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Failure of two or more genes to recombine at random as a result of their location on the same chromosome pair. Among the haploid products of a cell which has gone through meiosis, two genes located in the same chromosome pair remain in their two original combinations of alleles (“parental”) unless an odd number of exchanges of homologous segments occurred within the interval bounded by their loci. The incidence of exchanges of homologous segments at meiosis is roughly proportional to the length of the chromosome segment between two loci. The percentage of recombinants thus provides an estimate of this length and a basis for constructing gene maps on which linked loci are arranged in linear order and spaced out in proportion to the recombination percentages between them. See Meiosis
a mechanism consisting of links joined together in lower kinematic pairs. Linkages can be plane or spatial. In plane linkages, the links make contact around a circumference (hinges, turning pairs) or along a line (sliding pairs). In spatial linkages, the links are joined over cylindrical or spherical surfaces (turning pairs) or over planes (sliding pairs). Link gears and slider-crank mechanisms are also considered to be linkages. In technical literature, linkages are often referred to as pin-connected mechanisms.
Linkages are simpler to fabricate, stronger, and more resistant to wear than cam and gear mechanisms. As a result, they are employed to transmit large forces in, for example, presses, forging machines, internal combustion engines, and loaders.
(in Russian, sharnir), a movable connection between two bodies. In a cylindrical linkage, the relative movement of the two bodies is rotation about an axis; in a spherical linkage, the relative movement is rotation about a point.
A cylindrical linkage in a mechanism is called a rotary kinematic pair. It consists of a journal rotating in either a simple bearing or a rolling-contact bearing.
A spherical linkage in a mechanism is referred to as a spherical kinematic pair. It may be either a ball and socket or a ball bearing. Such a linkage is difficult to manufacture. Therefore, it is often replaced by a universal joint, which consists of two or three cylindrical linkages that are connected in tandem and whose axes of rotation intersect at a single point. Various modifications of the universal joint may be used instead of a spherical linkage; such modifications differ in the number of intermediate links and in the type of kinematic pairs formed by the intermediate links.
A linkage may be exposed or enclosed. An enclosed linkage is protected from dust or sand by a housing and is used in machinery that operates in air with a high dust content.
Linkages are used not only in machine mechanisms but also in building systems, such as trusses. In such systems, linkages are employed to relieve the components of bending stresses and to reduce thermal stresses.
N. I. LEVITSKII
A set of rigid bodies, called links, joined together at pivots by means of pins or equivalent devices. A body is considered to be rigid if, for practical purposes, the distances between points on the body do not change. Linkages are used to transmit power and information. They may be employed to make a point on the linkage follow a prescribed curve, regardless of the input motions to the linkage. They are also used to produce angular or linear displacement. See Mechanism
If the links are bars the linkage is termed a bar linkage. A common form of bar linkage is one for which the bars are restricted to a given plane, such as a four-bar linkage. A commonly occurring variation of the four-bar linkage is the linkage used in reciprocating engines (see illustration). Slider C is the piston in a cylinder, link 3 is the connecting rod, and link 4 is the crank. (Link 1 is the fixed base, A and D are pivots, R is the length of the crank, L is the length of the connecting rod, and Θ denotes the angle of the crank.) This mechanism transforms a linear into a circular motion, or vice versa. The straight slider in line with the crank center is equivalent to a pivot at the end of an infinitely long link. See Pantograph
hypertextA linkage between related information. Hypertext is the foundation of the World Wide Web, enabling users to click or tap a link in order to switch to another part of the same Web page, another page on the same site or to a website anywhere in the world. Hypertext is the umbrella term for all links, whether appearing as text (word, phrase or sentence) or as an icon or other graphical element, the latter technically called a "hypergraphic." The terms "hypertext," "hyperlink" and "link" are all used synonymously. See hypermedia, live link and virtual hypertext.
The term was coined by Ted Nelson in 1963, but his vision was more expansive than the one-way links of today's Web. Nelson proposed two-way linking and support for non-hierarchical organization (for more information, visit www.xanadu.com).
The World Wide Web = Hypertext
The Web was developed in the early 1990s by Tim Berners-Lee and Robert Cailliau at the CERN European Laboratory for Particle Physics in Geneva, Switzerland. Whether the Web embodied hypertext as Nelson envisioned it or not, the linking of one item to another created the largest information explosion the world has ever witnessed.