Sequencing Coverage Level

While it is easy to assume that whole-genome sequencing (WGS) involves sequencing each base in a genome one time, most applications require sequencing each base multiple times to achieve high confidence base calls. Sequencing coverage (or sequencing depth), at the nucleotide level, refers to the number of times a reference base is represented within a set of sequencing reads. When describing sequencing coverage at the whole genome level, this value is expressed as an average or median of all the per base coverage values. For example, a genome with 30 × coverage will have an average of 30 reads spanning any given position within the genome—some regions will have higher coverage and some will have lower coverage. Under certain assumptions, shotgun sequencing coverage follows a Poisson distribution. Therefore, if the average coverage is 30×, the data would be expected to fall to 15× or below about 0.2 % of the time.