Changes

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, 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.

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

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

One of the most surprising features of many variant lists in humans is that C->T changes (C reference, T variant) are more frequent than T->C changes. Likewise, G->A changes are more frequent than A->G changes.

One of the most surprising features of many variant lists in humans is that C->T changes (C reference, T variant) are more frequent than T->C changes. Likewise, G->A changes are more frequent than A->G changes.