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Identical or uniovular twins theoretically contain "identical determiners" in their chromosomes which constitute the chief physical basis of heredity; they are always of the same sex and of similar physical characteristics. They are therefore often of equally normal mental and physical pattern; likewise pathologic departures of close contrast, in both respects, are not unusual. Depiction of the spiritual communion manifest between twins has produced interesting reading; the Antipholus pair served by the Dromio identicals gave us Shakespeare's "Comedy of Errors." Numerous less prominent literatures and fictional authors have used the similarity to advantage.
Medical writers have interested themselves in the physiology of twinning and the study of twins in health and disease. Perusal of reference sources reveals case reports of diverse and seemingly innumerable pathologic entities common to identical twins. This survey, however, fails to produce a reported instance of a duodenal ulcer occurring in one or both of identical
McHardy G, Browne DC. DUODENAL ULCER CONCOMITANT IN IDENTICAL TWINS. JAMA. 1944;124(8):503. doi:10.1001/jama.1944.62850080002009a