Phage progenies from heterozygous particles (HETs) have been classified by means of a new method for identifying genetic markers. The contents of mottled plaques of T4D, marked at six sites in the rII region, were analyzed in a way which permitted classification of the heterozygotes. The results showed, in agreement with the earlier observations of Levinthal and Trautner, that phage particles heterozygous for the central one of three markers tend to be recombinant for the terminal markers. In general the structure of HETs seems to be simple, having zero, one, or two (rarely more) end points within the confines of the area marked in these crosses. The data permit a rough estimate of the average length of the heterozygous area in T4D, giving values of 11 and 14 map units for HETs derived from normal lysis and from early premature lysis, respectively (map units corrected for high negative interference). When a HET area ends within the region marked in these crosses, the r alleles adjacent to the missing ones tend to be replicated infrequently. This is referred to as polarized segregation. The interpretation is advanced that the heterozygous area is composed of a double number of strands of genetic material which overlap where they are homologous, the two component strands ending at opposite ends of the overlap. Repetitive copy-choice replication on such a structure would explain the polarized segregation data. Assuming that such a structure is composed of DNA, a tentative estimate can be made that, in average, about 1500 nucleotide pairs from each genetic strand are included in a double double-helix overlap of normal-lysis heterozygotes and about 1900 in an overlap of early-premature-lysis heterozygotes. A possible mechanism utilizing such genetic overlaps is suggested for crossing-over in organisms with a standard meiosis. The model includes an interpretation of gene conversion, which can explain both its characteristic nonreciprocality and its association with crossing-over. The interpretation of gene conversion makes a prediction that can be tested with Neurospora or similar ascomycetes.