 The 3 is to distinguish this from other third chromosome balancers , such as " TM1 " and " TM2 " . A line is said to be "double-balanced" if it is heterozygous for two different balancer chromosomes (for example, TM6,Tb/TM3,Ser) on a balancer chromosome and a homozygous lethal, heterozygous visible mutant on the other, wild-type chromosome (for example, D/TM3,Ser).
<fluency>  The 3 is to distinguish this from other third chromosome balancers , such as " TM1 " and " TM2 " . A line is said to be "double-balanced" if it is heterozygous for two different balancer chromosomes (for example, TM6,Tb/TM3,Ser) on a balancer chromosome and a homozygous lethal, heterozygous visible mutant on the other, wild-type chromosome (for example, D/TM3,Ser).
 The 3 is to distinguish this from other third chromosome balancers , such as " TM1 " and " TM2 . A line is said to be "double-balanced" if it is heterozygous for two different balancer chromosomes (for example, TM6,Tb/TM3,Ser) on a balancer chromosome and a homozygous lethal, heterozygous visible mutant on the other, wild-type chromosome (for example, D/TM3,Ser).