The number of human chromosomes observed at mitosis was incorrectly reported as 24 pairs by W. Flemming in 1898. It was not until 1956 that the correct number was determined as 23 pairs by J. H. Tjio and A. Levan. The mitotic chromosomes are generally grouped into seven classes (A-G) according to the following cytological criteria: Group A (chromosomes 1–3) large chromosomes with approximately median centromeres. Group B (chromosomes 4–5)—large chromosomes with submedian centromeres. Group C (chromosomes 6–12 and the X chromosome)—medium-sized chromosomes with submedian centromeres. Group D (chromosomes 13–15)—medium-sized acrocentric chromosomes. Chromosome 13 has a prominent satellite on the short arm. Chromosome 14 has a small satellite on the short arm. Group E (chromosomes 16–18)—rather short chromosomes with approximately median (in chromosome 16) or submedian centromeres. Group F (chromosomes 19 and 20)—short chromosomes with approximately median centromeres. Group G (chromosomes 21, 22, and the Y chromosome)—very short acrocentric chromosomes. See Chronology, 1956, Tjio and Levan; 1971, O'Riordan; 1981, Harper and Saunders; 1991, Ijdo et al.; high-resolution chromosome studies, human chromosome band designations, symbols used in human cytogenetics.
Subjects: Genetics and Genomics.