Single nucleotide polymorphisms (SNPs, pronounced “snips”) represent the simplest type of genetic variation between individuals. A SNP refers to a specific location in the genome where different people have been shown to have a different nucleotide. If more than one percent of the population has a different nucleotide at a particular location, that location is considered a SNP. If less than one percent of the population has a different nucleotide, it is considered a mutation (Alberts et al. 2010).
There are approximately 3 million SNPs throughout the human genome, yet they do not all behave independently. Groups of nearby SNPs are often inherited together in what are called haplotype blocks (see Figure 19-36 in your Alberts text). Depending on the region of the genome and the location of prior meiotic recombination, a haplotype block can span one or many genes.
Some SNPs appear to be benign, while others are associated with an increased risk of developing disease. To try to determine genetic risk factors for disease, scientists have compared SNPs in healthy individuals to SNPs in individuals with a particular disease. If a SNP is associated with an increased risk of disease, it’s important to keep in mind that it may not be causative. Furthermore, many SNPs that are associated with disease confer only a modest increase in disease risk. In this lab, you will have the opportunity to experimentally examine a SNP located within the TAS2R38 gene (also called the PTC Taster gene). While variations in TAS2R38 genotype had long been thought to be benign, recent work has suggested a correlation between TAS2R38 genotype and disease risk (Carrai et al. 2011, Lee et al. 2012).