Some of the most tantalizing clues to the genetic basis of human personality and behavior come from studies of MZ twins reared apart since birth. Such twins have the same genes but, presumably, different environments. A major study of more than 100 such twin pairs showed some remarkable coincidences. A pair of twins meeting for the first time at age thirty-nine each arrived wearing seven rings, two bracelets on one wrist, and a watch and one bracelet on the other wrist. Another twin pair discovered they each had dogs named Toy, had married and divorced women named Linda, remarried women named Betty, and named their sons James Allan and James Alan.
CONCORDANCE IN TWIN STUDIES
Number of twin pairs in which both are affected Total number of twin pairs
A proband is an independently ascertained twin with the disease; independently ascertained means the twin was NOT identified through the co-twin. c2 = the number of concordant pairs in which both twins are probands c1 = the number of concordant pairs in which only one twin is a proband d = the number of discordant pairs
Using concordance patterns to estimate the relative contributions of genetic and environmental determinants to a condition or disorder:
If MZ concordance = 100%
Only genetic determinants likely
If MZ > DZ concordance
Genetic determinants important Environmental modifiers likely
If MZ concordance = DZ concordance
While these coincidences are amazing, it is important to remember that many more twin pairs in this study did not have such parallel lives or habits. Such stories are curious and provocative but cannot by themselves tell us about the relative contributions of genetics and the environment in shaping personality, behavior, health, or other aspects of the self.
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