microchromosome

right|thumb|200px|Image of chicken chromosomes featuring the many microchromosomes (appearing as dots). The arrows indicate a stained gene locus on homologous macrochromosomes.

A microchromosome is a chromosome defined for its relatively small size. They are typical components of the karyotype of birds, some reptiles, fish, amphibians, and monotremes. As many bird genomes have chromosomes of widely different lengths, the name was meant to distinguish them from the comparatively large macrochromosomes. The distinction referred to the measured size of the chromosome while staining for karyotype, and while there is not a strict definition, chromosomes resembling the large chromosomes of mammals were called macrochromosomes (roughly 3 to 6 μm), while the much smaller ones of less than around 0.5 μm were called microchromosomes. In terms of base pairs, by convention, those of less than 20Mb were called microchromosomes, those between 20 and 40 Mb are classified as intermediate chromosomes, and those larger than 40Mb are macrochromosomes. By this definition, all normal chromosomes in organisms with relatively small genomes (less than 100-200Mb) would be considered microchromosomes.