 First, a balancer carries a recessive lethal mutation, which makes the organism non-viable if two copies of balancer are inherited ( ie no copy of the desired mutation).
<meaning-changed>  First, a balancer carries a recessive lethal mutation, which makes the organism non-viable if two copies of balancer are inherited ( ie no copy of the desired mutation).
 First, a balancer carries its own independent recessive lethal mutation, which makes the organism non-viable if two copies of balancer are inherited ( ie no copy of the desired mutation).