The underappreciated, underrecognized problem of fourth chromosome trisomy in stocks and a simple, general method for building diplo-4 stocks from triplo-4 stocks.

Trisomy of the small, fourth chromosome is common in stocks. Even though the presence of an extra chromosome can confound the interpretation of experimental crosses, many Drosophila geneticists are unaware of this potential problem. Here I describe a simple method employing a mutation in a haploinsufficient gene to recognize fourth chromosome trisomy and establish disomic stocks from trisomic stocks.