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The impetus for this article on vision (ophthalmological findings) in twins reared apart comes mainly from my research on a rare case of two, doubly switched-at-birth monozygotic (MZ) male twin pairs from Colombia, South America (Segal & Montoya, 2018; Segal et al., 2017; Segal et al., 2018). Colleagues and I are currently analyzing data from a comprehensive study of the switched twins’ visual functioning. As a prelude to that analysis, it is worth noting that a summary of relevant findings from early reared-apart twin studies has been completed (Farber, 1981). Furthermore, past and recent findings from the Minnesota Study of Twins Reared Apart (MISTRA), the only reared-apart twin project to include a formal ophthalmological component in the assessment battery, are available and add important information to that literature. I will focus on refraction, but reference other eye conditions as well.
Farber (1981) began her survey with reference to a ‘remarkable book’ on ophthalmological genetics by Sorssy (1970). Cited in the book is strong evidence of genetic influence on refraction, based on a study of 78 MZ twin pairs, 40 dizygotic (DZ) twin pairs and 48 control twin pairs. Refractive errors refer to optical deficiencies that prevent the eye from properly focusing light, causing blurred vision. The key refractive errors are nearsightedness (myopia), farsightedness (hyperopia) and astigmatism (blurred vision caused by light failing to find a single focus on the retina; Heiting, 2018). I was unable to obtain the original twin study (Sorsby et al., 1962), so the zygosity classification method is not known, but a subsequently published comment offers insight into the data analysis (Sorsby & Fraser, 1964). Specifically, correlations across the three pair types (MZ twins, DZ twins and controls), organized by refraction types, led Sorsby et al. (1962) to suggest a quantitative inheritance pattern consistent with additive genetic effects and no dominance — correlations for MZ twins were close to 1.0, correlations for DZ twins were close to .50, and correlations for the control pairs were close to 0. In later family studies, these investigators acknowledged that more than one mode of transmission for refraction errors was likely (Sorsby et al., 1966).
Ophthalmological data on 35 reared-apart MZ (MZA) twin pairs (from among the 121...