Genetics in ophthalmology / Arnold Sorsby.

  • Sorsby, Arnold, 1900-1980.
Date:
1951
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics in ophthalmology / Arnold Sorsby. Source: Wellcome Collection.

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    THE NATURE OF THE GENE AND OF ITS ACTION THE NATURE OF THE GENE AND OF ITS ACTION One view of the gene stresses its similarity to the virus. Both are ultra-microscopic, both probably consist of nucleo-proteins, both live inside cells, and both have the faculty of self-perpetuation with widespread biological effects. It is suggested that they both represent the most primitive form of life: the virus is regarded as an uncivilized gene with a predatory parasitical existence on other forms of life. The gene is its civilized counterpart, and lives in an organized community. Chemically the gene is regarded as a giant molecule in which each radical has a characteristic effect. Re-arrangement of the radicals can produce isomers in the same way as these are produced in simpler molecules with extensive changes in the chemical and physical properties of such molecules. Mutations may therefore be isomeric in character, and x-ray study of genes suggests that a mutation is essentially the effect of ionization confined to one atom. The discontinuity presented by mutations has its counterpart in the jumps of present-day quantum mechanics. Gene action is being visualized in terms of enzymes. Each gene is assumed to produce a specific enzyme. Albinism may be used as an illustration. Normal pigmented individuals produce melanin in pigmented areas; this pigment is derived from phenylalanine, an amino-acid derived from food. A complicated series of chemical reactions lie between phenylalanine and melanin; each of these steps requires the presence of a specific enzyme. The normal individual possesses all these enzymes; an albino cannot, however, convert one of the intermediate products (3, 4-dihydroxyphenylalanine—also known as dopa) into the next compound in the chain of reactions. Likewise in Tay-Sachs disease the patient lacks the necessary enzyme for oxydizing the lipoid sphingomyelin. In some lower animals, any of the cells composing the developing fertilized ovum before growth can be separated artificially and induced to develop into a complete young animal. This is no longer possible once differentiation has taken place. Development is visualized as consisting of the following sequence of events. All cells contain their full complement of chromosomes and genes, but different cells differ in the chemical stimuli ( organizers ) that they liberate activating the genes. Thus in the cells that ultimately form skin, only those genes are stimulated which cause the development of pigment, hair and other features of the skin. The remaining genes lie dormant, but can be stimulated artificially to produce their specific effects. Thus, if the developing eye is grafted on to the abdominal wall, it will stimulate the skin of the abdomen to produce a lens. These changes can be induced only in the early stages of development. Once the organizers have induced differentiation of the tissue the process proceeds independently of environment, so that experimentally the embryonic bud which will form, say, the leg of a developing chicken, will continue to grow and develop into a leg with normal bone development if kept in an artificial nutrient medium. 35