TABLE 7-2 A Lexicon of QTL Terminology

Term

Definition

Allele

A variant of a gene or genetic marker

Effect size

The amount of genetic variation explained by a particular QTL

Genetic marker

An easily-assessed DNA segment with a known genetic map position and many alleles. It should have no effect on the phenotype

Interval mapping

A method for QTL mapping in which one takes into account genotype information from markers on either side of the region being studied; this allows the detection of recombination events within the interval that can be used to position the QTL within the interval

Quantitative trait

A trait whose phenotypic variation is continuous (rather than discrete) and is determined by the segregation of many genes or QTLs

QTL

Quantitative trait locus; one of a suite of genes specifying any particular phenotype

QTL mapping

The process of determining probable sites and probable effect sizes of a QTL or QTLs

RI

Recombinant inbred: an inbred strain generated by 20 generations of sibling mating from the F2 of two inbred strains or by 10 generations of self-fertilization (hermaphroditic species)

SDP

Strain distribution pattern; the pattern of alleles at a locus across a series of RI strains derived from crosses between the same initial inbred parents

be genetically linked. For 75 years, single-gene traits have been "mapped" by taking advantage of such cosegregation and the fact that the frequency of recombination events that separate linked genes is proportional to the physical distance between genes on the chromosome. The mapping of quantitative traits, especially demographic traits, is more difficult because these traits are influenced by more than one gene. However, the basis of QTL mapping has been apparent for some time. If a QTL for mean life span, for example, is genetically close (linked) to a genetic marker, then, on average, progeny that carry the marker allele from the "low" parent will have a shorter life span than progeny carrying the marker allele from the "high'' parent. The Human Genome Project has provided "dense" genetic maps of the human and mouse with highly polymorphic marker loci; as a result, the mapping of QTLs over practically the entire human genome has become possible. Nevertheless, QTL mapping in humans is difficult and involves large population sizes. Several other animal model systems are also under intense study and are of more immediate relevance for our discussion. These include other mammalian models, particularly the mouse and rat. and invertebrate models such as fruit flies and the nematode C. elegans.

Mapping of QTLs involves (1) assaying the phenotype of interest in each offspring, (2) characterizing the pattern of genetic markers in each offspring, and (3) a statistical assessment of whether any part of the phenotypic variation is significantly associated with any marker. For example, a marker allele tightly



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