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Gene
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<p>A <strong>gene</strong> is a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions. <sup class="reference" id="_ref-Pearson_2006_0">[1]<br /sup><sup class="reference" id="_ref-Rethink_0">[2]<br /sup> The physical development and phenotype of organisms can be thought of as a product of genes interacting with each other and with the environment<sup class="reference" id="_ref-0">[3]</sup> . A concise definition of gene taking into account complex patterns of regulation and transcription, genic conservation and non-coding RNA genes, has been proposed by Gerstein et al.<sup class="reference" id="_ref-Gerstein_2007_0">[4]</sup> "A gene is a union of genomic sequences encoding a coherent set of potentially overlapping functional products".</p>
<p>Colloquially, the term <strong>gene</strong> is often used to refer to an inheritable trait which is usually accompanied by a phenotype as in ("tall genes" or "bad genes") -- the proper scientific term for this is allele.</p>
<p>In cells, genes consist of a long strand of DNA that contains a promoter, which controls the activity of a gene, and coding and non-coding sequence. Coding sequence determines what the gene produces, while non-coding sequence can regulate the conditions of gene expression. When a gene is active, the coding and non-coding sequence is copied in a process called transcription, producing an RNA copy of the gene's information. This RNA can then direct the synthesis of proteins via the genetic code. However, RNAs can also be used directly, for example as part of the ribosome. These molecules resulting from gene expression, whether RNA or protein, are known as gene products.</p>
<p>Genes often contain regions that do not encode products, but regulate gene expression. The genes of eukaryotic organisms can contain regions called introns that are removed from the messenger RNA in a process called splicing. The regions encoding gene products are called exons. In eukaryotes, a single gene can encode multiple proteins, which are produced through the creation of different arrangements of exons through <em>alternative splicing</em>. In prokaryotes (bacteria and archaea), introns are less common and genes often contain a single uninterrupted stretch of DNA, called a <em>cistron</em>, that codes for a product. Prokaryotic genes are often arranged in groups called operons with promoter and operator sequences that regulate transcription of a single long RNA. This RNA contains multiple coding sequences. Each coding sequence is preceded by a Shine-Dalgarno sequence that ribosomes recognize.</p>
<p>The total set of genes in an organism is known as its genome. An organism's genome size is generally lower in prokaryotes, both in number of base pairs and number of genes, than even single-celled eukaryotes. However, there is no clear relationship between genome sizes and complexity in eukaryotic organisms. One of the largest known genomes belongs to the single-celled amoeba <em>Amoeba dubia</em>, with over 670 billion base pairs, some 200 times larger than the human genome.<sup class="reference" id="_ref-Cavalier-Smith_0">[5]</sup> The estimated number of genes in the human genome has been repeatedly revised downward since the completion of the Human Genome Project; current estimates place the human genome at just under 3 billion base pairs and about 20,000–25,000 genes.<sup class="reference" id="_ref-IHSGC2004_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-IHSGC2004">[6]</a></sup> A recent <em><a title="Science (journal)" href="http://en.wikipedia.org/wiki/Science_%28journal%29">Science</a></em> article gives a final number of 20,488, with perhaps 100 more yet to be discovered .<sup class="reference" id="_ref-gene_count2007_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-gene_count2007">[7]</a></sup> The gene density of a genome is a measure of the number of genes per million base pairs (called a megabase, Mb); prokaryotic genomes have much higher gene densities than eukaryotes. The gene density of the human genome is roughly 12–15 genes/Mb.<sup class="reference" id="_ref-Watson_2004_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Watson_2004">[8]</a></sup></p>
