Tag SNP:
A group of SNPs in a region of a genome may be in high linkage disequilibrium (LD). In such case, one SNP, called a tag SNP, represents the whole group. As sequencing SNPs is costly, often only the tag SNP, instead of the all the SNPs in the group, is sequenced to find genetic variation that may be associated with a phenotype.
Singleton SNP:
Sometimes one tag SNP represents only itself i.e., it is not in high LD with any other SNPs in that region. Such tag SNP is called a singleton SNP.
References:
1) Wikipedia, Tag SNP, accessed on 29 March 2015.
2) Xiayi Ke et al. (2008), Singleton SNPs in the human genome and implications for genome-wide association studies, European Journal of Human Genetics, 16, 506–515.
