Changes

Human mutations don't occur randomly. In fact, transitions (changes from A <-> G and C <-> T) are expected to occur twice as frequently as transversions (changes from A <-> C, A <-> T, G <-> C or G <-> T). Thus, another useful diagnostic is the ratio of transitions to transversions in a particular set of SNP calls. This ratio is often evaluated separately for previously discovered and novel SNPs.

Human mutations don't occur randomly. In fact, transitions (changes from A <-> G and C <-> T) are expected to occur twice as frequently as transversions (changes from A <-> C, A <-> T, G <-> C or G <-> T). Thus, another useful diagnostic is the ratio of transitions to transversions in a particular set of SNP calls. This ratio is often evaluated separately for previously discovered and novel SNPs.

Across the entire genome the ratio of transitions to transversions is typically around 2. In protein coding regions, this ratio is typically higher, often a little above 3. The higher ratio occurs because, especially when they occur in the third base of a codon, transitions are much more likely to change the encoded amino acid. A refinement to this analysis, in protein coding regions, is to examine the transition to transversion ratio separately for non-degenerate, two-fold degenerate, three-fold degenerate and four-fold degenerate sites.

Across the entire genome the ratio of transitions to transversions is typically around 2. In protein coding regions, this ratio is typically higher, often a little above 3. The higher ratio occurs because, especially when they occur in the third base of a codon, transversions are much more likely to change the encoded amino acid. A refinement to this analysis, in protein coding regions, is to examine the transition to transversion ratio separately for non-degenerate, two-fold degenerate, three-fold degenerate and four-fold degenerate sites.

== Why Are Reciprocal Changes Not Equally Frequent? ==

== Why Are Reciprocal Changes Not Equally Frequent? ==

More often than not, we expect that the reference genome will include the most common allele, which is also likely to be the ancestral allele. Thus, if C->T mutations are more common than T->C mutations, we expect to see an imbalance of C->T versus T->C changes. Further, when comparing rare and common variants, we expect the imbalance to be stronger for lower frequency variants.

More often than not, we expect that the reference genome will include the most common allele, which is also likely to be the ancestral allele. Thus, if C->T mutations are more common than T->C mutations, we expect to see an imbalance of C->T versus T->C changes. Further, when comparing rare and common variants, we expect the imbalance to be stronger for lower frequency variants.