Meiosis is a great source of genetic variability for sexually reproducing species. Describe how crossing-over occurs during meiotic division. What does crossing-over increase for the reproducing individuals?
Crossing-over occurs during prophase I when chromosomes lay side by side closely intertwined. Corresponding parts of chromatids from homologous chromosomes break apart and exchange with each other. Later on in prophase I, the point at which the cross-over took place can be seen and is indicated by the presence of a discrete point where the chromatids are joined, called a chiasma. Then, during meiosis II, this switching the alleles of chromatids results in the production of four different types of haploid gametes instead of the two different types that would have resulted if cross-over had not occurred. Thus, cross-over and meiosis II increase the total number of genetic combinations possible.
