Quantitative Trait Locus (QTL) Analysis 1 Glossary


One of the different forms of a gene (or marker) that can exist at a single locus. A single allele for each locus is inherited separately from each parent.


The inherited association of alleles at loci due to their proximity on a chromosome. The degree of linkage is estimated as the percentage of recombination between loci. Linkage maps are thus based on mathematical distances among loci rather than actual physical distances. However, the order of loci along a chromosome in a linkage map should represent the actual order.

linkage disequilibrium

The condition whereby alleles at two loci occur together in a population at a greater frequency than that predicted by the product of the individual allele frequencies.


The site on a chromosome where a particular gene (or other sequence) is located (plural=loci).

molecular marker

An identifiable DNA sequence on a chromosome. A marker can be a gene, part of a gene, or a sequence in a non-gene region. SSR, RFLP, RAPD, and AFLP are acronyms for commonly used marker techniques.


Frequency distribution whose plot is bell-shaped and symmetrical.


Existing in multiple forms. When a marker locus is polymorphic, there are detectable differences in DNA sequence at that locus.


Quantitative trait locus. (1) a locus that influences the expression of a quantitative trait. (2) a chromosome region detected by statistical analysis that is significantly associated with variation for a quantitative trait.

segregation distortion

Deviation of the expected segregation ratio from that expected by Mendelian inheritance. Distorted segregation means that either the maternal or paternal allele was transmitted to the progeny at a higher frequency than expected.

single seed descent

Breeding procedure in which progenies of a segregating population are self-pollinated and advanced to the next generation by a single seed in each progeny line.

transgressive segregation

Segregation of progeny phenotypes that are more extreme than either parent.