. The cells of plants and animals look different but their construction and operation are very similar. Individual threads in the nucleus of the cells are called chromosomes.
There is another form of cell division called meiosis which only occurs in the reproductive organs where the sex cells, gametes, are formed. In the human body the female egg and male sperm contain 23 chromosomes which fuse together to form a zygote during reproduction which then contains the full amount of 46 chromosomes in 23 matching pairs. 
The chromosomes in each pair are called homologues. One member of each homologue is from the father and the other from the mother. There are two types of chromosome; the sex chromosomes and the autosomes. In humans as well as most animals males and females have two sex determining chromosomes. In the female they are both X chromosomes but in the male one is an X chromosome and the other a smaller Y chromosome. Back to Top Genetics & Treacher Collins SyndromeThe features of TCS are, however, variable so that whilst some people may be severely affected others can be so mildly affected that it is very difficult to say whether they have the condition or not. All human genetic information is carried on structures called chromosomes, of which there are 46 in most cells of the body. Of these 46 chromosomes, two are involved in determining the baby's sex whilst the remainder are made up of 22 pairs. Each chromosome contains many genes, each one having the ability to produce a certain characteristic. Under normal circumstances each of a pair of chromosomes looks alike and at the same position on each chromosome are genes determining the same characteristic. Usually these genes are identical although in some cases (for instance in TCS) one gene is different from its opposite number and cannot produce its normal characteristic - the genetic information is effectively broken. If either parent has TCS there is a 50-50 chance that each of their children may be affected. If the TCS (broken) gene is passed from parent to child the child will be affected. In approximately 60% of cases, however, there is no previous family history of the condition - neither parent has it. In this case the child has developed TCS as a result of genetic mutation, not through hereditary causes. Although the broken gene has now been found there is still a great deal of research to be done to gain a much better understanding of the mechanisms underlying TCS. Moreover, this may also lead to a better idea of the mechanisms responsible for other forms of conductive deafness. Identification of the gene should also ultimately aid early diagnosis of TCS, which will be particularly useful for those people with very mild TCS and in whom clinical diagnosis is correspondingly difficult. As a result of the earlier discovery of the chromosome upon which the gene responsible for causing TCS lies upon it is now possible to predict who is and who is not affected by TCS with a very high degree of accuracy (greater than 95%). Unfortunately a test such as this will not be applicable to all families until a later point in the ongoing research. The families that would be potentially suitable for testing are those families in which there is a clearly defined history of TCS and not those where TCS has arisen without a previous family history. At present there is no reliable prenatal testing for TCS. Specialist ultrasound can sometimes give an indication that a baby may be affected but cannot indicate the severity. X-rays can sometimes help to determine whether a person has TCS. Any genetic testing/consultation should be carried out by specially qualified geneticists who have experience of TCS. For more information about the TCS research project contact: Prof. Michael Dixon Department of Cell and Structural Biology University of Manchester School of Biological Sciences Stopford Building Oxford Road Manchester M13 9PT Back to Top |
