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<dl><dd><em>Main articles:</em> <a title="Classical genetics" href="http://en.wikipedia.org/wiki/Classical_genetics">Classical genetics</a>, <a title="Mendelian inheritance" href="http://en.wikipedia.org/wiki/Mendelian_inheritance">Mendelian inheritance</a> </dd></dl>

<p>Classical genetics consists of the techniques and methodologies of genetics that predate the advent of <a title="Molecular biology" href="http://en.wikipedia.org/wiki/Molecular_biology">molecular biology</a>. After the discovery of the genetic code and such tools of <a title="Clone (genetics)" href="http://en.wikipedia.org/wiki/Clone_%28genetics%29">cloning</a> as <a title="Restriction enzyme" href="http://en.wikipedia.org/wiki/Restriction_enzyme">restriction enzymes</a>, the avenues of investigation open to geneticists were greatly broadened. Some classical genetic ideas have been supplanted with the mechanistic understanding brought by molecular discoveries, but many remain intact and in use, such as <a title="Mendelian inheritance" href="http://en.wikipedia.org/wiki/Mendelian_inheritance">Mendel's laws</a> and <a title="Muller's morphs" href="http://en.wikipedia.org/wiki/Muller%27s_morphs">Muller's morphs</a>. Patterns of inheritance still remain a useful tool for the study of <a title="Genetic disease" href="http://en.wikipedia.org/wiki/Genetic_disease">genetic diseases</a>.</p>

<h3><span class="mw-headline">Behavioral genetics</span></h3>

<dl><dd><em>Main article:</em> <a title="Behavioral genetics" href="http://en.wikipedia.org/wiki/Behavioral_genetics">Behavioral genetics</a> </dd></dl>

<p>Behavioral genetics studies the influence of varying genetics on animal behavior. Behavioral genetics studies the effects of human disorders as well as its causes. Behavioral genetics has yielded some very interesting questions about the evolution of various behaviors, and even some fundamental principles of evolution in general. For example, guppies and meerkats seem to be genetically driven to post a lookout to watch for predators. This lookout stands a significantly slimmer chance of survival than the others, so because of the mechanism of <a title="Natural selection" href="http://en.wikipedia.org/wiki/Natural_selection">natural selection</a>, it would seem that this trait would be lost after a few generations. However, the gene has remained, leading evolutionary philosopher/scientists such as <a title="Richard Dawkins" href="http://en.wikipedia.org/wiki/Richard_Dawkins">Richard Dawkins</a> and <a title="W. D. Hamilton" href="http://en.wikipedia.org/wiki/W._D._Hamilton">W. D. Hamilton</a> to propose explanations, including the theories of <a title="Kin selection" href="http://en.wikipedia.org/wiki/Kin_selection">kin selection</a> and <a title="Reciprocal altruism" href="http://en.wikipedia.org/wiki/Reciprocal_altruism">reciprocal altruism</a>. The interactions and behaviors of gregarious creatures is partially genetic in cause and must therefore be approached by evolutionary theory.</p>

<h3><span class="mw-headline">Clinical genetics</span></h3>

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<p><a title="Physician" href="http://en.wikipedia.org/wiki/Physician">Physicians</a> who are trained as Geneticists diagnose, treat, and counsel patients with <a title="Genetic disorder" href="http://en.wikipedia.org/wiki/Genetic_disorder">genetic disorders</a> or <a title="Syndrome" href="http://en.wikipedia.org/wiki/Syndrome">syndromes</a>. These doctors are typically trained in a genetics <a title="Residency (medicine)" href="http://en.wikipedia.org/wiki/Residency_%28medicine%29">residency</a> and/or <a title="Fellowship" href="http://en.wikipedia.org/wiki/Fellowship">fellowship</a>.</p>

