Monoploid

Monoploid is a term used in genetics to denote a genome composed of a single complete chromosome set. In cytogenetics this is described by the monoploid chromosome number, x, which is the number of distinct chromosomes in a single monoploid set. The concept is closely related to, but not always identical with, the haploid number, n, which is the chromosome number in a gamete. In many species, n equals x; in others, particularly polyploids, n differs from x.

Examples help illustrate the distinction. Arabidopsis thaliana has 2n = 10 and x = 5, so n = x = 5. Maize (Zea mays) has 2n = 20, with x = 10 and n = 10, again matching. Bread wheat (Triticum aestivum) is hexaploid with 2n = 42, but its basic chromosome number is x = 7, while the haploid number is n = 21, showing a clear difference between n and x in polyploids.

Applications and significance. The monoploid concept provides a framework for understanding genome structure across species, especially when comparing diploids, autopolyploids, and allopolyploids. In plant breeding, haploid and doubled-haploid production are valuable because they enable rapid development of homozygous lines. Monoploids themselves are rarely viable in animals, whereas in plants they can be produced and then chromosome-doubled to restore diploidy for breeding and genetic analysis.