Examining Relationships in Heterozygous Triplets

• Activity Number: 166
• Type: Contributed
• Date/Time: Monday, August 9, 2004 : 2:00 PM to 3:50 PM
• Sponsor: Section on Statistics in Epidemiology
• Author(s): Gene S. Fisch*+ and Nina Kohn and Catherine Derom and Martin L. Lesser
• Companies: North Shore LIJ Research Institute and North Shore LIJ Research Institute and Catholic University, Leuven and North Shore LIJ Research Institute
• Keywords: intraclass correlation ; triplets ; twins

Abstract

The intraclass correlation coefficient (ICC) computes the correlation between k>2 unordered measures, e.g., IQ in twins. In triplets, the ICC is applied when triplets are monozygotic (MZ) or trizygotic (TZ). When the triplet is heterogeneous--identical MZ twins plus a DZ sib--the ICC is not valid; nor is there valid procedure to examine the relationship. We propose to measure relationship in n=7 such sets of triplets by constructing all possible 2^n vectors of MZ twins that could be paired with their respective DZ sib, and compute their ICCs. Mean ICC for all 27 ICCs was used to measure relationship between MZs and the DZ sib. Our heterogeneous dataset had higher than expected mean ICC=.75, the distribution of which was unexpectedly bimodal. We used a jackknife procedure to identify the source of bimodality and found one influential triplet. The DZ from that triplet had an unusually high IQ compared to its MZ siblings. In 6/7 triplets, the DZ IQ was either higher or lower than the MZ IQs (p=.26). To analyze the effect on the ICC distribution, we lowered DZ IQ score in that triplet and found that, as it decreased, the distribution of ICCs became increasingly unimodal.