Chromosomal and gene marker studies of somatic cells in a family with multiple chromosome translocations show apparent instability of the translocations in transmission from generation to generation. This instability is attributed to meiotic crossing-over between the involved chromosomes. Technical advances in chromosome analysis combined with human gene map information in the study of this unusual family demonstrate for the first time that crossing-over involves exchange of chromatin between nonsister chromatids of homologous chromosomes.
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