Phenotypic picture for two cytogenetically different variants of trisomy 4p (with and without involvement of the proximal part of 4q) obtained on the data of Minsk Teratologic Center and on the review of 64 cases from the world literature is presented. Mathematical evaluation of intrapair, within- and interfamilial similarity, depending on a duplicated segment, is given. It is shown that phenotypic similarity among patients in case of duplicated distal segments 4p15(16)----pter is significantly greater, while as the size of trisomic segment increases, the similarity goes down. Significant excess of within- over interfamilial similarity is shown, the fact that may be ascribed to a greater similarity of the genofonds in the group of relatives.