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

Trisomy of the small, fourth chromosome is common in <i>Drosophila melanogaster</i> 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.