Frequency of twinning in two Costa Rican ethnic groups: An update

Variation in the frequency of twinning among human populations has been pre sumed to reflect genetic differences. It has been commonly reported that po pulations of African ancestry have the highest, those of Asian ancestry the lowest, and those of European and Middle-Eastern ancestry intermediate fre quencies of twinning. Populations from the Americas have been reported to h ave intermediate twinning frequencies, presumably reflecting their admixtur e. In this context, Madrigal (1994. Am J Hum Biol 6:215-218) reported virtu ally identical land high) twinning frequencies in two Costa Rican ethnic gr oups, one of African, the other of Euro-Amerindian ancestry. These frequenc ies were interpreted in light of frequent inter-ethnic unions, and it was p redicted that the two groups would not differ substantially in gene frequen cies of several blood enzyme systems. This paper reports the gene frequenci es of both groups for such systems. The samples differ significantly for sy stems that have clearly different frequencies in African and European popul ations. Given that the groups are actually different in gene frequencies an d not homogenous as predicted earlier, the conclusion that twinning frequen cies are similar as a result of a similar genetic make up can be questioned . The results challenge the assumption that if populations have similar twi nning frequencies it is because they are genetically similar and argue for a stronger environmental component for twinning frequencies. Am. J. Hum. Bi ol. 13:220-226 2001. (C) 2001 Wiley-Liss, Inc.