Tag SNP and Singleton SNP

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.

Common Disease - Common Variant

Common disease - common variant (CD-CV) hypothesis says that the common disease-causing allele (or variant) will be found in all human populations commonly having that disease.

Common variants (not necessarily disease-causing) are known to exist in coding and regulatory sequence of genes. CD-CV says, some of these variants lead to cause that disease.

Common example is SNP (Single Nucleotide Polymorphism) - single nucleotide base change in DNA. SNP variants tend to be common in different human population. This polymorphism have been valuable as "markers," in search for common variants causing a common disease.

In complex disease, the effect (additive or multiplicative) of a variant at a gene to cause the disease will be very small and it will be evolutionarily neutral, as so many genes influence a complex disease.