Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 197

Molecular quantitative genetics mainly focuses on evaluating the coupling association of the polymorphic DNA sites with the phenotypic variations of quantitative and complex traits. In addition, whereas classical quantitative genetics deals with the holistic status of all genes, molecular quantitative genetics dissects the genetic architectures of quantitative genes (concerned with the analytical status of the major genes and holistic status of the minor genes).

The genetic architecture of quantitative traits entails the number of QTLs that influence a quantitative trait, the number of alleles that each QTL possesses, the frequencies of the alleles in the population, and the influence of each QTL and its alleles on the quantitative trait. Identifying and characterizing QTLs will provide a basis for selecting and improving plant species. The summation of QTL studies indicates that QTL alleles with large effects are rare; most quantitative traits are controlled by many loci with small effects.

Researchers commonly use one of two fundamental approaches to design and study quantitative traits. In what is called the top‐down approach, they start with the trait of interest and then attempt to draw inferences about the underlying genetics from examining the degree of trait resemblance among related subjects. It is usually the first step taken to determine if there is any evidence for a genetic component. It is also described as the unmeasured genotype approach because it focuses on the inheritance pattern without measuring any genetic variations. Typical statistical analyses employed in this approach are heritability and segregational analysis.

In the second approach, the bottom‐up (measured) approach, researchers actually measure QTLs and use the information to draw inferences about which genes might have a role in the genetic architecture of a quantitative trait. Typical statistical analyses employed in this approach are linkage analysis and association analysis. The second approach is becoming more assessable with the advent of newer, more efficient, and less expensive technologies to measure QTLs. These technologies include DNA microarrays and protein mass spectrometry. They allow researchers to quantitatively measure the expression levels of thousands of gene simultaneously, and thereby study gene expression at both the ribonucleic acid (RNA) and protein levels as a quantitative trait.