The above may be illustrated using a hypothetical example, where a heterozygote is +/m in which + and m differ by single base pair, their being G-C base pair in + allele and A-T base pair in m allele at the corresponding position. At meiosis, each of the four chromatids belonging to two paired homologous chromosomes, will have one DNA duplex thus making four DNA duplexes, of which, at a specific point, only two are involved in recombination process. After interaction of DNA molecules, in heteroduplex segments, mismatches may by represented by G-T pair in one duplex and by C-A base pair in the other (Fig. 13.5). If we consider the fate of only one base pair i.e. G-T, this mismatch may be repaired to restore G-C base pair i.e. + allele or may give rise to A-T base pair i.e. m allele (Fig. 13.6). If no mismatch repair is affected, then the mismatch may be corrected through replication at the first post meiotic mitotic division giving post meiotic segregation. Therefore, the heteroduplex segments may have any one of the following fates shown in Table 13.1 : (i) no mismatch repair or correction, leading to aberrant 4 : 4 segregation with two mixed spore pairs (mm, m+, m+, ++); this represents post-meiotic segregation; (ii) mismatch repair or correction in one heteroduplex DNA giving 3 : 5 ratio (mm, mm, m+, ++), or 5 : 3 ratio (mm, m+, ++, ++); this represents, both gene conversion and post meiotic segregation (pms); (iii) mismatch repair in both heteroduplexes giving a 2 : 6 ratio (mm, mm, mm, ++) or 6 : 2 ratio (mm, ++, ++, ++); this represents gene conversion without pms. Other uncommon ratios are also found and can be explained by mismatch repair at one or both heteroduplex segments in either of the two directions.

Thus the phenomenon of gene conversion provided actual clues for the formulation of hybrid or heteroduplex DNA model for explaining the mechanism of genetic recombination or crossing over at the molecular level. Several features or events involved in the hybrid DNA models have been experimentally demonstrated in vivo or in vitro. These include (i) nicking, (ii) strand displacement, (iii) branch migration, (iv) repair synthesis and (v) ligation. Recombination intermediate structures envisaged in Holliday's model and other hybrid DNA models have also been observed under the electron microscope in recombining phages and plasmids. Figure 13.7. shows how two duplex circles (as found in plasmids) may give rise to a dimeric circle through an intermediate structure resembling a figure of eight (8) and how a dimeric circle can give rise to two monomeric recombinant circles. Structures like figure of eight may be isolated and will give rise to chi (χ)structures due to cleavage with a restriction enzyme, that cuts each monomeric circle once. This structure will have four arms held together by a region of heteroduplex DNA at the point of fusion of four arms. Such a chi (χ) structure has actually been observed and one such structure is shown in Figure 13.8.