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[[Image:DNA Overview2.jpg|thumb|left|upright|[[DNA]], the molecular basis for inheritance. Each strand of DNA is a chain of [[nucleotides]], matching each other in the center to form what look like rungs on a twisted ladder.]]
'''Genetics''', a discipline of [[biology]], is the  [[science]] of [[heredity]] and [[Genetic variation|variation]] in living [[organism]]s.<ref>Griffiths et al. (2000)
</ref><ref name=Hartl_and_Jones>Hartl D, Jones E (2005)
</ref> Knowledge of the inheritance of characteristics has been implicitly used since prehistoric times for improving crop plants and animals through [[selective breeding]]. However, the modern science of genetics, which seeks to understand the mechanisms of inheritance, only began with the work of [[Gregor Mendel]] in the mid-nineteenth century.<ref name=Weiling>{{cite journal| author=Weiling F| title=Historical study: Johann Gregor Mendel 1822–1884|journal=American Journal of Medical Genetics| volume=40| issue=1|pages=1–25; discussion 26 |year=1991 |pmid=1887835}}</ref> Although he did not know the physical basis for heredity, Mendel observed that inheritance is fundamentally a discrete process where specific traits are inherited in an independent manner&mdash;these basic units of inheritance are now called [[gene]]s.

Genes correspond to regions within [[DNA]], a molecule composed of a chain of four different types of [[nucleotides]]&mdash;the sequence of this nucleotides is the genetic information organisms inherit. DNA naturally occurs in a double stranded form, with nucleotides on each strand complementary to each other. Each strand can act as a template for [[DNA replication|synthesis]] of a new partner strand&mdash;this is the physical mechanism for the copying and inheritance of genetic information.

The sequence of nucleotides in DNA is used by [[cell (biology)|cells]] to produce specific sequences of [[amino acid]]s, creating [[protein]]s&mdash;a correspondence known as the [[genetic code]]. This sequence of amino acids in a protein determines how it folds into a three-dimensional structure, this structure is, in turn, responsible for the protein's function. Proteins carry out almost all the functions needed for cells to live and reproduce. A change to DNA sequence can change a protein's structure and behavior, and this can have dramatic consequences in the cell and on the organism as a whole.

Although genetics plays a large role in determining the appearance and behavior of organisms, it is the interaction of genetics with the environment an organism experiences that determines the ultimate outcome. For example, while genes play a role in determining a person's [[human height|height]], the [[nutrition]] and [[health]] that person experiences in childhood also have a large effect.

==History ==
{{main|History of genetics}}

[[Image:Sexlinked inheritance white.jpg|left|thumb|Morgan's observation of sex-linked inheritance of a mutation causing white eyes in ''[[Drosophila]]'' led him to the hypothesis that genes are located upon chromosomes.]]

Although the science of genetics has its origins in the work of Gregor Mendel in the mid-nineteenth century, various theories of inheritance preceded Mendel. These theories generally assumed that there existed an [[inheritance of acquired characteristics]]: the belief that individuals inherit traits that have been strengthened in their parents. Today, the theory is commonly associated with Jean-Baptiste Lamarck, who used this pattern of inheritance to explain the evolution of various traits within species (these changes are now understood to be the product of [[natural selection]] rather than a product of [[soft inheritance]]).<ref>Lamarck, J-B (2008). In Encyclopædia Britannica.  Retrieved from [http://www.search.eb.com/eb/article-273180 Encyclopædia Britannica Online] on [[2008-03-16]]. </ref>

=== Mendelian and classical genetics ===

The modern science of genetics traces its roots to the observations made by Gregor Johann Mendel, a German-Czech Augustinian monk and scientist who made detailed studies of the nature of inheritance in plants. In his paper "Versuche über Pflanzenhybriden" ("[[Experiments on Plant Hybridization]]"), presented in 1865 to the Brunn Natural History Society, Gregor Mendel traced the inheritance patterns of certain traits in pea plants and showed that they could be described mathematically.<ref name="mendel">{{cite journal | author=Mendel, GJ | title=[[Experiments on Plant Hybridization|Versuche über Pflanzen-Hybriden]] | journal=Verhandlungen des naturforschenden Vereins Brünn| volume=4 | pages=3–47|year=1866}} (in English in 1901, J. R. Hortic. Soc. 26: 1–32) [http://www.mendelweb.org/Mendel.html|English translation available online]</ref> Although not all features show these patterns of [[Mendelian inheritance]], his work suggested the utility of the application of statistics to the study of inheritance.

The significance of Mendel's observations was not understood until early in the twentieth century, after his death, when his research was re-discovered by other scientists working on similar problems. The word ''genetics'' itself was coined in 1905 by William Bateson, a significant proponent of Mendel's work, in a letter to Adam Sedgwick.<ref>genetics, ''n.'', Oxford English Dictionary, 3rd ed. </ref><ref>{{cite web| url=http://www.jic.ac.uk/corporate/about/bateson.htm | title=Letter from William Bateson to Alan Sedgwick in 1905| publisher=The John Innes Centre| accessdate=2008-03-15| author=Bateson W}}</ref> (The adjective ''genetic'', derived from the Greek word ''genno'' (''γεννώ''): to give birth, predates the noun and was first used in a biological sense in 1860.<ref>genetic, ''adj.'', Oxford English Dictionary, 3rd ed. </ref>) Bateson publicly promoted and popularized usage of word ''genetics'' to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in 1906.<ref name="bateson_genetics">{{cite conference | author=Bateson, W |title=The Progress of Genetic Research |editor=Wilks, W (editor) | booktitle=Report of the Third 1906 International Conference on Genetics: Hybridization (the cross-breeding of genera or species), the cross-breeding of varieties, and general plant breeding|publisher=Royal Horticultural Society | location=London | year=1907}}
:Although the conference was titled "International Conference on Hybridisation and Plant Breeding", Wilks changed the title for publication as a result of Bateson's speech.</ref>

