Another major contribution to the Drosophila genetic toolkit was the discovery of ring chromosomes. Ring chromosomes were discovered from their unusual frequencies of recombination in an attached-X stock, which revealed a circularized X-chromosome upon cytological examination. Ring-X chromosomes are unstable in early development, a phenomenon that has been applied to generate mosaic tissues containing XX and XO cells during mitosis that bear recessive loss-of-function alleles of specific X-linked genes.
