Hermann Muller Invented the Balancer Chromosome

Since writing about Balancer Chromosomes, I've gotten several email messages pointing out things I missed. Thanks to everyone who responded. It's what makes this blog worthwhile.

Quite a few readers pointed out that balancer chromosomes were invented a very long time ago by Hermann Muller. Muller won the Nobel Prize in 1946 for discovering mutagenesis by X-rays.

Dale Hoyt, a fly geneticist, sent me a description of Muller's experiment and he has given me permission to post it.

The first Nobel laureate who used balancers in his work was Hermann J. Muller. He used a strain of D. melanogaster that was heterozygous for an X-chromosome inversion. This suppresses crossing over between the normal X and the X carrying the inversion during meiosis. A single crossover within the inverted segment will generate a "bridge" at meiosis I, causing the non-crossover chromatid to preferentially segregate to the future ovum. In Muller's work the inverted X was marked with the dominant eye shape mutation, Bar, and carried a recessive lethal allele.¹ A female heterozygous for the marked inverted chromosome and a "wild type" chromosome will produce only 1/2 the normal number of male progeny and they will all be wild type. This is because 1/2 the males die because they receive the Bar chromosome and are hemizygous for the lethal. The inversion heterozygosity prevents recombination between the Bar locus and the lethal locus. Muller used this stock, called "ClB", to show that X-irradiation increased the frequency of mutation to lethal genes on the X-chromosome. Irradiated male flies were individually mated to the ClB females. Their Bar-eyed female offspring (heterozygous for the inversion and the irradiated X-chromosome) were mated to their brothers. If no males were produced from this cross then the irradiated male transmitted an X chromosome with a lethal mutation. It was easy to score the crosses—just look at the bottle and if there were no males then Muller knew that he had a radiation induced lethal.

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1. l(1)C, associated with the left breakpoint of the inversions. Presumably the break disrupts a gene required for viability. The gene must be known by now.

[Photo Credit: WIRED]