In the decades following rediscovery and popularization of Mendel's work, numerous experiments sought to elucidate the molecular basis of DNA. In 1910 [[Thomas Hunt Morgan]] argued that genes reside on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies.<ref>{{cite journal| author=Moore JA| title=Thomas Hunt Morgan—The Geneticist| journal=American Zoologist| year=1983| volume=23| issue=4| pages=855–865| url=http://icb.oxfordjournals.org/cgi/content/abstract/23/4/855}}</ref> In 1913 his student [[Alfred Sturtevant]] used the phenomenon of [[genetic linkage]] and the associated recombination rates to demonstrate and map the linear arrangement of genes upon the chromosome.<ref>{{cite journal| author=Sturtevant AH| year=1913| title=The linear arrangement of six sex-linked factors in Drosophila, as shown by their mode of association| journal=Journal of Experimental Biology| volume=14| pages=43–59}} [http://www.esp.org/foundations/genetics/classical/holdings/s/ahs-13.pdf pdf from Electronic Scholarly Publishing]</ref>

[[Image:JamesDWatson.jpg|[[James D. Watson]] (''pictured'') and [[Francis Crick]] resolved the structure of DNA in 1953.|upright|thumb|left]]
=== Molecular genetics ===
Although chromosomes were known to contain genes and composed of both protein and DNA, it was unknown which was critical for heredity or how the process occurred. In 1928, [[Frederick Griffith]] published his discovery of the phenomenon of [[Transformation (genetics)|transformation]] (see [[Griffith's experiment]]); sixteen years later, in 1944, Oswald Theodore Avery, Colin McLeod and [[Maclyn McCarty]] used this phenomenon to isolate and identify the molecule responsible for transformation as [[DNA]].<ref name=Avery_et_al>{{cite journal| author=Avery OT, MacLeod CM, and McCarty M| title=Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Desoxyribonucleic Acid Fraction Isolated from Pneumococcus Type III| journal=Journal of Experimental Medicine| year=1944| volume=79| issue=1| pages=137–158}}[http://www.ncbi.nlm.nih.gov/pubmed/33226?dopt=AbstractPlus 35th anniversary reprint available]</ref> The [[Hershey-Chase experiment]] in 1952 identified DNA (rather than protein) as the genetic material of the viruses that infect bacteria, further evidence that DNA was the molecule responsible for inheritance.<ref>{{cite journal| author=Hershey AD, Chase M| year=1952| title=Independent functions of viral protein and nucleic acid in growth of bacteriophage| journal=The Journal of General Physiology| volume=36| pages=39–56| url=http://www.jgp.org/cgi/content/abstract/36/1/39}}</ref>

[[James D. Watson]] and [[Francis Crick]] solved the structure of DNA in 1953, using the [[X-ray crystallography]] work of [[Rosalind Franklin]] that indicated the molecule had a helical structure.<ref>{{cite book |title=The Eighth Day of Creation: Makers of the Revolution in Biology |last=Judson |first=Horace |middle=Freeland |authorlink=Horace Freeland Judson |year=1979 |publisher=Cold Spring Harbor Laboratory Press |isbn=0-87969-477-7 |pages=51–169}}</ref><ref name=watsoncrick_1953a>{{cite journal| author=Watson JD, Crick FHC| title=[[Molecular structure of Nucleic Acids]]: A Structure for Deoxyribose Nucleic Acid| journal=Nature| year=1953| volume=171| issue=4356| pages=737–738|format=PDF| url=http://www.nature.com/nature/dna50/watsoncrick.pdf}}</ref> Their double-helix model paired a sequence of nucleotides with complementary nucleotides on the other strand.<ref name=watsoncrick_1953b>{{cite journal| author=Watson JD, Crick FHC| title=Genetical Implications of the Structure of Deoxyribonucleic Acid| journal=Nature| volume=171| issue=4361| year=1953| pages=964–967|format=PDF|  url=http://www.nature.com/nature/dna50/watsoncrick2.pdf}}</ref> This structure not only provided a physical explanation for how information is contained within the order of the nucleotides, but also suggested a simple mechanism for duplication—through separation of strands and the reconstruction of partner strands based on the nucleotide sequences. Although the structure explained the process of inheritance, it was still unknown how DNA influenced the behavior of cells. In the following years scientists sought to understand how DNA controls the process of [[protein]] production within [[ribosome]]s, eventually discovering the transcription of DNA into [[messenger RNA]] and cracking the [[genetic code]], which links the nucleotide sequence of [[messenger RNA]] to the [[amino acid]] sequence of protein.

