Genetics for the clinician / C.A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1964
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics for the clinician / C.A. Clarke. Source: Wellcome Collection.

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    CHAPTER ONE THE NATURE OF THE GENE To think of Gregor Mendel is for many to recall only some dull and simple ratios, the result of breeding garden peas. There is often failure to realize that the fundamental importance of his work was the discovery that the characters of an organism are inherited as discrete units and that each retains its particulate nature from generation to generation. This still remains a good approximation of what geneticists believe today, yet a hundred years ago Darwin was ignorant of it and thought that inherit¬ ance was blended. He appreciated that blending would lead to uniformity whereas all around he saw variability but he explained the difficulty by assuming that mutations occurred very frequently—a view which we now know to be untrue. Mendel's work showed conclusively that blend¬ ing of the units of inheritance—the genes—did not occur, for though the first filial (Fi) generation often presented an intermediate appearance yet in the second filial (F2) generation the genes would segregate and the parental characters reappear unchanged. It is genes which do not blend— characters may be and often are intermediate. When Mendel's work was rediscovered in 1900 much more was known about the chromosomes and it was soon obvious that they provided exactly the right mechanism necessary for carrying the genes and for ensuring their segregation. Ford has pointed out a close parallel between the behaviour of the chromosomes as seen under the microscope and that of the genes whose action can be inferred by tracing the inheritance of the characters which they control. We cannot do better than quote a striking passage from his book: 'The genes are present in pairs (allelomorphs) as are the chromosomes (homologous chromosomes). The members of these pairs, both of genes and of chromosomes, are derived respectively from the two parents. Consequent upon Mendehan segregation, the genes constituting the allelomorphs separate from one another and pass into different gametes, as do the members of the homologous pairs of chromo¬ somes, owing to meiosis. The gametes then contain one member only of the pairs both of genes and of chromosomes; but these are restored by the additive nature of fertiHzation'. (Ford, 1961.) There is overwhelming evidence that genes are situated on the chromo¬ somes. For example, if we look at the salivary gland of Drosophila we see on each giant chromosome transverse bands, some wide and some narrow, I