Heredity in ophthalmology / Jules François.

  • François, J. (Jules)
Date:
1961
Catalogue details

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

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

    81/750 (page 77)
    THE STUDY OF TWINS 77 iris is nonexistent, (b) the color and structure of the iris are closely linked to inheritance, (c) they are polymeric characters since at least four genes contribute to their expression, and (d) in addition, at least in the European race, these characters show great variability. 13. Appearance of the fundus oculi, including the optic papilla and disposi¬ tion of the retinal vessels, which is identical in 85% of the patients (Jancke, 1940; Joly and Lavat, 1954). According to Tower (1955), the arrangement of retinal vessels generally agrees well around the papilla and the posterior pole (macular and perimacular region) but might differ more peripherally. It should be remarked, however, that monozygotic twins might sometimes present important differences, particularly in the following circumstances: 1. Effect of external factors during intrauterine life. For example, the con¬ ditions of fetal-placental circulation are obviously not always the same for the two fetuses but might favor one at the expense of the other. It is not exceptional to find a significant weight difference between monozygotic twins at birth. 2. Separation of the two embryos from the initial single egg at a rather late stage, i.e., when the embryo already has begun to differentiate. In this case the twins provide mirror images as follows: (a) one might be left-handed and the other right-handed, (b) the locks of hair might turn clockwise in one twin and anticlockwise in the other, and (c) certain abnormalities in the implantation of teeth, freckles, vascular or pigmentary nevi are found on the right side in one and on the left side in the other. 3. Twins in whom the aforementioned points are all identical are practically certain to be monozygotic. When, however, agreement is not complete and some doubt persists, the blood groups and factors of both the twins and their parents should be determined: groups O, A, B, AB, Ai, Аг, M, N, MN, C, D, E, c, P, and p by the use of anti-A, anti-B, anti-Ab, anti-Ai, anti-M, anti-N, Anti-C, anti-D, anti-E, anti-c and anti-P antisera. Finally, a definite proof that twins are monozygotic can be provided by a skin graft (1 cm.^) which in monozygotic twins heals by take, whereas in dizy¬ gotic twins it heals by necrosis and slow scar formation (Franceschetti, Bamatter, and Klein, 1948). Let us recall also that, if the twins are of different sexes, they must be dizy¬ gotic. One can say that in the case of monozygotic twins and thus isogenic twins, the following factors are true: 1. Identical characters are due to hereditary factors. 2. Characters that differ are either due to environment or have been modified by environment, for example, when the genes involved have low penetrance and expressivity. 3. A defect or disease caused exclusively by the presence of one or more genes of high penetrance and expressivity will necessarily appear in both twins. 4. The difference between the clinical manifestations of hereditary defects between two monozygotic twins measures the expressivity of the genes compared with the external environment.