With this molecular understanding of inheritance, an explosion of research that applied this new knowledge to biology became possible. One early development was chain-termination [[DNA sequencing]] in 1977, which enabled the determination of nucleotide sequences on DNA.<ref name=sanger_et_al>
{{cite journal| journal=Nature| year=1977| title=DNA sequencing with chain-terminating inhibitors| author=Sanger F, Nicklen S, and Coulson AR| volume=74| issue=12| pages=5463–5467}}</ref> In 1983 the [[polymerase chain reaction]], developed by Kary Banks Mullis, provided a simple method for isolating and amplifying segments of DNA.<ref name=saiki_et_al>{{cite journal| title=Enzymatic Amplification of β-Globin Genomic Sequences and Restriction Site Analysis for Diagnosis of Sickle Cell Anemia| year=1985| journal=Science| author=Saiki RK, Scharf S, Faloona F, Mullis KB, Horn GT, Erlich HA, Arnheim N| volume=230| issue=4732| pages=1350–1354 |pmid=2999980}}</ref> These and other techniques, through the pooled efforts of the [[Human Genome Project]] and parallel private effort by Celera Genomics, culminated in the sequencing of the human [[genome]] in 2003.<ref name=human_genome_project />

==Features of inheritance==
===Discrete inheritance and Mendel's laws===
{{main|Mendelian inheritance}}

[[Image:Punnett square mendel flowers.jpg|right|thumb|A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms]]
At its most fundamental level, inheritance in organisms occurs by means of discrete traits, called [[gene]]s.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.199 Chapter 2 (Patterns of Inheritance): Introduction]
</ref> This property was first observed by [[Gregor Mendel]], who studied the segregation of heritable traits in pea plants.<ref name="mendel" /><ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.200 Chapter 2 (Patterns of Inheritance): Mendel's experiments]
</ref> In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white&mdash;and never an intermediate between the two colors. These different, discrete versions of the same gene are called [[allele]]s.

In the case of pea plants, each organism has two alleles of each gene, and the plants inherit one allele from each parent.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.484 Chapter 3 (Chromosomal Basis of Heredity): Mendelian genetics in eukaryotic life cycles]
</ref> Many organisms, including humans, have this pattern of inheritance. Organisms with two copies of the same allele are called [[homozygous]], while organisms with two different alleles are [[heterozygous]].

The set of alleles for a given organism is called its [[genotype]], while the observable trait the organism has is called its [[phenotype]]. When organisms are heterozygous, often one allele is called [[Dominant allele|dominant]] as its qualities dominate the phenotype of the organism, while the other allele is called [[Recessive allele|recessive]] as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have [[Dominance relationship#Incomplete dominance|incomplete dominance]] by expressing an intermediate phenotype, or [[Dominance relationship#Codominance|codominance]] by expressing both alleles at once.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.630 Chapter 4 (Gene Interaction): Interactions between the alleles of one gene]
</ref>

When a pair of organisms [[sexual reproduction|reproduce sexually]], their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as [[Mendelian inheritance#Law of Segregation|Mendel's first law]] or the Law of Segregation.

[[Image:Pedigree-chart-example.svg|thumb|240px|left|Genetic pedigree charts help track the inheritance patterns of traits.]]

===Notation and diagrams===
Geneticists use diagrams and symbols to describe inheritance. A gene is represented by a letter (or letters)&mdash;the capitalized letter represents the dominant allele and the recessive is represented by lowercase.<ref>{{cite web| url=http://faculty.users.cnu.edu/rcheney/Genetic%20Notation.htm| title=Genetic Notation| author=Richard W. Cheney| accessdate=2008-03-18}}</ref> Often a "+" symbol is used to mark the usual, non-mutant allele for a gene.

In fertilization and breeding experiments (and especially when discussing Mendel's) the parents are referred to as the "P" generation and the offspring as the "F1" (first filial) generation. When the F1 offspring mate with each other, the offspring are called the "F2" (second filial) generation. One of the common diagrams used to predict the result of cross-breeding is the [[Punnett square]].

When studying human genetic diseases, geneticists often use [[pedigree chart]]s to represent the inheritance of traits.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.229 Chapter 2 (Patterns of Inheritance): Human Genetics]</ref> These charts map the inheritance of a trait in a family tree.

===Interactions of multiple genes===
[[Image:Galton-height-regress.png|thumb|right|Human height is a complex genetic trait. [[Francis Galton]]'s data from 1889 shows the relationship between offspring height as a function of mean parent height. While correlated, remaining variation in offspring heights indicates environment is also an important factor in this trait.]]

Organisms have thousands of genes, and in sexually reproducing organisms assortment of these genes are generally independent of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "[[Mendelian inheritance#Law of Independent Assortment|Mendel's second law]]" or the "Law of independent assortment", means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. (Some genes do not assort independently, demonstrating [[genetic linkage]], a topic discussed later in this article.)
 
Often different genes can interact in a way that influences the same trait. In the blue-eyed Mary, for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all: color or white. When a plant has two copies of this white allele, its flowers are white&mdash;regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called [[epistasis]], with the second gene epistatic to the first.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.644 Chapter 4 (Gene Interaction): Gene interaction and modified dihybrid ratios]</ref>

Many traits are not discrete features (eg. purple or white flowers) but are instead continuous features (eg. human height and skin color). These [[Quantitative trait locus|complex trait]]s are the product of interactions of many genes.<ref>{{cite journal| author=Mayeux R| title=Mapping the new frontier: complex genetic disorders| journal=The Journal of Clinical Investigation |volume=115 |issue=6 |pages=1404–1407 |year=2005 |pmid=15931374}}</ref> The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called [[heritability]].<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.4009 Chapter 25 (Quantitative Genetics): Quantifying heritability]</ref> Measurement of the heritability of a trait is relative, although&mdash;in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a complex trait with a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62%.<ref>{{cite journal|author=Luke A, Guo X, Adeyemo AA, Wilks R, Forrester T, Lowe W Jr, Comuzzie AG, Martin LJ, Zhu X, Rotimi CN, Cooper RS|title=Heritability of obesity-related traits among Nigerians, Jamaicans and US black people|journal=Int J Obes Relat Metab Disord|year=2001|volume=25|issue=7|pages=1034–1041}} [http://www.ncbi.nlm.nih.gov/pubmed/11443503?dopt=Abstract Abstract from NCBI]</ref>

==Molecular basis for inheritance==
===DNA and chromosomes===
{{Main|DNA|Chromosome}}
[[Image:DNA chemical structure with repositioned labels.png|thumb|right|The [[molecular structure]] of DNA. Bases pair through the arrangement of [[hydrogen bonding]] between the strands.]]

