Summary
I
Two loci, Star and asteroid, in the aeoond chromosome of D. melanogaster were found to be extremely closely linked with an estimated standard map distance ot 0.01 to 0.04 unit. It was demoanstrated that these two loci are included in the 21 E. 1-2 doublet structure of the salivary gland chromosomes. Not only do the Star and asteroid mutants affect the eye in a similar way, but they also show position effects depending on how they are distributed between the two chromosomes. An interpretation of the observed phenomena is made in terms of a naturally occurring repeat in the chromosomes.
II
The Star Duplication is a tandem repeat in direct order for the four bands, 21 D 3, 4 and 21 E 1-2. It includes the Star and asteroid loci and with the use of mutants at these loci as markers. it has been possible to study unequal crossing over in females heterozygous as well as homozygous for the duplication. The genetic composition of the Star Duplication has been varied in ten distinct ways; the results indicate that there is probably a position effect exerted on the asteroid locus 1n the left section of the duplication causing the originally present asteroid change to act like a reversion to normal. It is likely that there is little or no position effect exerted by tho Star and asteroid 1oci in one section on those loci in the other section.
