2n

In genetics, "2n" denotes the diploid number of chromosomes in a cell. The symbol n represents a single set of chromosomes (the haploid number), so 2n means two sets. For example, if n = 23 for humans, 2n = 46. A 2n cell has paired chromosomes (one set from each parent).

• Think of n as the count of unique chromosomes. In humans, n = 23 (unique chromosomes 1 through 22, plus X or Y). Therefore 2n (two of each) = 46.
• When solving problems, if you see "2n = X," you can immediately find n = X/2. Likewise, if n is given, 2n is just double it.
• Different species have different n values. The concept of 2n applies to all diploid organisms: for example, a pea plant has 2n = 14 (so n = 7). Always clarify which organism's context you're in.

• An exam may present something like "Organism Y has 2n = 18" and then ask how many chromosomes are in a gamete or in a somatic cell after mitosis, etc. Recognize 2n as the full set and n as half.
• If a cell is described as 2n, it's a diploid cell. For instance, a question might ask: "Is a liver cell 2n or n?" Since a liver cell is a body (somatic) cell, it's 2n (diploid).