Chromosome Organization and Gene Function in Drosophila 
vation at sites of transposons carrying the brown 
gene, but only for paired copies of the gene. In 
addition, we have found that even very small le- 
sions that disrupt pairing in the immediate vicin- 
ity of the gene also reduce trans-inactivation. Our 
current efforts are aimed at identification of the 
cis- and trans-acting components of trans- 
inactivation. 
Position effect in general and trans-inactivation 
in particular are phenomena that are easily ob- 
served in Drosophila, where powerful tools are 
available for genetic dissection. Related phenom- 
ena are known to occur in mammals, such as X 
chromosome inactivation, in which one of the 
female's X chromosomes becomes heterochro- 
matic. The many similarities between chro- 
mosomes in organisms as diverse as flies and 
mammals lead to the expectation that an under- 
standing of position effects in Drosophila will 
have general implications. 
The classical phenomenon of dominant position-effect variegation reproduced at a site of inser- 
tion of the Drosophila brown gene present on a transposon. The brown gene is necessary for 
transport of a precursor needed for red eye pigment production. The fly with a uniform red eye has 
a single copy of the transposed gene on one chromosome, while the other fly has both a normal 
copy and a position-affected copy. The mutant phenotype results from cis-inactivation of the 
position- affected copy and trans-inactivation of the paired copy. 
Research of Steven Henikoff. 
194 
