Chromosome inversion polymorphism in Drosophila pseudoobscura

    D. pseudoobscura is a wild relative of D. melanogaster. The Standard (ST) arrangement of the third chromosome is characterized by a particular polytene banding pattern. A paracentric inversion involving about a third of the chromosome produces the Arrowhead (AR) arrangement. Chromosome pairing in meiosis requires formation of a characteristic inversion loop in order to bring homologus gene segments (region C-G) together. Individuals with two different third chromosomes are called heterokaryons (by analogy with "heterozygotes"; individuals with identical chromosomes are homokaryons). Other chromosome polymorphisms (such as Chiricahua [CH]) involve multiple inversions, and inversions-within-inversions, resulting in complicated loops (below). [chromosome figures from Minkoff 1983 after Dobzhansky & Sturtevant (1938)]

Text material © 2005 by Steven M. Carr