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# Locus

Sometimes it is useful in mathematics to describe the path that a point traces as it moves in a plane to meet certain conditions. For example, what is the path that a point on the end of the second hand of a clock traces in 60 seconds? This answer, of course, is a circle.

One way to define a circle is to say that a circle is the locus of all the points in a plane that are a given distance from a fixed point, called the center.

A locus in a plane can be thought of as all the possible locations or positions that a point can take as it moves to meet certain stated conditions. What is the locus of, or path traced by, a point in a plane that moves so that it is always three inches from point A? This locus will be a circle, with point A as the center and a radius of three inches.

What is the locus in a plane of all points that are 2 centimeters from a given line? This locus is made up of two lines, each parallel to the given line, one on each side, and at a distance of 2 centimeters from it, as illustrated in the figure below. The two dashed lines form the locus.

What is the locus of all points in a plane that are the same distance from point D and from point E? To answer this, one might draw some example points that are equidistant from D and E, such as the points marked with a star in the left-hand illustration of the figure below.

These example points indicate that the locus of all the points in a plane that are the same distance from D as they are from E is a line that is the perpendicular bisector of the line segment that joins D and E, as shown in the right-hand illustration.

The idea of a locus can be used not just in a plane but also in three-dimensional space. For example, the preceding example, extended into space, becomes the locus of all points that are equidistant from points D and E. This locus will be the entire plane that is perpendicular to the plane containing DE and its perpendicular bisector and that contains the entire perpendicular bisector.

In space, the locus of all points at a given distance from a specific point is a sphere with a center at the point and a radius equal to the given distance. In space, the locus of all points at a given distance from a line segment is a cylinder with a hemisphere at each end.

The idea of locus can also be used to define the conic sections. In a plane, a circle is the locus of all points at a given distance from a specific point; a parabola is the locus of all points such that each point on the curve is the same distance from a specific point as its distance from a specific line; an ellipse is the locus of all points such that, for each point on the curve, the sum of the distances from each of two separate specific points, called the foci, remains the same; and a hyperbola is the locus of all points such that, for each point on the curve, the absolute value of the difference of the distances to each of two separate specific points, called the foci, remains the same.

Lucia McKay

## Bibliography

Hogben, Lancelot. Mathematics in the Making. New York: Crescent Books, Inc., 1940.

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## locus

lo·cus / ˈlōkəs/ • n. (pl. lo·ci / ˈlōˌsī; -ˌsē; -ˌkē; -ˌkī/ ) 1. technical a particular position, point, or place: it is impossible to specify the exact locus in the brain of these neural events. ∎  the effective or perceived location of something abstract: the real locus of power is the informal council. ∎  Genetics the position of a gene or mutation on a chromosome. 2. Math. a curve or other figure formed by all the points satisfying a particular equation of the relation between coordinates, or by a point, line, or surface moving according to mathematically defined conditions.

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"locus." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 16 Dec. 2017 <http://www.encyclopedia.com>.

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## locus

locus (pl. loci) The specific place on a chromosome where a gene is located. In diploids, loci pair during meiosis and unless there have been translocations, inversions, etc., the homologous chromosomes contain identical sets of loci in the same linear order. At each locus is one gene; if that gene can take several forms (alleles), only one of these will be present at a given locus.

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"locus." A Dictionary of Zoology. . Encyclopedia.com. 16 Dec. 2017 <http://www.encyclopedia.com>.

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## locus

locus (pl. loci) A specific place on a chromosome where a gene is located. In diploids, loci pair during meiosis and, unless there have been translocations, inversions, etc., the homologous chromosomes contain identical sets of loci in the same linear order. At each locus is 1 gene; if that gene can take several forms (alleles), only 1 of these will be present at a given locus.

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"locus." A Dictionary of Plant Sciences. . Encyclopedia.com. 16 Dec. 2017 <http://www.encyclopedia.com>.

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"locus." A Dictionary of Plant Sciences. . Retrieved December 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/locus

## locus

locus In geometry, the path traced by a specified point when it moves to satisfy certain conditions. For example, a circle is the locus of a point in a plane moving in such a way that its distance from a fixed point (the centre) is constant.

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"locus." World Encyclopedia. . Encyclopedia.com. 16 Dec. 2017 <http://www.encyclopedia.com>.

"locus." World Encyclopedia. . Encyclopedia.com. (December 16, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/locus

"locus." World Encyclopedia. . Retrieved December 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/locus

## locus

locus (pl. loci) The position of a gene on a chromosome or within a nucleic acid molecule. The alleles of a gene occupy the same locus on homologous chromosomes.

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# LOCUS

[Latin, Place; place where a thing is performed or done.]

For example, the locus delicti is the place where an accident or crime occurred.

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"Locus." West's Encyclopedia of American Law. . Encyclopedia.com. 16 Dec. 2017 <http://www.encyclopedia.com>.

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## locus

locus (loh-kŭs) n.
1. (in anatomy) a region or site.

2. (in genetics) the region of a chromosome occupied by a particular gene.

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"locus." A Dictionary of Nursing. . Encyclopedia.com. 16 Dec. 2017 <http://www.encyclopedia.com>.

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## locus

locus XVIII. — L., ‘place’.

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## locus

locusBacchus, Caracas, Gracchus •Damascus •Aristarchus, carcass, Hipparchus, Marcus •discus, hibiscus, meniscus, viscous •umbilicus • Copernicus •Ecclesiasticus • Leviticus • floccus •caucus, Dorcas, glaucous, raucous •Archilochus, Cocos, crocus, focus, hocus, hocus-pocus, locus •autofocus •fucus, Lucas, mucous, mucus, Ophiuchus, soukous •ruckus • fuscous • abacus •diplodocus • Telemachus •Callimachus • Caratacus • Spartacus •circus