For example human chromosomes 21 and 22 are almost indentical in length on the long arm and the very small short arm is of variable length. Lejeune described what is now known as Down's syndrome as trisomy 21 and when techniques showed this to be demonstrably shorter than 22 the the original numbers were retained. The problems with cows and sheep, with their numerous acrocentrics and centric-fusion differences, and man and great apes with human chromosome 2 being a fusion product of two acrocentrics, impose different numbers for homologues. The convention in mice was to number bands by capital letters from the combined telomere and centromere downwards. In human chromosome, most of which do not have a terminal centromere, the convention is to use a Dewey type classification numbered from the centromere outwards.
Centric fusions are without phenotypic effect.
The human nomenclature uses in lower case, for 'petit bras' and 'queue', 'little arm' and 'tail' in French. This replaced proposed l and s usage with its ambiguity over large or long and small or short at the time of French leadership in clinical cytogenics. Positions are named by a number system of 1,2, for ordered bands outward from the centromere, with 1.1, 1.2, 1.3 for sub-namds and occasionally 1.2.1 for sub.sub.bands. This hierarchy is entirley arbitrary and subjective following advances in technique. Simple numeric sorts invert the short arms.