crossing 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. The two homologous chromosomes of a pair synapse, or come together. While the chromosomes are synapsed, breaks occur at corresponding points in two of the non-sister chromatids, i.e., in one chromatid of each chromosome. Since the chromosomes are homologous, breaks at corresponding points mean that the segments that are broken off contain corresponding genes , i.e., alleles. The broken sections are then exchanged between the chromosomes to form complete new units, and each new recombined chromosome of the pair can go to a different daughter sex cell. Crossing over results in recombination of genes found on the same chromosome, called linked genes, that would otherwise always be transmitted together. Because the frequency of crossing over between any two linked genes is proportional to the chromosomal distance between them, crossing over frequencies are used to construct genetic, or linkage, maps of genes on chromosomes. Mutations , temperature changes, and radiation all affect crossing over frequency. Under the microscope, a crossover has the appearance of an X and is called a chiasma.
Author not available, CROSSING OVER.,
The Columbia Encyclopedia, Sixth Edition 2008
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Don't blame railroad for Waukesha truck-train accident; Railroads need to spend more on safety devices I encourage the efforts of the state railroad commissioner in seeking additional state funding for automatic gates at dangerous crossings throughout the state. I am also aware of crossing upgrades in Waukesha funded by the federal government that propose to increase motorist safety. While this constitutes good safety efforts by these tax-funded agencies, it's only half the way toward optimal railroad crossing safety. Railroad companies such as Wisconsin Central Ltd. have substantially increased train speed and train volume in the Waukesha rail corridor throughout the 1990s. They have done so at an increase in profits without a commensurate expenditure for increased safety devices. For example, Wisconsin statutes permit a railroad to conduct safety studies, reduce train speed and travel times, petition for upgrades and even pay for crossing protections. Instead, the railroads wait for the taxpayers to foot the bill rather than funding gates or signals while motorists are injured and killed at hazardous crossings. In the early 1900s, railroads were considered primarily responsible for safety at crossings. Sadly, this is not the case today since state and federal agencies have accepted part of the responsibility and allowed railroads to reduce their fair share. Aren't railroads really in the best position to assess crossing safety since they control their trains' volume and speed? This is not to say that motorists shouldn't exercise care in traveling over crossings. Most do, and it is hard to conceive of a truck driver willingly putting himself and his pregnant wife in harm's way. But the simple fact remains that many crossings are ill-conceived and poorly designed so as not to afford the unsuspecting motorist much warning of oncoming trains. Flashing lights do not have the visibility or voltage that other traffic signals do. Gates have been known to reduce train-vehicle accidents by 90%. Waukesha, according to your interview with the commissioner, has two of the 10 most dangerous crossings in the entire state. The population of drivers is growing immensely, and the railroad is running numerous trains per day over crossings designed for the safety of a fraction thereof. Jay A. Urban Milwaukee; Blame truck driver for train derailment In your article about the train wreck at Waukesha, I found it most interesting that you wish to blame the railroad for this wreck, because of the increase in train traffic. Would having only one train per day have avoided this incident? Not if a semi-truck pulled out in front of it. State law gives the train the full right of way. The railroad has been there since the 1880s, contributing significantly to Wisconsin's economy. People live near railroads because of the economy they bring, not the noise, pollution or traffic tie-ups. Believe me, if the original Wisconsin Central Railroad had never been built, many of the towns along the line would not be there today. This concept is nothing new. Railroads will always be there, as long as there are companies that need to ship goods. The accident could have been avoided if the truck driver had acted responsibly. This driver allegedly risked the life of his pregnant wife and his own to save maybe five minutes of travel time. His alleged irresponsibility resulted in near gridlock on the streets of Waukesha. Red lights mean stop. Erik Coleman Champaign, Ill.; How many others? There is nothing wrong with a railroad crossing that has a flashing red light or a stop sign. The problem comes in when you have a driver who has taught himself or herself that one doesn't have to stop for a flashing red light. Instead of fixing the crossing, how are you going to fix someone who has taught his or her subconscious mind that speed limits and flashing red lights aren't important? Would anyone like to hazard a guess as to how many more are driving with the same mindset? John Hickey Sr. Milwaukee Editor's note: The articles on the derailment made it clear that o
The Milwaukee Journal Sentinel; 9/12/1998; 345 words;
... the article covering the recent grade crossing accident and subsequent derailment ... the truck blatantly ignored operating crossing signals and proceeded into the train ... Central, and I run trains every day over this same stretch of track through ... said that there should be gates at all crossings, ...
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Relative Influences of Crossing Over and Gene Conversion on the Pattern of Linkage Disequilibrium in Arabidopsis thaliana
Genetics; 4/1/2006; Plagnol, Vincent; Padhukasahasram, Badri; Wall, Jeffrey D; Marjoram, Paul; Nordborg, Magnus; 5946 words;
... the rates of gene conversion and crossing over in Arabidopsis thaliana from population ... The ratio of gene conversion to crossing over is estimated to be around one ... for fine-scale variations of the crossing-over rate. SEVERAL recent articles ...
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QWEST SUBPOENAED OVER FIBER DEALS FEDS PROBE GLOBAL CROSSING'S REVENUE FIGURES IN SWAPS WITH TELCOS FOR NETWORK CAPACITY
Denver Rocky Mountain News; 2/12/2002; Jeff Smith; News Staff Writer; 856 words;
... QWEST SUBPOENAED OVER FIBER DEALS FEDS PROBE GLOBAL CROSSING'S REVENUE FIGURES ... Bermuda-based Global Crossing also maintains it ... practices. Global Crossing bought capacity on ... capacity on Global Crossing's transatlantic and ... business with Global Crossing, didn't ...
