What is a quantitative trait locus (QTL)? Suppose you wanted to search for QTLs influencing fruit size in tomatoes. Describe the general structure of a QTL experiment, including the kind of tomato strains you would use, how molecular markers should be distributed in the genome, how the genetic marker alleles should differ between the two strains, and how you would use the F₁ progeny in a subsequent cross to obtain information about the possible location(s) of QTLs of interest.

In 1988, Horst Wilkens investigated blind cavefish, comparing them with members of a sibling species with normal vision that are found in a lake [Wilkens, H. (1988). Evol. Biol. 25:271–367]. We will call them cavefish and lakefish. Wilkens found that cavefish eyes are about seven times smaller than lakefish eyes. F₁ hybrids have eyes of intermediate size. These data, as well as the F₁×F₁ cross and those from backcrosses (F₁×cavefish and F₁×lakefish), are depicted below. Examine Wilkens's results and respond to the following questions:

Based strictly on the F₁ and F₂ results of Wilkens's initial crosses, what possible explanation concerning the inheritance of eye size seems most feasible? 

Both QTL mapping and association (GWA) mapping are used to locate genes responsible for a phenotype. Which of the two techniques does NOT require crosses to produce a mapping population

Both QTL mapping and association mapping are used to locate genes responsible for a phenotype. Which of the following typically tests two differing alleles between the parents of a mapping population?

True or False:Association (GWA) mapping definitively proves that the gene identified is responsible for the trait variation or phenotype?

Many traits of economic or medical significance are determined by quantitative trait loci (QTLs) in which many genes, usually scattered throughout the genome, contribute to expression.

What is meant by the term cosegregate in the context of QTL mapping? Why are markers such as RFLPs, SNPs, and microsatellites often used in QTL mapping?

Many traits of economic or medical significance are determined by quantitative trait loci (QTLs) in which many genes, usually scattered throughout the genome, contribute to expression.

What general procedures are used to identify such loci?

A 3-inch plant was crossed with a 15-inch plant, and all F₁ plants were 9 inches. The F₂ plants exhibited a 'normal distribution,' with heights of 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 inches.

What will be the outcome if the F₁ plants are testcrossed with plants that are homozygous for all nonadditive alleles?