Any lethal recessive gene is likely to be lost from a population, because it confers a disadvantage on the progeny of heterozygotes. Imagine that we discover a lethal allele of our gene, vha55-. How can we keep it alive, perhaps for years? Here's what happens when we try to breed two heterozygotes:
The population does not breed true, because in the next generation the wild-type (+/+) appears. In subsequent generations, this will come to dominate the population, because 100% of the progeny of a wild-type fly survives, whereas only 75% of the progeny of a vha55-/+ fly survives. So our only option for keeping this line alive is to select for mutant heterozygotes (assuming we can recognise them!) and perform virgin crosses in each generation. This is so labour intensive as to be impracticable for a large lab with a lot of mutant lines.
But what if we immediately cross our mutant fly to another recessive lethal on the same chromosome? We're not interested in the second lethal, it's just a "balancer" for reasons that will become clear below:
Now, the line DOES breed true, because the only class of progeny which survives is identical to the parents. So once we've gone to the initial effort of crossing-in our balancer chromosome (in this case, TM3), there is no need to select particular flies in subsequent generations, and the mutation can be kept for many years with minimal effort.
Crossing-over and recombinationIn fact, there are a couple of other wrinkles associated with a balancer chromosome. The first is that the order of genes in the balancer chromosome has been jumbled up by a series of previous mutagenic events. The effect of this is to prevent recombination between the two chromosomes: otherwise, the two lethal alleles would eventually recombine onto the same chromosome, and a wild-type chromosome would then occur, leading to the problems discussed earlier.
MarkersThe second desirable property of the balancer is that it should carry a clearly visible dominant marker, that allows us to follow the fate of the balancer chromosome in any cross we should subsequently choose to do. In the case of TM3, this is Sb (Stubble), which conspicously makes the hairs on the back of the fly short and stubbly. Beleive it or not, this is a relatively easy marker to score!
This may sound complex, but you're about to see it in action, when it should become more obvious.
The population does not breed true, because in the next generation the wild-type (+/+) appears. In subsequent generations, this will come to dominate the population, because 100% of the progeny of a wild-type fly survives, whereas only 75% of the progeny of a vha55-/+ fly survives. So our only option for keeping this line alive is to select for mutant heterozygotes (assuming we can recognise them!) and perform virgin crosses in each generation. This is so labour intensive as to be impracticable for a large lab with a lot of mutant lines.
But what if we immediately cross our mutant fly to another recessive lethal on the same chromosome? We're not interested in the second lethal, it's just a "balancer" for reasons that will become clear below:
Now, the line DOES breed true, because the only class of progeny which survives is identical to the parents. So once we've gone to the initial effort of crossing-in our balancer chromosome (in this case, TM3), there is no need to select particular flies in subsequent generations, and the mutation can be kept for many years with minimal effort.
Crossing-over and recombinationIn fact, there are a couple of other wrinkles associated with a balancer chromosome. The first is that the order of genes in the balancer chromosome has been jumbled up by a series of previous mutagenic events. The effect of this is to prevent recombination between the two chromosomes: otherwise, the two lethal alleles would eventually recombine onto the same chromosome, and a wild-type chromosome would then occur, leading to the problems discussed earlier.
MarkersThe second desirable property of the balancer is that it should carry a clearly visible dominant marker, that allows us to follow the fate of the balancer chromosome in any cross we should subsequently choose to do. In the case of TM3, this is Sb (Stubble), which conspicously makes the hairs on the back of the fly short and stubbly. Beleive it or not, this is a relatively easy marker to score!
This may sound complex, but you're about to see it in action, when it should become more obvious.
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