The color and structure of the human iris. 2. Studies of 200 twins]

Subject of the present report is an investigation of the heredity of 30 iris characteristics. The material basis of this sample of twins (100 monozygotic and 100 dizygotic pairs), the largest ever taken as a basis for the purpose of iris research, consists of standardized colour photographs of the examined persons' irises and iris-microscopic observations. The pairs of twins are compared to 100 non-related casual pairs in order to render a better estimation of the degree of heredity of the features possible. On every occasion several classes were constituted for the concordance verification. These classes result from the number of the classes of the forms of markedness and present different levels of similarity. The different distribution of the twin pairs and the control pairs on the concordance classes are always examined for their statistical significance, and for each group of pairs a concordance value is computed. The intervals between the concordance values of the monozygotic and dizygotic pairs and the control pairs indicate the degree of heredity of the investigated feature. Taking all the compiled factors and conditions into consideration, an evaluation of the features as heredity features is made. Though a hereditary component can be observed for all iris characteristics, only about a third could be called heredity features. Quantity of pigment and the hitherto undescribed limiting layer folds were evaluated as "very good", whereas pigment colour in the fine analysis, quality of the anterior stroma leaf, frequency of iris crypts and iris frill position were judged as "good". Tone of blue, markedness of Woelfflin nodules, quantity and markedness of contractional rings could be evaluated as "moderately good" as to their heredity. On the basis of the prior concordance investigations for each of the 300 pairs, a "similarity index" and a "concordance index" were computed in order to arrive at evidence as to the degree of similarity of the single pairs; i.e. in order to detect the limits of the similarity between non-identical twins and the dissimilarity between identical twins. This concluding investigations lead to the question of the possibility of a diagnosis of genetic identicalness solely on the basis of the comparison of irises.