Mapping Qualitative and Quantitative Traits

Mapping is an empirical approach to finding genes, that is, it is a technique that makes use of naturally existing or experimentally arranged phenotypic variation to find associated genetic variation, rather than the direct experimental manipulation of known genes. Traits can be treated as qualitative or quantitative, but they are approached in logically similar ways.

In mapping a qualitative trait, our statistical analysis searches for associations between the presence, or probability of presence, of a trait quality with specific underlying alleles or genotypes. The idea here is classically mendelian—directly related to Mendel's experiments with peas. We use genome-spanning markers to find the gene involved. If more than one gene can generate a qualitative outcome, things become more complicated but are conceptually related.

In mapping a quantitative trait, however, the idea is to find mendelian segregation, but of alleles or genotypes that contribute to a quantitative trait measure. Usually, this means finding genotype-specific shifts in the mean or variance of the trait measure. This is a dose-like concept, in which one gene does not make or break the trait. A locus whose allelic variation in a given sample contributes to statistically meaningful fractions of the observed phenotypic variance is called a Quantitative Trait Locus (QTL). It does not "cause" the trait in the general way that we conceive of a particular allele causing wrinkled vs. smooth peas.

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