<p><span class="mw-headline"><font size="5"><br />
History</font></span></p>
<p>The existence of genes was first suggested by <a title="Gregor Mendel" href="http://en.wikipedia.org/wiki/Gregor_Mendel">Gregor Mendel</a> (1822-1884), who, in the <a title="1860s" href="http://en.wikipedia.org/wiki/1860s">1860s</a>, studied inheritance in <a title="Pea" href="http://en.wikipedia.org/wiki/Pea">pea</a> plants and <a title="Hypothesis" href="http://en.wikipedia.org/wiki/Hypothesis">hypothesized</a> a factor that conveys traits from parent to offspring. He spent over 10 years of his life on one experiment. Although he did not use the term <em>gene</em>, he explained his results in terms of inherited characteristics. Mendel was also the first to hypothesize <a class="mw-redirect" title="Independent assortment" href="http://en.wikipedia.org/wiki/Independent_assortment">independent assortment</a>, the distinction between <a class="mw-redirect" title="Dominant gene" href="http://en.wikipedia.org/wiki/Dominant_gene">dominant</a> and <a class="mw-redirect" title="Recessive" href="http://en.wikipedia.org/wiki/Recessive">recessive</a> traits, the distinction between a <a class="mw-redirect" title="Heterozygote" href="http://en.wikipedia.org/wiki/Heterozygote">heterozygote</a> and <a class="mw-redirect" title="Homozygote" href="http://en.wikipedia.org/wiki/Homozygote">homozygote</a>, and the difference between what would later be described as <a title="Genotype" href="http://en.wikipedia.org/wiki/Genotype">genotype</a> and <a title="Phenotype" href="http://en.wikipedia.org/wiki/Phenotype">phenotype</a>. Mendel's concept was given a name by <a title="Hugo de Vries" href="http://en.wikipedia.org/wiki/Hugo_de_Vries">Hugo de Vries</a> in 1889, who, at that time probably unaware of Mendel's work, in his book <em>Intracellular Pangenesis</em> coined the term "pangen" for "the smallest particle [representing] one hereditary characteristic"<sup class="reference" id="_ref-pangen_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-pangen">[9]</a></sup>. <a title="Wilhelm Johannsen" href="http://en.wikipedia.org/wiki/Wilhelm_Johannsen">Wilhelm Johannsen</a> abbreviated this term to "gene" ("gen" in Danish and German) two decades later.</p><p>In the early 1900s, Mendel's work received renewed attention from scientists. In 1910, <a title="Thomas Hunt Morgan" href="http://en.wikipedia.org/wiki/Thomas_Hunt_Morgan">Thomas Hunt Morgan</a> showed that genes reside on specific <a title="Chromosome" href="http://en.wikipedia.org/wiki/Chromosome">chromosomes</a>. He later showed that genes occupy specific locations on the chromosome. With this knowledge, Morgan and his students began the first chromosomal map of the fruit fly <em><a title="Drosophila melanogaster" href="http://en.wikipedia.org/wiki/Drosophila_melanogaster">Drosophila</a></em>. In 1928, <a title="Frederick Griffith" href="http://en.wikipedia.org/wiki/Frederick_Griffith">Frederick Griffith</a> showed that genes could be transferred. In what is now known as <a title="Griffith's experiment" href="http://en.wikipedia.org/wiki/Griffith%27s_experiment">Griffith's experiment</a>, injections into a mouse of a deadly strain of <a title="Bacteria" href="http://en.wikipedia.org/wiki/Bacteria">bacteria</a> that had been heat-killed transferred genetic information to a safe strain of the same bacteria, killing the mouse.</p><p>In 1941, <a title="George Wells Beadle" href="http://en.wikipedia.org/wiki/George_Wells_Beadle">George Wells Beadle</a> and <a title="Edward Lawrie Tatum" href="http://en.wikipedia.org/wiki/Edward_Lawrie_Tatum">Edward Lawrie Tatum</a> showed that mutations in genes caused errors in certain steps in <a title="Metabolic pathway" href="http://en.wikipedia.org/wiki/Metabolic_pathway">metabolic pathways</a>. This showed that specific genes code for specific proteins, leading to the "one gene, one enzyme" hypothesis.