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<p>A <strong>haplotype</strong> is the <a title="Genetics" href="http://en.wikipedia.org/wiki/Genetics">genetic</a> constitution of an individual <a title="Chromosome" href="http://en.wikipedia.org/wiki/Chromosome">chromosome</a>. <em>Haplotype</em> may refer to only one <a title="Locus (genetics)" href="http://en.wikipedia.org/wiki/Locus_%28genetics%29">locus</a> or to an entire <a title="Genome" href="http://en.wikipedia.org/wiki/Genome">genome</a>. In the case of <a title="Diploid" href="http://en.wikipedia.org/wiki/Diploid">diploid</a> organisms such as humans, a genome-wide haplotype comprises one member of the pair of <a title="Allele" href="http://en.wikipedia.org/wiki/Allele">alleles</a> for each locus (that is, half of a <a title="Diploid" href="http://en.wikipedia.org/wiki/Diploid">diploid</a> genome). An organism's haplotype is studied using a <a title="Genealogical DNA test" href="http://en.wikipedia.org/wiki/Genealogical_DNA_test">genealogical DNA test</a>. The term <em>haplotype</em> is a <a title="Contraction (linguistics)" href="http://en.wikipedia.org/wiki/Contraction_%28linguistics%29">contraction</a> of <em>&quot;<a title="Ploidy" href="http://en.wikipedia.org/wiki/Ploidy">haploid</a> <a title="Genotype" href="http://en.wikipedia.org/wiki/Genotype">genotype</a>&quot;</em>.</p>

<p>In a second meaning, haplotype is a set of <a title="Single nucleotide polymorphism" href="http://en.wikipedia.org/wiki/Single_nucleotide_polymorphism">single nucleotide polymorphisms</a> (SNPs) on a single <a title="Chromatid" href="http://en.wikipedia.org/wiki/Chromatid">chromatid</a> that are <a title="Association (statistics)" href="http://en.wikipedia.org/wiki/Association_%28statistics%29">statistically associated</a>. It is thought that these associations, and the identification of a few alleles of a haplotype block, can unambiguously identify all other polymorphic sites in its region. Such information is very valuable for investigating the genetics behind common <a title="Diseases" href="http://en.wikipedia.org/wiki/Diseases">diseases</a> and is collected by the <a title="International HapMap Project" href="http://en.wikipedia.org/wiki/International_HapMap_Project"><font color="#800080">International HapMap Project</font></a>.</p>

<h2><span class="mw-headline">Relation to genotypes</span></h2>

<p>An organism's genotype may not uniquely define its haplotype. For example, consider two <a title="Locus (genetics)" href="http://en.wikipedia.org/wiki/Locus_%28genetics%29">loci</a> on the same chromosome, each locus with two possible alleles: the first locus being either <em>A</em> or <em>a</em>, the second locus being <em>B</em> or <em>b</em>. If the organism's genotype is <em>AaBb</em>, there are two possible sets of haplotypes, corresponding to which pairs occur on the same <a title="Chromosome" href="http://en.wikipedia.org/wiki/Chromosome">chromosome</a>:</p>

<p>Unlike the UEPs, the Y-STRs mutate much more easily, which gives them much more resolution to distinguish recent genealogy. But it also means that rather than the population of descendents of a genetic event all sharing the <em>same</em> result, the Y-STR haplotypes are likely to have spread apart, to form a <em>cluster</em> of more or less similar results. Typically, this cluster will have a definite most probable centre, the <strong>modal haplotype</strong> (presumably close to the haplotype of the original founding event); and also a <strong>haplotype diversity</strong> - the degree to which it has become spread out. The longer ago the defining event occurred, and the more that subsequent population growth occurred early, the more will be the haplotype diversity for a particular number of descendents. On the other hand, if the haplotype diversity is smaller for a particular number of descendents, this may indicate a more recent common ancestor, or that a population expansion has occurred more recently.</p>

<p>Importantly, unlike for UEPs, there is no guarantee that two individuals with a similar Y-STR haplotype will necessarily share a similar ancestry. There is no uniqueness about Y-STR events. Instead, the clusters of Y-STR haplotype results inheriting from different events and different histories all tend to overlap.</p>

<p>Similarly for surnames. A cluster of similar Y-STR haplotypes may indicate a shared common ancestor, with an identifiable modal haplotype; but only if the cluster is sufficently distinct from what may have arisen by chance from different individuals historically having adopted the same name independently. This may require the typing of quite an extensive haplotype to establish, which has fuelled DNA testing companies to offer ever larger sets of markers - 24 then 37 then 63 and perhaps soon even more.</p>

<p>Plausibly establishing relatedness between different surnames data-mined from a database is significantly harder, because now one must establish not that a <em>randomly selected</em> member of the population is unlikely to have such a close match by accident; but rather that the <em>very nearest</em> member of the population in question, chosen purposely from the population for that very reason, would even under those circumstances be unlikely to match by accident. This is for the foreseeable future likely to be impossible except in special cases where there is further information to drastically limit the size of that population of candidates under consideration.</p>

<h2><span class="mw-headline">External links</span></h2>

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