The [[molecular]] basis for genes is [[deoxyribonucleic acid]] (DNA). DNA is composed of a chain of [[nucleotides]], of which there are four types: [[adenine]] (A), [[cytosine]] (C), [[guanine]] (G), and [[thymine]] (T). Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.<ref name=Pearson_2006>{{cite journal |author=Pearson H |title=Genetics: what is a gene? |journal=Nature |volume=441 |issue=7092 |pages=398–401 |year=2006 |pmid=16724031}}</ref> [[Virus]]es are the only exception to this rule&mdash;sometimes viruses use the very similar molecule [[RNA]] instead of [[DNA]] as their genetic material.<ref>{{cite book | title=Microbiology| last=Prescott| first=L| date=1993| publisher=Wm. C. Brown Publishers| id=0-697-01372-3}}</ref>

DNA normally exists as a double-stranded molecule, coiled into the shape of a [[double-helix]]. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: [[DNA replication]] duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.1523 Chapter 8 (The Structure and Replication of DNA): Mechanism of DNA Replication]</ref>

Genes are arranged linearly along long chains of DNA sequence, called [[chromosomes]]. In [[bacteria]], each cell has a single circular chromosome, while [[eukaryote|eukaryotic]] organisms (which includes plants and animals) have their DNA arranged in multiple linear chromosomes. These DNA strands are often extremely long; the largest human chromosome, for example, is about 247&nbsp;million [[base pair]]s in length.<ref>{{cite journal|title=The DNA sequence and biological annotation of human chromosome 1|author=Gregory SG et al.|year=2006|journal=Nature|volume=441|page=315–321}} [http://www.nature.com/nature/journal/v441/n7091/full/nature04727.html free full text available]</ref> The DNA of a chromosome is associated with structural proteins that organize, compact, and control access to the DNA, forming a material called [[chromatin]]; in eukaryotes [[chromatin]] is usually composed of [[nucleosome]]s, repeating units of DNA wound around a core of [[histone]] proteins.<ref>Alberts et al. (2002),  [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.608|II.4. DNA and chromosomes: Chromosomal DNA and Its Packaging in the Chromatin Fiber]</ref> The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the [[genome]].

While [[haploid]] organisms have only one copy of each chromosome, most animals and many plants are [[diploid]], containing two of each chromosome and thus two copies of every gene.<ref name=haploid_diploid>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.484 Chapter 3 (Chromosomal Basis of Heredity): Mendelian genetics in eukaryotic life cycles]</ref> The two alleles for a gene are located on identical [[Locus (genetics)|loci]] of sister [[chromatids]], each allele inherited from a different parent.

[[Image:Zell-substanz-book-illustrations.jpg|thumb|left|[[Walther Flemming]]'s 1882 diagram of eukaryotic cell division. Chromosomes are copied, condensed, and organized. Then, as the cell divides, chromosome copies separate into the daughter cells.]]
An exception exists in the [[sex chromosome]]s, specialized chromosomes many animals have evolved that play a role in determining the sex of an organism.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.222 Chapter 2 (Patterns of Inheritance): Sex chromosomes and sex-linked inheritance ]</ref> In humans and other mammals the Y chromosome has very few genes and triggers the development of male sexual characteristics, while the X chromosome is similar to the other chromosomes and contains many genes unrelated to sex determination. Females have two copies of the X chromosome, but males have one Y and only one X chromosome&mdash;this difference in X chromosome copy numbers leads to the unusual inheritance patterns of [[sex linked]] disorders.

=== Reproduction ===
{{main|Asexual reproduction|Sexual reproduction}}

When cells divide, their full genome is copied and each daughter cell inherits one copy. This is the simplest form of reproduction and is the basis for [[asexual reproduction]]. Asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Offspring that are genetically identical to their parents are called clones.

[[Eukaryotic]] organisms often use [[sexual reproduction]] to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of sexual reproduction generally alternates between forms that contain single copies of the genome ([[haploid]]) and double copies ([[diploid]]).<ref name=haploid_diploid /> Haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell [[gamete]]s.

Although they do not use the haploid/diploid method of sexual reproduction, [[bacteria]] have many methods of acquiring new genetic information. Some bacteria can undergo [[Bacterial conjugation|conjugation]], transferring a small circular piece of DNA to another bacterium.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.1304 Chapter 7 (Gene Transfer in Bacteria and Their Viruses): Bacterial conjugation]</ref> Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genome, a phenomenon known as [[transformation (genetics)|transformation]].<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.1343 Chapter 7 (Gene Transfer in Bacteria and Their Viruses): Bacterial transformation]</ref> This processes result in [[horizontal gene transfer]], transmitting fragments of genetic information between organisms that would otherwise be unrelated.

[[Image:Morgan crossover 2 cropped.jpg|thumb|right|[[Thomas Hunt Morgan]]'s 1916 illustration of a double crossover between chromosomes]]

===Recombination and linkage ===
{{Main|Chromosomal crossover|Genetic linkage}}

The diploid nature of chromosomes allows for genes on different chromosomes to [[independent assortment|assort independently]] during sexual reproduction, recombining to form new combinations of genes. Genes on the same chromosome would theoretically never recombine, however, were it not for the process of [[chromosomal crossover]]. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.929 Chapter 5 (Basic Eukaryotic Chromosome Mapping): Nature of crossing-over]</ref> This process of chromosomal crossover generally occurs during [[meiosis]], a series of cell divisions that creates haploid [[germ cells]] that later combine with other germ cells to form child organisms.

