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Encyclopedia > Chromosomal crossover
Thomas Hunt Morgan's illustration of crossing over (1916)
Thomas Hunt Morgan's illustration of crossing over (1916)

Homologous Recombination is the process by which two chromosomes, paired up during prophase I of meiosis, exchange some distal portion of their DNA. Crossover occurs when two chromosomes, normally two homologous instances of the same chromosome, break and then reconnect but to the different end piece. If they break at the same place or locus in the sequence of base pairs, the result is an exchange of genes, called genetic recombination. This outcome is the normal way for crossover to occur. If they break at slightly different loci, the result can be a duplication of genes on one chromosome and a deletion of these on the other. This is known as an unequal crossover. If chromosomes break and rejoin on opposite sides of the centromere, the result can be one chromosome being lost during cell division. Image File history File links Download high resolution version (1174x910, 65 KB) Summary An early of the genetic phenomenon of crossing over, from Thomas Hunt Morgans 1916 A Critique of the Theory of Evolution, page 132. ... Image File history File links Download high resolution version (1174x910, 65 KB) Summary An early of the genetic phenomenon of crossing over, from Thomas Hunt Morgans 1916 A Critique of the Theory of Evolution, page 132. ... This article is about the biological chromosome. ... Prophase, with the mitotic spindles stained green and the cell nucleus and chromatin stained blue. ... In biology, meiosis is the process that transforms one diploid cell into four haploid cells in eukaryotes in order to redistribute the diploids cells genome. ... The general structure of a section of DNA Deoxyribonucleic acid (DNA) is a nucleic acid —usually in the form of a double helix— that contains the genetic instructions specifying the biological development of all cellular forms of life, and most viruses. ... Figure 1: Chromosome. ... Two or more structures are said to be homologous if they are alike because of shared ancestry. ... The word locus (plural loci) is Latin for place: In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... Genetic recombination is the transmission-genetic process by which the combinations of alleles observed at different loci (plural of locus) in two parental individuals become shuffled in offspring individuals. ... The centromere is a region on chromosomes with a special sequence and structure. ... This article or section does not cite its references or sources. ...

A double crossing over
A double crossing over

Any pair of homologous chromosomes may be expected to cross over multiple times during meiosis, depending on the species and length of the chromosome. The recombination is activily assisted in the cell by a, through evolution, well conserved machinery. This reduces the genetic linkage between genes on the same chromosome. The genetic variation of a population is thereby increased through chromosomal crossover. Independent assortment is a somewhat related process operating on the complete set of chromosomes. Image File history File links Download high resolution version (987x740, 57 KB) Summary An illustration of double crossing over, from Thomas Hunt Morgans A Critique of the Theory of Evolution (1916). ... Image File history File links Download high resolution version (987x740, 57 KB) Summary An illustration of double crossing over, from Thomas Hunt Morgans A Critique of the Theory of Evolution (1916). ... Genetic linkage occurs when particular alleles are inherited together. ... Independent assortment is a term in genetics for the independent segregation and assortment of chromosomes during sexual reproduction. ...


Crossing over was first described by Thomas Hunt Morgan, and the physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931. Thomas Hunt Morgan in 1891. ... Harriet B. Creighton (d. ... Barbara McClintock (June 16, 1902 – September 2, 1992) was a pioneering American scientist and one of the worlds most distinguished cytogeneticists. ... 1931 (MCMXXXI) was a common year starting on Thursday (link is to a full 1931 calendar). ...


The production of knockout mice, for example, uses this machinery to incorporate altered DNA sequences into the genome. A knockout mouse is a genetically engineered mouse one or more of whose genes have been made inoperable. ...


See also


  Results from FactBites:
 
ScienceDaily: Chromosomal crossover (1550 words)
Chromosome Centers Defined And Sequenced For Model Plant (December 29, 1999) -- The serendipitous discovery of a rare mutation and five years of concentrated effort have allowed a team of American and British researchers to define the centromeres of the five chromosomes of...
Chromosomal crossover -- Homologous recombination is the process by which two chromosomes, paired up during prophase 1 of meiosis, exchange some distal portion of their DNA.
Chromosome -- The DNA which carries genetic information in cells is normally packaged in the form of one or more large macromolecules called chromosomes.
Chromosome (786 words)
Sexually reproducing species have somatic cells[?] (body cells), which are diploid [2n] (they have two sets of chromosomes, one from the mother, one from the father) or polyploid [Xn] (more than two sets of chromosomes), and germ line cells[?] (reproductive cells) which are haploid [n] (they have only one set of chromosomes).
During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent.
Chromosomal aberrations[?] or Partial chromosomal dysplasia [?](malformation), which are usually the result of a defective crossover.
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