Crossing Over
One of the key differences between meiosis and mitosis is the synapsis of homologous chromosomes during prophase I of meiosis. Synapsis is the process in which homologous chromosomes carefully pair. The pairing allows for an orderly first division to send one chromosome from each pair to separate cells. The close association of the homologous chromosomes also allows for crossing over between non-sister chromatids (Fig. 7). During this process sections of the chromosomes break off and are exchanged between non-sister chromatids. When non-sister chromatids crossover, chromatids can be made that have a new combination of genes compared to the original combination on the chromosome. The original combination is inherited from the organism’s parents and is called the parental combination of genes. The new combinations made are called the recombinant combinations. In Figure 7, a crossover occurs but the original or parental combination of CS (Red and Plump) and cs (White and Shrunken) stay together. Crossing over can cause new gene combinations to occur on a chromosome if the crossover occurs between the linked genes.
Figure 7. Crossover event for an ACS / acs parent. This crossover event occurred between A,a and C,c. No crossover happened between C,c and S,s. This means with respect to the C,c and S,s loci, only CS and cs parental gametes are made. (Redrawn from Linkage Part 1, D. Lee, 2001. M. Sutter, 2023, PowerPoint v. 16.7. Redrawn with permission)
Figure 8. Crossover event for an ACS / acs parent. This crossover event occurred between C,c and S,s. This results in a Cs and cS recombinant plus a CS and cs parental combination. (Redrawn from Linkage Part 1, D. Lee, 2001. M. Sutter, 2023, PowerPoint v. 16.7. Redrawn with permission)
When a crossover occurs between genes, chromatids with both the parental combination and chromatids with a new combination are made. We can see this in Figure 8. Two of the chromatids are not involved in the crossing over. These chromatids will maintain the parental combination and when meiosis is complete, the two gametes made that have these chromosomes will be called parental gametes. The gametes made that have the other two chromosomes, those that went through crossing over and have the new gene combination, are called recombinant gametes (Fig. 9).
Figure 9. Parental and recombinant gametes from a crossover between C,c and S,s. The parentals are ACS and acs. The recombinants are ACs and acS. (Redrawn from Linkage Part 1, D. Lee, 2001. M. Sutter, 2023, PowerPoint v. 16.7. Redrawn with permission)
When crossing over occurs between two non-sister chromatids, cells will make equal numbers of recombinant and parental gametes. In looking back at the data, the number of plants that inherited recombinant type gametes was far below 50%. Why are parental combinations of linked genes made more frequently than new combinations? Understanding this requires us to imagine many cells going through gamete formation.