chiasma

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chiasma

(kīăz`mə): see crossing overcrossing over,
process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis. At that stage each chromosome has replicated into two strands called sister chromatids.
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chiasma

[kī′az·mə]
(anatomy)
A cross-shaped point of intersection of two parts, especially of the optic nerves.
(cell and molecular biology)
The point of junction and fusion between paired chromatids or chromosomes, first seen during diplotene of meiosis.
References in periodicals archive ?
Post chiasmal visual pathway: Post chiasmal interference in visual pathway produces homonymous hemianopia in visual field from a single lesion.
Chiasmal compression typically leads to bitemporal hemianopia, (18) and post-chiasmal compression generally leads to homonymous hemianopia.
Previously limited to opticospinal involvement and seropositivity of anti-Aquaporin 4 (AQP4) or NMO IgG antibody, NMOSD also includes clinical syndromes and MRI findings related to the area postrema, other brainstem, diencephalic, or cerebral presentations in addition to seronegative opticospinal cases with optic chiasmal involvement and longitudinally extensive spinal cord lesions (LESCLs) [5].
Forty two patients with optic chiasmal craniopharyngioma, confirmed by computerized tomography (CT) and/or magnetic resonance imaging (MRI) scan, were included in the study.
Visual field defects can broadly arise from lesions occurring pre-chiasmal, chiasmal or postchiasmal.
b) The defect suggests a lesion that is chiasmal in origin
Most commonly a nasal defect of the optic disc may be seen, with hypopigmentation of the inferonasal retina, and these individuals may have a supero-temporal visual field defect2 that does not observe the vertical midline, which distinguishes it from a visual field defect that is due to chiasmal compression.
Gross defects such as hemianopias, which can be caused by chiasmal tumours, can be detected this way.