[Possible cytogenetic mechanisms of direct paternal influence on the human twinning tendency and their consequences: a hypothesis].

Certain cytogenetic mechanisms are suggested to explain the puzzling cases of the direct male influence on the repeated twin births in mammals including humans. The hypothesis is based on the peculiarities of female oogenesis and meiosis, the peculiarities of fertilization and on the established facts of the occurrence of true viable chimaeras produced by separate fertilization of two meiotic products of oogenesis. We postulate that definite genetic factors are transferred from the paternal side whose products become active in male gametes and promote penetration of two spermatozoa (polyspermy) or appearance of two male pronuclei in the egg cytoplasm. The results of such events may be twinning and occurrence of chimaeric or heteroploid individuals. The appearance of viable twins produced by male-dependent polyspermy may be considered as a fortunate outcome of various possible cytogenetic anomalies of fertilization, meiosis, and cleavage divisions. The existence of non-canonical cases of twins, except mono-and dizygotic ones is postulated, according to the hypothesis. Twins pairs produced by two paternal and one maternal genomes may be called "one and halfzygotic or sesquizygotic". The different types of twins may be classified in an order, according to the degree of genetic similarity; monozygotic, chimaeric, sesquizygotic chimaeric, sesquizygotic and dizygotic. This gives an opportunity to explain the appearance of 2 to 3% of "doubtful cases" in mass classification of twin pairs into mono- and dizygotic. The verification of the hypothesis involves the special thorough genetic and cytogenetic analysis of all twin sibs and their parents in families with the direct paternal influence on twin births.(ABSTRACT TRUNCATED AT 250 WORDS)