We reviewed the SC phocomelia syndrome (SCS) and the Roberts syndrome (RS) to demonstrate techniques of nosologic analysis based primarily on the phenotype analysis. We considered type, localization, severity, and variability of the manifestations. In this patient sample these techniques are not sensitive enough to rule out any one of the three most likely etiologic hypotheses, namely whether the SCS and the RS are due to different recessive genes, different alleles, or the same recessive gene. However, this study does suggest certain implications for each of these possibilities.