<sup class="reference" id="_ref-Gerstein_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Gerstein">[10]</a></sup> <a title="Oswald Avery" href="http://en.wikipedia.org/wiki/Oswald_Avery">Oswald Avery</a>, <a class="mw-redirect" title="Collin Macleod" href="http://en.wikipedia.org/wiki/Collin_Macleod">Collin Macleod</a>, and <a title="Maclyn McCarty" href="http://en.wikipedia.org/wiki/Maclyn_McCarty">Maclyn McCarty</a> showed in 1944 that DNA holds the gene's information. In 1953, <a title="James D. Watson" href="http://en.wikipedia.org/wiki/James_D._Watson">James D. Watson</a> and <a title="Francis Crick" href="http://en.wikipedia.org/wiki/Francis_Crick">Francis Crick</a> demonstrated the molecular structure of <a title="DNA" href="http://en.wikipedia.org/wiki/DNA">DNA</a>. Together, these discoveries established the <a title="Central dogma of molecular biology" href="http://en.wikipedia.org/wiki/Central_dogma_of_molecular_biology">central dogma of molecular biology</a>, which states that proteins are translated from <a title="RNA" href="http://en.wikipedia.org/wiki/RNA">RNA</a> which is transcribed from DNA. This dogma has since been shown to have exceptions, such as <a title="Reverse transcription" href="http://en.wikipedia.org/wiki/Reverse_transcription">reverse transcription</a> in <a title="Retrovirus" href="http://en.wikipedia.org/wiki/Retrovirus">retroviruses</a>.</p><p>In <a title="1972" href="http://en.wikipedia.org/wiki/1972">1972</a>, <a title="Walter Fiers" href="http://en.wikipedia.org/wiki/Walter_Fiers">Walter Fiers</a> and his team at the Laboratory of Molecular Biology of the <a class="mw-redirect" title="University of Ghent" href="http://en.wikipedia.org/wiki/University_of_Ghent">University of Ghent</a> (<a title="Ghent" href="http://en.wikipedia.org/wiki/Ghent">Ghent</a>, <a title="Belgium" href="http://en.wikipedia.org/wiki/Belgium">Belgium</a>) were the first to determine the sequence of a gene: the gene for <a title="Bacteriophage MS2" href="http://en.wikipedia.org/wiki/Bacteriophage_MS2">Bacteriophage MS2</a> coat protein.<sup class="reference" id="_ref-Min_1972_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Min_1972">[11]</a></sup> <a title="Richard J. Roberts" href="http://en.wikipedia.org/wiki/Richard_J._Roberts">Richard J. Roberts</a> and <a class="mw-redirect" title="Phillip Sharp" href="http://en.wikipedia.org/wiki/Phillip_Sharp">Phillip Sharp</a> discovered in 1977 that genes can be split into segments. This leads to the idea that one gene can make several proteins. Recently (as of <a title="2003" href="http://en.wikipedia.org/wiki/2003">2003</a>-<a title="2006" href="http://en.wikipedia.org/wiki/2006">2006</a>), <a title="Biology" href="http://en.wikipedia.org/wiki/Biology">biological</a> results let the notion of gene appear more slippery. In particular, genes do not seem to sit side by side on <a title="DNA" href="http://en.wikipedia.org/wiki/DNA">DNA</a> like discrete beads. Instead, <a title="Region" href="http://en.wikipedia.org/wiki/Region">regions</a> of the DNA producing distinct proteins may overlap, so that the idea emerges that "genes are one long <a title="Continuum (theory)" href="http://en.wikipedia.org/wiki/Continuum_%28theory%29">continuum</a>".<sup class="reference" id="_ref-Pearson_2006_1"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Pearson_2006">[1]</a></sup></p><p><a id="Mendelian_inheritance_and_classical_genetics" name="Mendelian_inheritance_and_classical_genetics"></a> </p><h2><span class="editsection">[<a title="Edit section: Mendelian inheritance and classical genetics" href="http://en.wikipedia.org/w/index.php?title=Gene&action=edit&section=2">edit</a>]</span> <span class="mw-headline">Mendelian inheritance and classical genetics</span></h2>
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<div class="noprint relarticle mainarticle"><em>Main articles: <a title="Mendelian inheritance" href="http://en.wikipedia.org/wiki/Mendelian_inheritance">Mendelian inheritance</a> and <a title="Classical genetics" href="http://en.wikipedia.org/wiki/Classical_genetics">Classical genetics</a></em></div>
