chromosome each of the bases appear). Determination of haplotypes can be done through molecular methods (Allele Specific PCR, Somatic cell hybrids). These methods distinguish which allele is present at which chromosome by separating the chromosomes before genotyping. They can be very time-consuming and expensive, so statistical inference methods have been developed as a less expensive and automated option. These statistical-inference software packages utilize parsimony, maximum likelihood, and Bayesian algorithms to determine haplotypes. Disadvantage of statistical-inference is that a proportion of the inferred haplotypes could be wrong. Population differences When haplotypes are used for genome wide association studies, it is important to note the population being studied. Often different populations will have different patterns of LD. One example of differentiating patterns are African-descended populations vs. European and Asian-descended populations. Since humans originated in Africa and spread into Europe and then the Asian and American continents, the African populations are the most genetically diverse and have smaller regions of LD while European and Asian-descended populations have larger regions of LD due to founder effect. When LD patterns differ in populations, SNPs can become disassociated with each other due to the changes in haplotype blocks. This means that tag SNPs, as representatives of the haplotype blocks, are unique in populations and population differences should be taken into account when performing association studies. Application GWAS Almost every trait has both genetic and environmental influence. Heritability is the proportion of phenotypic variance that is inherited from our ancestors. Association studies are used to determine the genetic influence on phenotypic presentation. Although mostly used for mapping diseases to genomic areas, they can also be used to map heritability of any phenotype like height, eye color etc. Genome-wide association studies (GWAS) use single-nucleotide polymorphisms (SNPs) to identify genetic associations with clinical conditions and phenotypic traits. They are hypothesis free and use a whole-genome approach to investigate traits by comparing a large group of individuals that express a phenotype with a large group of people that don't. The ultimate goal of GWAS is to determine genetic risk factors that can be used to make predictions about who is at risk for a disease, what are the biological underpinnings of disease susceptibility and creating new prevention and treatment strategies. The National Human Genome Research Institute and the European Bioinformatics Institute publishes the GWAS Catalog, a catalog of published genome-wide association studies that highlights statistically significant associations between hundreds of