The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between them. For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated. For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate [[genetic linkage]]&mdash;alleles for the two genes tend to be inherited together. The amounts of linkage between a series of genes can be combined to form a linear [[Genetic linkage#Linkage mapping|linkage map]] that roughly describes the arrangement of the genes along the chromosome.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.899 Chapter 5 (Basic Eukaryotic Chromosome Mapping): Linkage maps]</ref>

==Gene expression==
[[Image:Genetic code.svg|thumb|right|The [[genetic code]]: DNA, through a [[messenger RNA]] intermediate, codes for protein with a triplet code.]]
=== Genetic code ===
{{Main|Genetic code}}

[[Image:Sickle_cell_hemoglobin_shortened.jpg|right|frame|200px|A single amino acid change causes hemoglobin to form fibers.]]

Genes generally express their functional effect through the production of [[proteins]], which are complex molecules responsible for most functions in the cell.<ref>Some genes are transcribed into RNA, but their RNA products are never used to produce protein. These RNA products may fold into forms with enzymatic properties (eg. [[ribosomal RNA]] and [[transfer RNA]]), or they may have a regulatory effect through hybridization interactions with other RNA molecules (eg. [[microRNA]]).</ref> Proteins are chains of [[amino acid]]s, and the DNA sequence of a gene (through an RNA intermediate) is used to produce a specific protein sequence. Each group of three nucleotides in the sequence, called a [[codon]], corresponds to one of the twenty possible amino acids in protein&mdash;this correspondence is called the [[genetic code]].<ref>{{cite book| title=Biochemistry| author=Berg JM, Tymoczko JL, Stryer L, Clarke ND| edition=5th edition| year=2002| publisher=W. H. Freeman and Company| location=New York}} [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.section.685 I. 5. DNA, RNA, and the Flow of Genetic Information: Amino Acids Are Encoded by Groups of Three Bases Starting from a Fixed Point]</ref> The flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but never from protein back into the sequence of DNA&mdash;a phenomenon [[Francis Crick]] called the [[central dogma of molecular biology]].<ref name="crick1970">Crick, F (1970): [http://www.nature.com/nature/focus/crick/pdf/crick227.pdf Central Dogma of Molecular Biology] (PDF). ''Nature'' 227, 561–563. PMID 4913914 </ref>

[[Image:Hb-animation2.gif|upright|right|frame|The dynamic structure of hemoglobin is responsible for its ability to transport oxygen within mammalian blood.]]

The specific sequence of amino acids [[protein folding|results]] in a unique three-dimensional structure for that protein, and the three-dimensional structures of protein are related to their function.<ref>Alberts et al. (2002), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.388|I.3. Proteins: The Shape and Structure of Proteins]</ref><ref>Alberts et al. (2002), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.452|I.3. Proteins: Protein Function]</ref> Some are simple structural molecules, like the fibers formed by the protein [[collagen]]. Proteins can bind to other proteins and simple molecules, sometimes acting as [[enzyme]]s by facilitating chemical reactions within the bound molecules (without changing the structure of the protein itself). Protein structure is dynamic; the protein [[hemoglobin]] bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood.

A single nucleotide difference within DNA can cause a single change in the amino acid sequence of a protein. Because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. For example, [[sickle-cell anemia]] is a human genetic disease that results from a single base difference within the coding region for the β-globin section of hemoglobin, causing a single amino acid change that changes hemoglobin's physical properties.<ref>{{cite web| title=How Does Sickle Cell Cause Disease?| url=http://sickle.bwh.harvard.edu/scd_background.html| date=2002-04-11| accessdate=2007-07-23| publisher=Brigham and Women's Hospital: Information Center for Sickle Cell and Thalassemic Disorders}}</ref> Sickle-cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of red blood cells carrying the protein. These sickle-shaped cells no longer flow smoothly through blood vessels, having a tendency to clot or degrade, causing the medical problems associated with the disease.

=== Nature vs. nurture ===
Although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotype&mdash;a dichotomy often referred to as "[[nature vs. nurture]]". The phenotype of an organism depends on the interaction of genetics with the environment. One example of this is the case of temperature-sensitive mutations. Often, a single amino acid change within the sequence of a protein does not change its behavior and interactions with other molecules, but it does destabilize the structure. In a high temperature environment, where molecules are moving more quickly and hitting each other, this results in the protein [[Denaturation (biochemistry)|losing its structure]] and failing to function. In a low temperature environment, however, the protein's structure is stable and functions normally. This type of mutation is visible in the coat coloration of Siamese cats, where a mutation in an enzyme responsible for pigment production causes it to destabilize and lose function at high temperatures.<ref>{{cite journal| author=Imes DL, Geary LA, Grahn RA, Lyons LA| year=2006 | title=Albinism in the domestic cat (''Felis catus'') is associated with a ''tyrosinase'' (''TYR'') mutation| journal=Animal Genetics| volume=37| issue=2| pages=175| url = http://www.blackwell-synergy.com/doi/full/10.1111/j.1365-2052.2005.01409.x| format=Short Communication| accessdate=2006-05-29}}</ref> The protein remains functional in areas of skin that are colder&mdash;legs, ears, tail, and face&mdash;and so the cat has dark fur at its extremities.

