For such discrete traits, one can infer the "genotype" (SS or Ss versus ss) by observing the "phenotype" (large or small). If fruit size, for example, is controlled by a single gene with alleles “s” for small and “S” for large, then the progeny of crosses between the two parents would segregate in to 3:1 ratios of large- to small- fruited plants. Quantitative traits complicate the works of breeders because performance only partially reflects the genetic values of the individuals. Instead, their phenotypes typically vary along a continuous gradient depicted by a bell curve. Unlike monogenic traits, polygenic traits do not follow patterns of Mendelian inheritance (qualitative traits). Consequently, several QTLs regulate the expression of a single phenotypic trait (in this paper, QTL refers to a single region of DNA associated with a particular trait while QTLs refers to the situation when two or more regions of DNA from the same or different chromosomes are associated with a particular trait). Since the proposal of the multiple-factor hypothesis by Nillson-Ehle (1909) and East (1916), the genetic variation of a quantitative trait is assumed to be controlled by the collective effects of numerous genes, known as quantitative trait loci (QTLs) ( Bulmer, 1985 Edwards et al. A quantitative trait is a measurable trait that depends on the cumulative action of many genes and their interaction with the environment that can vary among individuals over a given range to produce a continuous distribution of phenotypes ( Sham et al. Most of the traits of interest in plant breeding ( e.g., yield, height, drought resistance, disease resistance in many species, etc.) are quantitative, also called polygenic, continuous, multifactorial or complex traits. Figure 1 summarizes the different breeding methods that are commonly employed in crop improvement programs. Plant breeding is a three step process, wherein populations or germplasm collections with useful genetic variation are created or assembled, individuals with superior phenotypes are identified, and improved cultivars are developed from selected individuals ( Moose and Mumm, 2008).
#SIMS 4 TRAITS ABBREVIATIONS SOFTWARE#
This review provides an overview of the two QTL mapping methods, including mapping population type and size, phenotypic evaluation of the population, molecular profiling of either the entire or a subset of the population, marker-trait association analysis using different statistical methods and software as well as the future prospects of using markers in crop improvement. Linkage analysis and association mapping are the two most commonly used methods for QTL mapping. Many QTL mapping studies conducted in the last two decades identified QTLs that generally explained a significant proportion of the phenotypic variance, and therefore, gave rise to an optimistic assessment of the prospects of markers assisted selection. Exploiting molecular markers in breeding involve finding a subset of markers associated with one or more QTLs that regulate the expression of complex traits. The genetic variation of a quantitative trait is assumed to be controlled by the collective effects of quantitative trait loci (QTLs), epistasis (interaction between QTLs), the environment, and interaction between QTL and environment. Most traits of interest in plant breeding show quantitative inheritance, which complicate the breeding process since phenotypic performances only partially reflects the genetic values of individuals. RFLP: Restriction fragment length polymorphism
Keywords: association mapping, linkage disequilibrium, markers assisted selection, molecular breeding, molecular markers, quantitative trait, QTL mapping, QTL analysis. International Maize and Wheat Improvement Centerĭepartment of Plant and Environmental Sciencesįinancial support: The Bill & Melinda Gates Foundation. The genetic dissection of quantitative traits in crops © 2010 by Pontificia Universidad Católica de Valparaíso - Chile Electronic Journal of Biotechnology ISSN: 0717-3458
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