The genetic analyses of indices relating to distance alignment and accommodative convergence are presented. This approach is important in understanding the contribution of genetic factors to observed individual differences for these measures. Abnormalities of either of these two components results in a tendency towards either a convergent or divergent position of the eyes (relative to the position of fusional demand) and thus places an additional load or strain on the other components of the binocular mechanism which must compensate for this potential disruption. If compensation is inadequate to maintain alignment of the eyes, a manifest deviation will result. Consequently, an understanding of the aetiology of such factors underlies an understanding of the aetiology of nonparalytic strabismus. The three populations displayed different tendencies in the cover test measure. The average tendency for each of the populations was consistent with the type of deviation common to all propositi of families within each of the respective populations. The heritability of this character was similar in the three populations and was dependent on the contribution of the female parent only, but to a substantial degree (h2 = 0-42 +/- 0-12). The nature of the sex difference is at present open to speculation. The population means within generations were similar for the gradient measure of AC/A ratio. The heritability of this character (0-38 +/- 0-09) suggests that the genetic component is of substantial importance underlying individual differences for AC/A ratio.
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