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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    some general concepts fact. The conception of modifying genes has been used in human genetics to explain irregular dominance as opposed to the assumption that environmental influences suppress the manifestation of the dominant gene. It is assumed that in addition to the main gene responsible for an abnormality there is an auxiliary gene which itself has physiological or slight effects and is widely scattered throughout the population. A gene for a dominant abnormality would be expressed in the presence of such an auxiliary gene and suppressed in its absence. Alternatively it is assumed that the modifying gene contributes an effect which brings out the manifestation of the dominant gene. Proof for these conceptions is lacking in human genetics. Actually every gene is subject to modifying influences from every other gene, for a gene exerts its effect in a community of genes. The significance of the total genie environment has been brought out experimentally in a number of different ways. In the first place the actual position of the gene in the chromosome is of importance, as seen when chromosomes are experimentally damaged by x-rays, and exchange of segments between non-homologous chromosomes occurs; there result substantial somatic changes. Secondly, the addition of a segment of the X chromosome at a particular stage in the development of Drosophila will deter¬ mine the development of females. Thirdly, there is the evidence from the formation of intersexes as in the chocolate moth, studied by Goldschmidt, in which X chromosomes are expressed differently in different autosomal environments. Change in Mode of Inheritance A gene is so subject to total genie balance that the conception of dominance and recessiveness serves well for particular individuals or generations only, but not as an absolute measure. A gene, by itself, is neither recessive nor dominant. These states are the result of the interaction with the other genes and with environ¬ mental factors. Under experimental conditions one and the same gene in two different genetic environments may be either recessive or dominant. There are good grounds for believing that a mutant gene is frequently dominant on its first appearance, but becomes either completely eliminated or steadily more recessive as the accumulation of modifying factors by natural selection aids the necessary conditions which prevent the dominant mutant from expressing itself. The gene reaches full recessiveness when it can express itself only in the duplex state. CHROMOSOME MECHANISMS Linkage If each chromosome is transmitted intact, it carries a whole group of genes en bloc. Should two of these genes determine dominant abnormalities, the offspring possessing this chromosome would show two clinical conditions—gene linkage producing linked affections. Fig. 12 shows the two possibilities with linked genes on autosomes. In Fig. 12 a, the two genes are carried on the same chromosome. In the absence of cross-over (discussed below) the offspring resemble their parents, that is, if one parent is a double heterozygote for two dominant genes (shown in capital letters), and the 26