THE COMPOUND X CHROMOSOMES IN DROSOPHILA

Abstract

HROMOSOMES formed by the union of homologous elements provide C a situation in which somewhat more can be learned about the nature of the meiotic phenomena in Drosophila than may be deduced from a study of normal chromosomes. The classic example is, of course, the attached X chro-mosome; there are, however, five other types of compound X chromosomes theoretically realizable. All five are now known and have been studied to some extent. It is the purpose of this paper to describe the origin and synaptic properties of the newer compounds and to indicate the kinds of information that have come from a study of them. TERMINOLOGY There are four ways in which two X chromosomes may be joined to give a compound with two ends. The centromere of the compound may be either median or terminal and, in each case, the two component chromosomes may be in reversed order with respect to each other or they may be arranged in tandem. A simple self-descriptive terminology is achieved if the compounds with median centromeres are referred to as metacentrics and those with termi-nal or subterminal centromeres as acrocentrics, and if these terms are preceded by tandem or reversed, depending on the relative order oi the two components. The four possibilities are, then, the tandem metacentric, the tandem acro-centric, the reversed metacentric and the reversed acrocentric. It should be noted that the attached X chromosome is the reversed metacentric compound ; the latter name is introduced here not to supplant the well established attached X designation, but to indicate the structural relation of the attached X to the other types. Finally, there are two possible compounds in which the two X chromosomes are joined to form a continuous ring. Here also the two components may be in reversed or tandem order. The first is the reversed ring, the second, the tandem ring.