A genetic strain of any of several lab organisms (like fruit flies) which can be maintained as heterozygous individuals without requiring constant artificial selection for the heterozygotes, due to a number of lethal recessive genes with the result that the homozygous females are sterile (unable to produce offspring) and the males with the wrong recessive alleles are dead. (Male fruit flies are not homozygous or heterozygous because they only have one allele per particular genetic loci in question).
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.