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locus, pl. loci (lo´kus, lo´sI)

1. A place; usually, a specific site. 2. The position that a gene occupies on a chromosome lod score. In genetics, the log of the odds ratio of observed to expected distribution of genetic markers. 3. The position of a point, as defined by the coordinates on a graph. [L.]
l. ceru´leus [NA] a shallow depression, of a blue color in the fresh brain, lying laterally in the most rostral portion of the rhomboidal fossa near the cerebral aqueduct; it lies near the lateral wall of the fourth ventricle and consists of about 20,000 melanin-pigmented neuronal cell bodies whose norepinephrine-containing axons have a remarkably wide distribution in the cerebellum as well as in the hypothalamus and cerebral cortex.substantia ferruginea [NA], l. cinereus, l. ferrugineus;
l. cine´reus l. ceruleus
cis-acting l. a section of DNA that affects the activity of DNA sequences on that same molecule of DNA.
complex l. a set of closely linked genetic loci with a common function, as in the major histocompatibility complex l.
l. of control a theoretical construct designed to assess a person's perceived control over his/her own behavior; classified as internal if the person feels in control of events, external if others are perceived to have that control.
l. ferrugin´eus l. ceruleus
genetic l. the set of homologous parts of a pair of chromosomes that may be occupied by allelic genes. The l. thus comprises a pair of locations (except in the X chromosome in males). The concept of a l. is somewhat idealized, not taking into account accidents that may occur in meiosis such as duplication of loci as a result of unequal crossing-over, translocations, inversions, etc.
marker l. a l. on a chromosome or in a stretch of DNA that can be identified (e.g., a restriction fragment length polymorphism) and can serve in linkage analysis and in the isolation of a disease gene. See also linkage marker.
l. ni´ger substantia nigra
l. perfora´tus anti´cus anterior perforated substance
l. perfora´tus posti´cus posterior perforated substance
sex-linked l. any l. that in normal karyotypes is borne on a heterosome; commonly but incorrectly applied to an X-linked l.
X-linked l. any l. that in normal karyotypes is borne on the X chromosome.
Y-linked l. any (haploid) l. that in normal karyotypes is borne on the Y chromosome. The known content is so far small.


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