Heredity in ophthalmology / Jules François.

  • François, J. (Jules)
Date:
1961
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Heredity in ophthalmology / Jules François. Source: Wellcome Collection.

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    62 GENERAL GENETICS I Q P я Í 0 ж □ etш i ¿¿¿¿¿¿б ж СмГ1~[Г^ Fig. 51. Mutation. An isolated case of retinal glioma appears in the second generation due to a mutation in one of the two parents, and it is transmitted through three subsequent generations. (After Nonnenmacher, 1955.) CAN ONE DECIDE THE PERIOD WHEN A MUTATION HAS ARISEN WITHIN A GIVEN FAMILY? The determination of the time of appearance of a mutation is easy if the abnormality is dominant, less easy if it is a sex-linked recessive, and very difficult if it is a simple recessive. Dominant Abnormality. A dominant abnormality is the rarest kind but is easily detectable. Any individual affected by a dominant abnormality must have received it from one of his parents. If the parents and their own ancestry are normal, they are certainly no carriers of the gene. One has therefore to assume that in one of the parents a mutation of a normal gene to a pathologic gene has occurred, and since this mutation is dominant, it necessarily expresses itself in the offspring. For example, Fig. 51 outlines the case of a mutation which appeared in a family affected by glioma of the retina inherited as a dominant. According to Haldane, the proportion of mutations for retinal glioma should be 1.4 X 10®, that is, 1.4 mutations for every 100,000 gametes, or one mutation for every 71,000 genes (35,500 births). The same author calculates the percentage of mutations for aniridia as 1.2 X 10^, that is, 1.2 mutations for every 100,000 gametes, or 1 for every 83,000 genes (41,500 births). Recessive Abnormality. A recessive abnormality certainly occurs more frequently but easily passes unnoticed since the offspring of an individual who carries it are phenotypically normal. A normal gene which has mutated to a pathologic recessive gene can be transmitted through many generations without manifesting itself. Since it is carried by heterozygotes, it remains hidden under the normal allelomorph until by chance a union with another heterozygote carrying the same gene occurs and a homozygous individual showing the ab¬ normality is born. An impression of the frequency of recessive mutations can be gained from the fact that pigmentary retinopathy is observed in one out of every 5,000 persons. Recessive Sex-Linked Abnormality. A recessive sex-linked abnormality is more easily detectable. In fact, a woman carrying the gene will pass the affected X chromosome to her male offspring; one son in every two will be phenotypically affected. It may be recalled in this connection that dyschromatopsia is found in 8.37% of all men (Francois and collaborators, 1956).