Similarities and differences in a set of 7-year-old triplets, two of whom are monozygotic.

This paper describes three sisters, triplets, age 7 years, 2 months, two of whom derive from a single ovum, the third from a second ovum. As such, they provide an opportunity to examine the relative impact of heredity on certain of their physical, developed, and acquired abilities and characteristics. In addition to a comparison of such physical traits as height, weight, refractive status, axial length, and so on, certain (developed) sensory-motor and (acquired) cognitive and language measures are reported, such as binocular status, visual and auditory analysis skills, intelligence quotient, and (expressive and receptive) language abilities. Because all three children have lived in (more or less) the same pre- and post-natal environment, we assume that whatever concordance the monozygotic (MZ) pair displays that is not shared by their dizygotic (DZ) sister is attributable to heredity. (It does not necessarily follow, however, that any discordance between the two MZ sisters or any concordance between one of them and their DZ sister is attributable to environment.