In the context of HLA typing, what does the term "haplotype" refer to?

In the context of HLA typing, the term "haplotype" refers to a combination of alleles at multiple loci on a single chromosome that are inherited together. HLA (human leukocyte antigen) typing is essential for understanding how these genes can match between donors and recipients in transplantation, as well as in various autoimmune diseases and blood transfusion compatibility.

A haplotype represents a specific set of polymorphic markers that are located close to each other on the same chromosome, making them likely to be inherited together because of their proximity. This is particularly important because certain combinations of HLA alleles can strongly influence the immune response. By analyzing haplotypes, researchers can determine more about the genetic makeup of individuals and the compatibility necessary for medical purposes.

In contrast, a single allele at one locus identifies only one variant of a gene and cannot provide the same level of information about inherited combinations. Genetic mutations refer to alterations in DNA sequences that may affect single genes rather than combinations of them. The total number of alleles would not specifically capture the idea of combinations at multiple loci, as a haplotype is specific to the alleles inherited together on one chromosome rather than a simple count of allele instances.