[[Image:Niobe050905.jpeg|thumb|left|upright|Siamese cats have a temperature-sensitive mutation in pigment production.]]
Environment also plays a dramatic role in effects of the human genetic disease [[phenylketonuria]].<ref>{{cite web| url=http://www.nlm.nih.gov/medlineplus/phenylketonuria.html| title=MedlinePlus: Phenylketonuria| accessdate=2008-03-15| publisher=NIH: National Library of Medicine}}</ref> The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid [[phenylalanine]], causing a toxic build-up of an intermediate molecule that, in turn, causes severe symptoms of progressive mental retardation and seizures. If someone with the phenylketonuria mutation is kept on a strict diet that avoids this amino acid, however, they remain normal and healthy.

=== Gene regulation ===
{{Main|Regulation of gene expression}}

The genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. A gene is [[gene expression|expressed]] when it is being transcribed into mRNA (and translated into protein), and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. [[Transcription factor]]s are regulatory proteins that bind to the start of genes, either promoting or inhibiting the transcription of the gene.<ref>{{cite journal |author=Brivanlou AH, Darnell JE Jr |title=Signal transduction and the control of gene expression |journal=Science |volume=295 |issue=5556 |pages=813–818 |year=2002 |pmid=11823631}}</ref> Within the genome of ''[[Escherichia coli]]'' bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid [[tryptophan]]. However, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. The presence of tryptophan directly affects the activity of the genes&mdash;tryptophan molecules bind to the [[trp repressor|tryptophan repressor]] (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. The tryptophan repressor blocks the transcription and expression of the genes, thereby creating [[negative feedback]] regulation of the tryptophan synthesis process.<ref>Alberts et al. (2002), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.1269#1270|II.3. Control of Gene Expression - The Tryptophan Repressor Is a Simple Switch That Turns Genes On and Off in Bacteria]</ref>

[[Image:Zinc finger DNA complex.png|thumb|upright|right|Transcription factors bind to DNA, influencing the transcription of associated genes.]]
Differences in gene expression are especially clear within [[multicellular organism]]s, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. All the cells in a multicellular organism derive from a single cell, differentiating into different cell types in response to external and [[Cell signaling|intercellular signals]] and gradually establishing different patterns of gene expression to create different behaviors. No single gene is responsible for the [[development (biology)|development]] of structures within multicellular organisms, these patterns arise from the complex interactions between many cells.

Within [[eukaryote]]s there exist structural features of [[chromatin]] that influence the transcription of genes, often in the form of modifications to DNA and chromatin that are stably inherited by daughter cells.<ref>{{cite journal| author=Jaenisch R, Bird A| title=Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals| journal=Nature Genetics| volume=33| issue=3s| pages=245–254}}</ref> These features are called "[[epigenetic]]" because they exist "on top" of the DNA sequence and retain inheritance from one cell generation to the next. Because of epigenetic features, different cell types [[cell culture|grown]] within the same medium can retain very different properties. Although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of [[paramutation]], have multigenerational inheritance and exist as rare exceptions to the general rule of DNA as the basis for inheritance.<ref>{{cite journal| title=Paramutation: From Maize to Mice| author=Chandler VL| journal=Cell| volume=128| pages=641–645| year=2007}}</ref>

==Genetic change==
[[Image:Gene-duplication.png|thumb|upright|Gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original function without harming the organism.]]
===Mutations===
{{main|Mutation}}
During the process of [[DNA replication]], errors occasionally occur in the polymerization of the second strand. These errors, called [[mutations]], can have an impact on the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Error rates are usually very low&mdash;1 error in every 10–100&nbsp;million bases&mdash;due to the "proofreading" ability of DNA polymerases.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2706  Chapter 16 (Mechanisms of Gene Mutation): Spontaneous mutations]</ref><ref name=Kunkel>{{cite journal| title=DNA Replication Fidelity| author=Kunkel TA| year=2004| journal=[[Journal of Biological Chemistry]]| volume=279| issue=17| pages=16895–16898}}</ref> (Without proofreading error rates are a thousand-fold higher; because many viruses rely on DNA and RNA polymerases that lack proofreading ability they experience higher mutation rates.) Processes that increase the rate of changes in DNA are called [[mutagenic]]: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while [[UV radiation]] induces mutations by causing damage to the DNA structure.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2727 Chapter 16 (Mechanisms of Gene Mutation): Induced mutations]</ref> Chemical damage to DNA occurs naturally as well, and cells use [[DNA repair]] mechanisms to repair mismatches and breaks in DNA—nevertheless, the repair sometimes fails to return the DNA to its original sequence.

In organisms that use [[chromosomal crossover]] to exchange DNA and recombine genes, errors in alignment during [[meiosis]] can also cause mutations.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2844 Chapter 17 (Chromosome Mutation I: Changes in Chromosome Structure): Introduction]</ref> Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence—[[gene duplication|duplications]], [[chromosomal inversion|inversions]] or [[gene deletion|deletions]] of entire regions, or the accidental exchanging of whole parts between different chromosomes (called [[chromosomal translocation|translocation]]).

===Natural selection and evolution===
{{main|Evolution}}

Mutations produce organisms with different genotypes, and those differences can result in different phenotypes. Many genetic mutations have a negligible effect on an organism's phenotype, health, and reproductive [[fitness (biology)|fitness]]. Mutations that do have an effect are often deleterious, but occasionally mutations arise that are beneficial in the current environmental context of the organism.

