Why are the yeast chromosomes oriented and numbered the way they are?
The current orientation is nothing more than a historical accident. The first map with chromosome numbers was published by Hawthorne and Mortimer (Genetics (1960) 45: 1085-1110). There were earlier maps by Lindgren and others but they did not name the chromosomes. It is clear from the first map the chromosomes were oriented with the long arm to the right of the centromere and this orientation has not changed. Since that 1960 paper, chromosomes I through X have been in the same orientation. Four more chromosomes (chromosomes XI through XIV) were added in 1966 (Mortimer and Hawthorne, Genetics (1966) 53: 165-173). For these early maps many chromosomes only had one or two markers. Thus it was just chance which markers were mapped first and thus fixed the orientation. By 1973 there were seventeen chromosomes (Mortimer and Hawthorne, Genetics (1973) 74: 33-54). Between 1980 and 1985 it was realized that chromosome XVII was the killer plasmid and not a nuclear chromosome; luckily no renumber
