when we first see chiasmata under a microscope, we know that. Chiasmata are genetically determined to appear either highly localized or more uniformly distributed along the length of the paired chromosomes. There may be just one chiasma next to the centromere in some cases, while in closely related species there may be several chiasmata spread out along the length of each chromosome pair.
Sometimes, a closely related species will have a more uniform distribution, and the single chiasma will be found exclusively at the chromosome ends. Clearly, natural selection favored one method over the other. It has been hypothesized that chiasma localization protects large stretches of DNA that are resistant to recombination. Possible biological adaptation to long-term, consistent environmental conditions.
Crossings, or chiasma, between chromatids that are not identical sisters mark the sites of successful swaps. Distances between neighboring chiasmata can be measured in cytological units (such as microns) and a frequency distribution of inter-chiasmata distances can be determined in organisms with cytologically favorable meiotic chromosomes.
Instead of being exponential like linkage distance distributions, these distributions typically take on a modal shape. Radio-labeled antibodies can also be used to detect foci, or clusters, of proteins involved in fixing double-strand breaks. Modal distributions of inter-foci distances suggest that the cytological distance can be used as a surrogate data for the detection of crossover interference, albeit with some uncertainty.
Primary chemotherapy is increasingly used to treat optic chiasma gliomas, which are typically pilocytic astrocytomas, especially in young children. While RT is not as effective as chemotherapy, it is still used to treat children who relapse after chemotherapy and as primary therapy for older children and adults, with a control rate of 80% to 90% at 1020 years (Tao et al., 1997). Stereotactic techniques are commonly used in the treatment of benign tumors, including the less common brain tumors neurocytomas and hemangioblastomas that have progressed after surgery.