Zygosity for same sex twin pairs was determined using a 9-item assessment of physical characteristics completed by interviewers (Nichols and Bilbro, Jr., 1966), and by genotyping a minimum of 11 informative short tandem repeat polymorphisms (STRPs) using DNA obtained from buccal cells. Twin pairs with similar physical characteristics and concordant markers were categorized as MZ, and twin pairs with dissimilar physical characteristics and discordant markers were categorized as DZ. Discrepancies between the zygosity determination by interviewer ratings and genotyping were re-evaluated and resolved.