[[Image:Eukaryote tree.svg|thumb|left|An [[evolutionary tree]] of eukaryotic organisms, constructed by comparison of several [[orthologous gene]] sequences]]
[[Population genetics]] research studies the distributions of these genetic differences within populations and how the distributions change over time.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3842 Chapter 24 (Population Genetics): Variation and its modulation]</ref> Changes in the frequency of an allele in a population can be influenced by [[natural selection]], where a given allele's higher rate of survival and reproduction causes it to become more frequent in the population over time.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3886 Chapter 24 (Population Genetics): Selection]</ref> [[Genetic drift]] can also occur, where chance events lead to random changes in allele frequency.<ref>Griffiths et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3906 Chapter 24 (Population Genetics): Random events]</ref>

Over many generations, the genomes of organisms can change, resulting in the phenomenon of [[evolution]]. Mutations and the selection for beneficial mutations can cause a species to [[evolution|evolve]] into forms that better survive their environment, a process called [[adaptation]].<ref name=Darwin>{{cite book |last=Darwin |first=Charles |authorlink = Charles Darwin |year=1859 |title=On the Origin of Species |place=London |publisher=John Murray |edition=1st |pages=1 |url=http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16}}. Related earlier ideas were acknowledged in {{cite book |last=Darwin |first=Charles |authorlink = Charles Darwin |year=1861 |title=On the Origin of Species |place=London |publisher=John Murray |edition=3rd |pages=xiii |url=http://darwin-online.org.uk/content/frameset?itemID=F381&viewtype=text&pageseq=20}}</ref>  New species are formed through the process of [[speciation]], a process often caused by geographical separations that allow different populations to genetically diverge.<ref name=Gavrilets>{{cite journal |author=Gavrilets S |title=Perspective: models of speciation: what have we learned in 40 years? |journal=Evolution |volume=57 |issue=10 |pages=2197–2215 |year=2003 |pmid=14628909}}</ref>

As sequences diverge and change during the process of evolution, these differences between sequences can be used as a [[molecular clock]] to calculate the evolutionary distance between them.<ref>{{cite journal |author=Wolf YI, Rogozin IB, Grishin NV, Koonin EV |title=Genome trees and the tree of life |journal=Trends Genet. |volume=18 |issue=9 |pages=472&ndash;479 |year=2002 |pmid=12175808}}</ref> Genetic comparisons are generally considered the most accurate method of characterizing the relatedness between species, an improvement over the sometimes deceptive comparison of phenotypic characteristics. The evolutionary distances between species can be combined to form [[evolutionary tree]]s&mdash;these trees represent the [[common descent]] and divergence of species over time, although they cannot represent the transfer of genetic material between unrelated species (known as [[horizontal gene transfer]] and most common in bacteria).

==Research and technology==

[[Image:Drosophila melanogaster - side (aka).jpg|thumb|right|The common fruit fly (''Drosophila melanogaster'') is a popular [[model organism]] in genetics research.]]
===Model organisms and genetics===
Although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms. The fact that significant research already existed for a given organism would encourage new researchers to choose it for further study, and so eventually a few [[model organisms]] became the basis for most genetics research.<ref>{{cite web| url=http://www.loci.wisc.edu/outreach/text/model.html| title=The Use of Model Organisms in Instruction| accessdate=2008-03-15| publisher=University of Wisconsin: Wisconsin Outreach Research Modules}}</ref> Common research topics in model organism genetics include the study of [[gene regulation]] and the involvement of genes in [[Morphogenesis|development]] and [[cancer]].

Organisms were chosen, in part, for convenience&mdash;short generation times and facile genetic manipulation made some organisms popular genetics research tools. Widely used model organisms include the gut bacterium ''[[Escherichia coli]]'', the plant ''Arabidopsis thaliana'', baker's yeast (''Saccharomyces cerevisiae''), the nematode ''Caenorhabditis elegans'', the common fruit fly (''Drosophila melanogaster''), and the common house mouse (''Mus musculus'').

===Medical genetics research===
[[Medical genetics]] seeks to understand how genetic variation relates to human health and disease.<ref>{{cite web| url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=gnd&ref=sidebar| title=NCBI: Genes and Disease| publisher=NIH: National Center for Biotechnology Information| accessdate=2008-03-15}}</ref> When searching for an unknown gene that may be involved in a disease, researchers commonly use [[genetic linkage]] and genetic [[pedigree chart]]s to find the location on the genome associated with the disease. At the population level, researchers take advantage of [[Mendelian randomization]] to look for locations in the genome that are associated with diseases, a technique especially useful for [[quantitative trait locus|multigenic trait]]s not clearly defined by a single gene.<ref>{{cite journal| author=Smith GD, Ebrahim S| journal=International Journal of Epidemiology| title=‘Mendelian randomization’: can genetic epidemiology contribute to understanding environmental determinants of disease?| year=2003| volume=32| pages=1–22| url=http://ije.oxfordjournals.org/cgi/content/full/32/1/1}}</ref> Once a candidate gene is found, further research is often done on the same gene (called an [[homology (biology)#Orthology|orthologous]] gene) in model organisms. In addition to studying genetic diseases, the increased availability of genotyping techniques has led to the field of [[pharmacogenetics]]&mdash;studying how genotype can affect drug responses.<ref>{{cite web |url=http://www.nigms.nih.gov/Initiatives/PGRN/Background/FactSheet.htm |title=Pharmacogenetics Fact Sheet |accessdate=2008-03-15 |publisher=NIH: National Institute of General Medical Sciences}}</ref>

Although it is not an inherited disease, [[cancer]] is also considered a genetic disease.<ref>{{cite book| author=Strachan T, Read AP| title=Human Molecular Genetics 2| year=1999| publisher=John Wiley & Sons Inc.| edition=second edition}}[http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=hmg.chapter.2342 Chapter 18: Cancer Genetics]</ref> The process of cancer development in the body is a combination of events. [[Mutation]]s occasionally occur within cells in the body as they divide&mdash;while these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. There are biological mechanisms that attempt to stop this process&mdash;signals are given to inappropriately dividing cells that should trigger [[apoptosis|cell death]], but sometimes additional mutations occur that cause cells to ignore these messages. An internal process of [[natural selection]] occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous tumor that grows and invades various tissues of the body.

