**6. The major application for DNA-based genetic markers**

DNA-based molecular markers are such powerful tools for mapping human diseases and discover many multifactorial diseases and disorders.

#### **6.1. Mapping human diseases and risk prediction**

Genetic mapping and linkage: The mapping of the human genome has made possible to develop a haplotype map in order to better define human SNV variability. The haplotype map or HapMap will be a tool for the detection of human genetic variation that can affect health and diseases [23]. The HapMap project is far more useful because it will reduce the number of SNVs required to examine the entire genome for association with a phenotype or diseases from the 10 million SNPs that are expected to exist to approximately tag 500,000 SNPs [38]. The first large-scale effort to produce a human genetic map was performed mainly using RFLP; other several projects are underway to identify more markers in humans and to make this data publicly available to scientists worldwide. Many groups that are involved in these massive efforts through DNA polymorphisms discovery resource include the SNP consortium (TSC) http://snp.cshl.org [49, 50]. The reason for the current enormous interest in SNPs is the hope that they could be used as markers to identify genes that predispose individuals to common, multifactorial disorders by using linkage disequilibrium (LD) mapping.

"The HapMap Project (http://hapmap.ncbi.nlm.nih.gov/), and other approaches, such as genome wide association studies, have been widely reported for complex polygenic diseases, with some interesting novel genes affecting disease susceptibility now identified. Genome Wide Association; the GWAS has now been used for a large range of traits and diseases e.g. baldness and eye color" [51, 52].

#### **6.2. Quantitative trait loci mapping, candidate genes, and complex traits**

The identification of genes affecting complex trait is a very difficult task. For many complex traits, the observable variation is quantitative, and loci affecting such traits are generally termed quantitative trait loci (QTL). (SNVs) can be used as genetic markers for constructing high-density genetic maps and to carry out association studies related to complex traits and diseases [14].

#### **6.3. Pharmacogenetics**

Individual response to a drug is governed by many factors such as genetics, age, sex, environment, and disease. The influence of genetic factors on the response of a drug is a known fact. Polymorphic STRs, together with SNPs and CNVs, can explain variability in response to pharmacotherapy because of their prevalence in the human genome and their functional role as regulators of gene expression and its applications. Pharmacogenetics is the study of the influence of genetics factors on drug response and metabolism. The science of pharmacogenetics when applied can be used to evade adverse drug reactions, predict toxicity and therapeutic failure, and refine therapeutic efficiency and improve clinical outcomes [53].