<h3><span class="mw-headline">Molecular genetics</span></h3>

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<p>Molecular genetics builds upon the foundation of classical genetics but focuses on the structure and function of genes at a <a title="Molecule" href="http://en.wikipedia.org/wiki/Molecule">molecular</a> level. Molecular genetics employs the methods of both classical genetics (such as <a title="Hybridization" href="http://en.wikipedia.org/wiki/Hybridization">hybridization</a>) and <a title="Molecular biology" href="http://en.wikipedia.org/wiki/Molecular_biology">molecular biology</a>. It is so-called to differentiate it from other sub fields of genetics such as <a title="Ecological genetics" href="http://en.wikipedia.org/wiki/Ecological_genetics">ecological genetics</a> and <a title="Population genetics" href="http://en.wikipedia.org/wiki/Population_genetics">population genetics</a>. An important area within molecular genetics is the use of molecular information to determine the patterns of descent, and therefore the correct <a title="Scientific classification" href="http://en.wikipedia.org/wiki/Scientific_classification">scientific classification</a> of organisms: this is called <a title="Molecular systematics" href="http://en.wikipedia.org/wiki/Molecular_systematics">molecular systematics</a>. The study of inherited features not strictly associated with changes in the <a title="DNA" href="http://en.wikipedia.org/wiki/DNA">DNA</a> sequence is called <a title="Epigenetics" href="http://en.wikipedia.org/wiki/Epigenetics">epigenetics</a>.</p>

<p><a id="Population.2C_quantitative_and_ecological_genetics" name="Population.2C_quantitative_and_ecological_genetics"></a></p>

<h3><span class="mw-headline">Population, quantitative and ecological genetics</span></h3>

<p>Population, quantitative and ecological genetics are all very closely related subfields and also build upon classical genetics (supplemented with modern molecular genetics). They are chiefly distinguished by a common theme of studying <a title="Population" href="http://en.wikipedia.org/wiki/Population">populations</a> of organisms drawn from nature but differ somewhat in the choice of which aspect of the organism on which they focus. The foundational discipline is population genetics which studies the distribution of and change in <a title="Allele frequency" href="http://en.wikipedia.org/wiki/Allele_frequency">allele frequencies</a> of genes under the influence of the four evolutionary forces: <a title="Natural selection" href="http://en.wikipedia.org/wiki/Natural_selection">natural selection</a>, <a title="Genetic drift" href="http://en.wikipedia.org/wiki/Genetic_drift">genetic drift</a>, <a title="Mutation" href="http://en.wikipedia.org/wiki/Mutation">mutation</a> and <a title="Migration" href="http://en.wikipedia.org/wiki/Migration">migration</a>. It is the theory that attempts to explain such phenomena as <a title="Adaptation (biology)" href="http://en.wikipedia.org/wiki/Adaptation_%28biology%29">adaptation</a> and <a title="Speciation" href="http://en.wikipedia.org/wiki/Speciation">speciation</a>.</p>

<p>The related subfield of quantitative genetics, which builds on population genetics, aims to predict the response to <a title="Selection" href="http://en.wikipedia.org/wiki/Selection">selection</a> given data on the <a title="Phenotype" href="http://en.wikipedia.org/wiki/Phenotype">phenotype</a> and relationships of individuals. A more recent development of quantitative genetics is the analysis of <a title="Quantitative trait loci" href="http://en.wikipedia.org/wiki/Quantitative_trait_loci">quantitative trait loci</a>. Traits that are under the influence of a large number of genes are known as quantitative traits, and their mapping to a location on the <a title="Chromosome" href="http://en.wikipedia.org/wiki/Chromosome">chromosome</a> requires accurate phenotypic, pedigree and marker data from a large number of related individuals.</p>

<h3><span class="mw-headline">Genomics</span></h3>

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<p>A more recent development is the rise of <a title="Genomics" href="http://en.wikipedia.org/wiki/Genomics">genomics</a>, which attempts the study of large-scale genetic patterns across the <a title="Genome" href="http://en.wikipedia.org/wiki/Genome">genome</a> for (and in principle, all the DNA in) a given species. The field typically depends on the availability of whole genome sequences, computational tools and <a title="Sequence profiling tool" href="http://en.wikipedia.org/wiki/Sequence_profiling_tool">Sequence profiling tool</a> using <a title="Bioinformatics" href="http://en.wikipedia.org/wiki/Bioinformatics">bioinformatics</a> approaches for analysis of large sets of data.</p>