Haplotype

A haplotype (haploid genotype) is a group of alleles in an organism that are inherited together from a single parent. Many organisms contain genetic material (DNA) which is inherited from two parents. Normally these organisms have their DNA organized in two sets of pairwise similar chromosomes. The offspring gets one chromosome in each pair from each parent. A set of pairs of chromosomes is called diploid and a set of only one half of each pair is called haploid. The haploid genotype (haplotype) is a genotype that considers the singular chromosomes rather than the pairs of chromosomes. It can be all the chromosomes from one of the parents or a minor part of a chromosome, for example a sequence of 9000 base pairs. However, there are other uses of this term. First, it is used to mean a collection of specific alleles (that is, specific DNA sequences) in a cluster of tightly linked genes on a chromosome that are likely to be inherited together—that is, they are likely to be conserved as a sequence that survives the descent of many generations of reproduction. A second use is to mean a set of linked single-nucleotide polymorphism (SNP) alleles that tend to always occur together (i.e., that are associated statistically). It is thought that identifying these statistical associations and a few alleles of a specific haplotype sequence can facilitate identifying all other such polymorphic sites that are nearby on the chromosome. Such information is critical for investigating the genetics of common diseases; which in fact have been investigated in humans by the International HapMap Project. Thirdly, many human genetic testing companies use the term in a third way: to refer to an individual collection of specific mutations within a given genetic segment; (see short tandem repeat mutation). The term 'haplogroup' refers to the SNP/unique-event polymorphism (UEP) mutations that represent the clade to which a collection of particular human haplotypes belong. (Clade here refers to a set of haplotypes sharing a common ancestor.