Parenting Twins and the Genetics
University of Calgary
Litters of offspring among animals must have been a familiar phenomenon to our nomadic and agricultural ancestors; hence multiple births (MBs) among humans cannot have been a surprising or mysterious occurrence. Nevertheless, twins figure as special human beings in myth and literature, and interest in them goes back as far as recorded history. The Bible tells of Jacob and Esau, who presumably were fraternal twins, since they displayed different physical characteristics (Esau being hairy and Jacob smooth). Greek mythology tells of Castor and Pollux, who form the Gemini constellation in the sky. Rome had its mythical twin founders: Romulus and Remus. Twins also figure in much of literature, from a Plautus comedy to Shakespeare to Thornton Wilder's (himself a twin) The Bridge of San Luis Rey.
Sometimes twin gods provide benefits (e.g., the Indian twin gods Acvin look after the weak and oppressed), but often twins are regarded as products of evil spirits, especially in Africa. On that continent (and among Native Americans too), one of a twin pair was sometimes killed (particularly the female partner), perhaps out of a fear of incest or the belief that incest had already occurred in utero. However, for some African tribes, a twin birth also is the occasion of a special joyous ceremony of welcome. Therefore it appears that there is no universal attitude to twins, either as omens of good or ill (see Scheinfeld, 1973).
Certainly by the late nineteenth century it was known how identical or “one-egg” (monozygotic or MZ) twins and fraternal or “two-egg” (dizygotic or DZ) twins were created, and hence their different genetic relatedness was appreciated. The idea of using twins in genetic analyses probably stems from Sir Francis Galton who, in 1875, wrote about twins as a criterion of the relative powers of nature and nurture. Since the days of Galton and Gesell, McGraw, and Newman near the turn of the twentieth century, innumerable investigations have compared MZ and DZ twins' intrapair correlations for height, weight, physiological and autonomic nervous system measures, and cognitive