The LVEEG phenotype described above was chosen for the first genetic linkage study of a human EEG phenotype (Anokhin et al., 1992; Steinlein et al., 1992). While no significant linkage was detected in the entire sample, admixture test for linkage heterogeneity was highly significant, indicating a strong linkage to chromosome 20q in a subset of families and thus suggesting a genetic heterogeneity of this EEG phenotype. Genetic heterogeneity refers to the possibility that the same phenotype may have different genetic underpinnings in different individuals and families. Different allelic variants in the same or different genes may lead to the same phenotypic effect, e.g. mutations in different genes causing disruptions at different stages of a metabolic chain can lead to the deficiency of the end product, and different mutation may be responsible for the transmission of the “deficient” phenotype in different families. Overall, these findings provide further support for the transmission of LVEEG phenotype in families as a discrete, categorical trait and point to the chromosomal location of the underlying genetic locus, however, specific genes contributing to the LVEEG phenotype remain