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Hutchison and ST Telemedia return to claim Global Crossing: Hutchison's and ST Telemedia's successful second bid for Global Crossing last month was a masterpiece of ironic suspense. Now, dealmakers still have a maze of restructure details, regulatory requirements and not a little political thunder over Red Communists and "national security nightmares" to navigate before the deal's official. (News Analysis).
Telecom Asia; 9/1/2002; Tanner, John C.; 871 words;
When Global Crossing rejected a $750 million joint bid from ... of the original offer), with Global Crossing's majority stake in Asia Global Crossing included in the bargain. The deal bodes ... Miscalculation Of The 21st Century from Global Crossing's standpoint, landing a better offer ... communications ...
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Call to speed up progress on crossing after OAP run over
Evening News - Scotland; 8/15/2007; Hazel Mollison; 549 words;
... long-delayed pedestrian crossing after a pensioner was ... after she was hit while crossing Colinton Road, near the ... leaders are demanding a crossing is installed, more than ... it was promised. The crossing was supposed to be built ... have been left with no crossing facilities for the ...
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Crossing where wife was run over fails safety experts' tests
Daily Mail; 5/19/2007; BETH HALE; 686 words;
... claims his wife was killed on a pedestrian crossing because it was dangerous has been backed ... Transport guidelines. Anyone stepping on the crossing at the end of the green pedestrian phase ... for traffic came on. But the 14.3-metre crossing should have a 12-second safety margin ...
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&type=1 "AAP General News (Australia)")
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Vic: Debate over level crossing safety
AAP General News (Australia); 6/6/2007; 583 words;
AAP General News (Australia) 06-06-2007 Vic: Debate over level crossing safety By Catherine Best MELBOURNE, June 6 AAP - Debate raged today over the safety of Victoria's level crossings following the Kerang train disaster, but the state government questioned whether any measures could have ...
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&type=1 "South Wales Echo (Cardiff, Wales)")
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SAFETY FEARS OVER PENCOED CROSSING.(News)
South Wales Echo (Cardiff, Wales); 3/15/2006; 372 words;
... way a main line level crossing operates have sparked ... year, control of the crossing in Pencoed will be transferred ... there will monitor the crossing area on Hendre Road via ... the safety of the level crossing at Pencoed will not be ... that there's another crossing in Wales that has ...
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&type=1 "The Daily Mail (London, England)")
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Crossing where wife was run over fails safety experts' tests.
The Daily Mail (London, England); 5/19/2007; 689 words;
... claims his wife was killed on a pedestrian crossing because it was dangerous has been backed ... Transport guidelines. Anyone stepping on the crossing at the end of the green pedestrian phase ... for traffic came on. But the 14.3-metre crossing should have a 12-second safety margin ...
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Red alert over worst crossing
Evening News - Scotland; 10/14/2002; MICHAEL HOWIE; 471 words;
... offences has sparked safety concerns as the crossing is over the busy Edinburgh-Glasgow line, via Shotts ... clampdown on reckless motorists who misuse level crossings. Speaking on a visit to the level crossing, he said it was vital that drivers understood ...
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US DoD's JITC conducts IPv6 testing over Global Crossing's IP network.(CONTRACTS)
Submarine Fiber Optic Communications; 11/1/2006; 453 words;
Global Crossing announced that the Joint Interoperability Test Command (JITC) is using the Global Crossing IP network to test the global capabilities ... information systems and equipment. Global Crossing's IPv6 team is working with the JITC ...
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Global Crossing reports significant increase in IP data traffic over its Global Network. (Top Stories).
Fiber Optics Weekly Update; 7/5/2002; 362 words;
Global Crossing announced that IP data ... since January 2002, Global Crossing also announced network ... process continues. Global Crossing's network, which was completed ... include traffic on Global Crossing's Voice Over IP (VoIP) network, which ... minutes per month Global Crossing now ...
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Squabble over rail 'danger crossing'
Yorkshire Post; 8/23/2005; 505 words;
... injured at a notorious level crossing in Hull have attacked two ... upkeep of the Spring Bank West crossing, although both admit it is ... demanded a redesign of the crossing. A campaign included launching ... bureaucratic nightmare. Spring Bank crossing is known to be very dangerous ... Rail does agree ...
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Death crossing may shut in row over train noise; Fury at 'excessive' horn blasts at spot where man was killed
Evening Times; 3/3/2004; Graeme Murray; 421 words;
A RAILWAY crossing where a man was killed trying ... dog may be closed in a row over noisy trains. Rail bosses are considering shutting the foot crossing at Cardross, near Helensburgh ... warning as they approach the crossing near Cardross Station. Mr ... 's believed he ran on to the crossing to ...
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Crossing Over and out.(talk show)(Universal cancels talk show 'Crossing Over')
Broadcasting & Cable; 1/12/2004; Albiniak, Paige; 336 words;
... is canceling psychic strip Crossing Over With John Edward after three ... of this, its third season, Crossing Over underwent a makeover. The changes ... last year's season average. Crossing Over entered syndication in August ... stations opted to put more news on the air after the ...
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