## 2 Quantitative Trait Loci (QTL) and Its Role in Breeding Programs  
Quantitative trait loci (QTL) mapping was initially proposed by [PERSON] [12], who found that bean seed size (characteristic) was linked to seed coat color (a simple, monogenic trait). [PERSON] [13] extended this idea by highlighting that, if segregation of simple inherited monogenes could be used to identify connected QTLs, it should be possible to map and describe all QTLs involved in complicated characteristics. Traits exhibiting quantitative variation were previously explored using statistical analysis of suitable experimental populations based on means, variances, and covariances of relatives, without knowing the number and location of the underlying genes [14]. Such studies examined the distribution of phenotypes in populations, and associations between related individuals or lines. Investigations of morphological and agronomic aspects of maize [15] indicated that certain molecular markers explained a considerable part of the phenotypic variation of quantitative variables.  
Plant breeding involves creating or assembling populations or germplasm collections with high genetic diversity, in order to identify individuals with robust or beneficial traits and use those individuals to develop improved cultivars [16]. The majority of important plant breeding qualities (e.g., height, yield, drought tolerance, etc.) are quantitative, also known as continuous, multifactorial, or complex traits. Quantitative traits are based on the combined activity of many genes and their interactions with the environment, and may vary across individuals over a given range to form a continuous distribution of phenotypes [17]. Since [PERSON] [18] and [PERSON] [19] proposed the multiple-factor theory, it has been considered that the genetic variation of a quantitative trait is controlled by the collective effects of various genes--QTLs [20; 21]. As a result, many QTLs control the expression of a single phenotypic feature. Quantitative features increase difficultly for breeders, since performance only represents a portion of an individual's hereditary values. Hence, since multiple genes or QTLs govern quantitative characteristics, plants with the same phenotype might have distinct alleles at each of these genes or QTLs. Conversely, plants with identical QTL genotypes may have distinct phenotypes when grown in different settings, and the impact of one QTL can be influenced by the plant's allelic composition at other QTLs. As a result, the genotype cannot be inferred from the phenotype, necessitating the creation of specialized genetic stocks and their cultivation in carefully regulated conditions.  
The two main aims of QTL mapping in plants are: to (i) improve our understanding of quantitative trait inheritance and genetic architecture within and between species, and (ii) discover markers that may be utilized as indirect selection tools in breeding [22]. Extensive QTL mapping studies in most commercially significant crops were conducted to identify molecular markers for marker-aided selection [21; 23; 24]. It is known that a phenotypic trait can be controlled by many independent loci of little effect, or a few genes of large effect. Moreover, beneficial QTL alleles are known to be inherited from parents who have beneficial QTL alleles [25]. Figure 1 shows the general flow of information from collecting phenotypic and genotypic data to QTL and association analyses.