= Genomic copy number
{title2=$C$}
<Genomic copy number> is the count of copies of a specified <DNA> sequence in a defined biological unit, such as a cell or a haploid-equivalent <genome>. The sequence and denominator matter: a two-copy autosomal locus in a diploid cell is one copy per haploid genome, while a deletion or duplication can alter that locus without changing the ploidy of the whole cell. Such segment-level differences underlie <copy number variation>.<Talking Glossary of Genomic and Genetic Terms by NHGRI>[Copy Number Variation entry: segment copy differences arising through duplication, deletion and other structural changes]{ref}
<DNA replication> also changes the number of DNA molecules during the <cell cycle>. A replicated chromosome contains sister copies without thereby becoming two independently counted chromosomes under the usual centromere-based convention. Report the cell-cycle stage when comparing absolute DNA copies per cell.
<Sequencing depth> can estimate relative <genomic copy number> by comparing coverage with a reference region, but library preparation, sequence-dependent mapping and population mixtures affect that comparison. In a hypothetical mixture where half the cells carry two copies and half carry four, the mean is three copies per cell; that average does not show that any individual cell carries three. <Genomic copy number> also differs from the number of expressed <RNA> or <protein> molecules. This distinction matters when connecting gene dosage to a molecular abundance model.
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