Microchromosome

[1] As many bird genomes have chromosomes of widely different lengths, the name was meant to distinguish them from the comparatively large macrochromosomes.

[3] 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.

While originally thought to be insignificant fragments of chromosomes, in species where they have been studied they have been found to be rich in genes and high in GC content.

Their small size and poor condensation into heterochromatin means they generally lack the diagnostic banding patterns and distinct centromere locations used for chromosome identification.

Comparison between lancelet and modern vertebrate chromosomes shows that the macrochromosomes were the result of fusion between ancestral microchromosomes.

[4][8] Microchromosomes represent approximately one third of the total genome size, and have been found to have a much higher gene density than macrochromosomes.

[8] Birds (except Falconidae) usually have karyotypes of approximately 80 chromosomes (2n = 80), with only a few being distinguishable macrochromosomes and an average of 60 being microchromosomes.

[7] Comparative genomic analysis shows that microchromosomes contain genetic information which has been conserved across multiple classes of chromosomes.

For the many small linkage groups in the chicken genome which have not been placed on chromosomes, the assumption has been made that they are located on the microchromosomes.