The study of 151 reciprocal translocations associated with abnormal probands shows that the mode of imbalance at birth is determined by the nature of the involved chromosomes and by the position of the breakpoints. For each of the three modes (adjacent-1, adjacent-2, and 3:1) there is a corresponding pachytene diagram, so that for each translocation variety it is possible to predict the most probable mode of imbalance. The determining factor is the relative length of the different branches of the cross formed by the tetravalent. However, some heterochromatic regions (9qh, short arms of acrocentric chromosomes) and possibly R-negative regions have a minor role. The factors involved in these mechanisms seem to be the selection and the chiasma position; their respective roles are discussed.

Human sperm chromosomes from a 46,XY/47,XXY male were obtained using the technique of in vitro penetration of zona-free hamster eggs. The analysis of 543 sperm complements shows a significantly increased incidence (0.9%) of hyperhaploid gonosomal 24,XY sets, with a lack of the expected corresponding gonosomal hypohaploidies, and a normal rate of autosomal non-disjunctions. These results support the suggestion that 47,XXY cells are able to go through meiosis and to form spermatozoa. Only 24,XY sperm chromosomal constitutions were observed suggesting a preferential pairing of homologous sex chromosomes in 47,XXY spermatocytes.

Cytogenetic analysis of 78 spermatozoa from a man heterozygous for a t(13;14) Robertsonian translocation was performed. R banding was applied for chromosomal identification. Incidence of normal and balanced complements were respectively 50% and 41.3%. Six unbalanced complements (7.7%) were observed, resulting from adjacent segregation. Although alternate segregation is the most common mode of distribution, the possibility of producing unbalanced zygotes exists. The frequency of abnormalities unrelated to the translocation was 16.5% including 12.8% hypohaploïdy, 2.5% hyperhaploidy, and 1.2% of structural aberrations. An excess of t(13;14) X complements was observed (24 with X versus 14 with Y). This may result from the close association between trivalent (13;14) and X chromosome observed in the pachytene spermatocyte nucleus.

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