Chromosomes

Types of chromosomes

Autosomal or somatic chromosomes (i.e. not sex chromosomes) are the chromosomes that are identical in male and female members of the same species. They are numbered in ascending order:

chromosome 21 [lower-case c; arabic numerals]

Allosomes (sex chromosomes) use letters instead of numbers. There are 2 main sex-determination systems: XY (humans, most other mammals, some insects, some plants) and ZW (birds; some fish, crustaceans, insects and reptiles). These are generally written as:

X chromosome [capital X and lower-case c]     Z chromosome

Chromosome numbering conventions

Conventions for chromosome numbers include:

• 2n – zygotic number of chromosomes. Also known as the somatic number of chromosomes or the diploid number, this is the number of chromosomes in an organism that reproduces sexually. For example, humans have 46 chromosomes (23 pairs of chromosomes), so 2n = 46.
• n – gametic number of chromosomes. Also known as the haploid number of chromosomes, this is the number of chromosomes in the gamete (the cell that fuses with another cell in sexually reproducing organisms, such as a sperm cell or an egg cell). Human gametes have 23 chromosomes, so n = 23.
• x – basic number of chromosomes. This is the number of chromosomes in each cell of an organism, whether it reproduces sexually or asexually. In humans, x = 2n.

Italicise x and n , and insert a nonbreaking space each side of the equals sign.

Chromosomal location

The chromosomal location is the physical (cytogenetic) location of a gene or region on a chromosome. In humans, it is written as the chromosome number, the p or q arm, and the position. The ‘short arm’ is referred to as p and the ‘long arm’ as q:

If the band position is not known precisely, a range can be indicated:

Other species have different ways of mapping chromosomes; see the nomenclature websites listed under Genes and gene products for guidance.