[[Image:Ecoli colonies.jpg|thumb|left|''[[E coli]]'' [[colony (biology)|colonies]] on a plate of [[agar]], an example of [[Cloning#Cellular cloning|cellular cloning]] and often used in [[molecular cloning]]]]

In an effort to speed up the progress of [[medical research]], ten individuals made their genetic information, [[medical histories]], ethnic backgrounds, and other [[phenotype]]s publicly available on October 21, 2008. The ten individuals, all of whom have at least the equivalent of masters degree in genetics, are the first set of participants in the [http://www.personalgenomes.org| Harvard based Personal Genome Project]]. Project leader George Church hopes to change the public perception that individuals should keep their genetic information private by exploring the consequences of making genetic and medical information publicly accessible. Officials from the project hope to eventually enroll up to 100,000 participants.<ref>http://rs6.net/tn.jsp?t=cpxyzscab.0.0.g7akgrbab.0&p=http%3A%2F%2Fwww.nytimes.com%2F2008%2F10%2F20%2Fus%2F20gene.html%3Fpagewanted%3D1%26_r%3D2%26hp&id=preview</ref>

===Research techniques===
DNA can be manipulated in the laboratory. [[Restriction enzymes]] are a commonly used [[enzyme]] that cuts DNA at specific sequences, producing predictable fragments of DNA.<ref>Lodish et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.1582 Chapter 7: 7.1. DNA Cloning with Plasmid Vectors]</ref> The use of [[DNA ligase|ligation enzyme]]s allows these fragments to be reconnected, and by ligating fragments of DNA together from different sources, researchers can create [[recombinant DNA]]. Often associated with [[genetically modified organism]]s, recombinant DNA is commonly used in the context of [[plasmids]]&mdash;short circular DNA fragments with a few genes on them. By inserting plasmids into bacteria and growing those bacteria on plates of agar (to isolate [[Cloning#Cellular cloning|clones of bacteria cell]]s), researchers can clonally amplify the inserted fragment of DNA (a process known as [[Cloning#Molecular cloning|molecular cloning]]). (Cloning can also refer to the creation of [[Cloning#Organism|clonal organism]]s, through various techniques.)

DNA can also be amplified using a procedure called the [[polymerase chain reaction]] (PCR).<ref>Lodish et al. (2000), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=PCR&rid=mcb.section.1718 Chapter 7: 7.7. Polymerase Chain Reaction: An Alternative to Cloning]</ref> By using specific short sequences of DNA, PCR can isolate and exponentially amplify a targeted region of DNA. Because it can amplify from extremely small amounts of DNA, PCR is also often used to detect the presence of specific DNA sequences.

===DNA sequencing and genomics===

One of the most fundamental technologies developed to study genetics, [[DNA sequencing]] allows researchers to determine the sequence of nucleotides in DNA fragments. Developed in 1977 by [[Frederick Sanger]] and coworkers, chain-termination sequencing is now routinely used to sequence DNA fragments.<ref>{{cite book|author=Brown TA| title=Genomes 2| edition=2nd edition| year=2002| isbn=ISBN 1 85996 228 9}}[http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=genomes.section.6452 Section 2, Chapter 6: 6.1. The Methodology for DNA Sequencing]</ref> With this technology, researchers have been able to study the molecular sequences associated with many human diseases. As sequencing has become less expensive and with the aid of computational tools, researchers have [[Genome project|sequenced the genomes]] of many organisms by stitching together the sequences of many different fragments (a process called [[genome assembly]]).<ref>Brown (2002), [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=genomes.section.6481 Section 2, Chapter 6: 6.2.  Assembly of a Contiguous DNA Sequence]</ref> These technologies were used to sequence the human genome, leading to the completion of the [[Human Genome Project]] in 2003.<ref name=human_genome_project>{{cite web| url=http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml| title=Human Genome Project Information| accessdate=2008-03-15| publisher=Human Genome Project}}</ref>

The large amount of sequences available has created the field of [[genomics]], research that uses computational tools to search for and analyze patterns in the full genomes of organisms. Genomics can also be considered a subfield of [[bioinformatics]], which uses computational approaches to analyze large sets of biological data.
{{-}}

==References==
* {{cite book| author=Alberts B, Johnson A, Lewis J, Raff M, Roberts K, and Walter P| title=Molecular Biology of the Cell| edition=4th edition| year=2002| isbn=0-8153-3218-1}} 
* {{cite book| author=Griffiths AJF, Miller JH, Suzuki DT, Lewontin RC, and Gelbart WM| title=An Introduction to Genetic Analysis| year=2000| publisher=W.H. Freeman and Company| location=New York| isbn=0-7167-3520-2}}
* {{cite book| author=Hartl D, Jones E| title=Genetics: Analysis of Genes and Genomes, 6th edition| publisher=Jones & Bartlett| year=2005| isbn=0-7637-1511-5}}
* {{cite book| author=Lodish H, Berk A, Zipursky LS, Matsudaira P,  Baltimore D, and Darnell J| title=Molecular Cell Biology| edition=4th edition| year=2000| isbn=0-7167-3136-3}}

==Notes==
{{reflist|2}}

==External links==


*{{dmoz|Science/Biology/Genetics/}}

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