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A haplotype (from the Greek word haploƻs, "onefold, single, simple") in genetics is a combination of alleles (DNA sequences) at different places (loci) on the chromosome that are transmitted together. A haplotype may be one locus, several loci, or an entire chromosome depending on the number of recombination events that have occurred between a given set of loci.

A haplotype is sometimes referred to by genetic genealogists as a genetic signature or a DNA signature, particularly with regard to a Y-DNA Y-STR haplotype.

In a second meaning, haplotype is a set of single-nucleotide polymorphisms (SNPs) on a single chromatid (half a chromosome pair) that are statistically associated. 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 diseases, and has been investigated in the human species by the International HapMap Project.

Many genetic testing companies use the term 'haplotype' to refer to an individual collection of short tandem repeat (STR) allele mutations within a genetic segment, while using the term haplogroup to refer to the SNP/unique-event polymorphism (UEP) mutations which represent the subhaplogroup to which a collection of potential haplotypes belong.

Y-DNA haplotypes from genealogical DNA tests

Unlike other chromosomes, Y chromosomes do not come in pairs. Every human male has only one copy of that chromosome. This means that there is no lottery as to which copy to inherit, and also (for most of the chromosome) no shuffling between copies by recombination; so, unlike autosomal DNA haplotypes, there is therefore effectively no randomisation of the Y-chromosome haplotype between generations, and a human male should largely share the same Y chromosome as his father, give or take a few mutations.

In particular, the Y-DNA that is the numbered results of a Y chromosome DNA test should match, barring mutations. Within genealogical and popular discussion, this is sometimes referred to as the "DNA signature" of a particular male human, or of his paternal bloodline.

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GNU head This article is licensed under the GNU Free Documentation Licence. It uses material from the Wikipedia article "Haplotype".