Robertsonian translocations constitute the most common structural chromosomal abnormalities in the general population, with an incidence of approximately 0.1%. The chromosome exchange takes place at the centromere, in the short arms, stalks, or satellite regions of the acrocentric chromosomes, resulting in either mono- or dicentric elements depending on the translocation site. The objective of this extensively revised proposal is to utilize molecular genetic approaches to better understand the formation and segregation of balanced chromosome rearrangements. Robertsonian translocations involving chromosomes 14 and 21 will be studied using fluorescence in situ hybridization (FISH) with alpha-satellite, beta-satellite, satellite I and III, and ribosomal DNA probes to determine their exact structure. Structural differences in the translocations will be compared to which centromere region in dicentrics is active to test the hypothesis that the functional status of a centromere is dependent on its adjacent structure. In addition, sperm from at least 20 male carriers of t(14q21q) will be studied both using FISH and single sperm PCR. The frequency of abnormal segregants will be determined by FSH, while the frequency of crossing over in meioses will be determined by single sperm PCR. In combination, these approaches will allow direct assessment of the relationships among centromere structure, meiotic crossing over and chromosome segregation. The studies proposed in this revised application will test the specific hypothesis that the meiotic behavior of one class of Robertsonian translocation, t(14q21q), can be explained by the particular pericentromeric structure and function of each translocation. Examination of this hypothesis will not only provide insight into the mechanisms and consequences of an important class of human chromosome abnormality, but will permit analysis of structural and functional features of the centromere that influence genetic recombination and chromosome segregation. As such, the specific goals of this project are complementary to and synergistic with those of Projects I, III, and IV.