Geneticists approach biological problems from a somewhat odd angle. Instead of physically dissecting a biological specimen as an anatomist does to see how the organism is constructed, or separating a cell into molecular components as a biochemist would be prone to do, a geneticist dissects a biological process by asking, What can I do to disrupt the system? They address this question by first generating mutations in genes that control the process of interest. They then deduce the function of a particular gene by examining the nature of defects resulting from the absence of that gene activity. This strategy is similar to that used by electricians or mechanics when trying to diagnose a problem with a malfunctioning piece of electronics or a broken-down car.
The goal of the massive mutant screen carried out by the Nüsslein-Volhard, Wieschaus, and Jürgens "gene team" was to generate a large collection of random mutants (about 30,000 in all). Each of these mutants was analyzed to determine whether mutant embryos exhibited morphological defects along the A/P or D/V axes. The screen identified about 750 mutants that had interpretable patterning defects. The investigators then asked how many different genes had been disrupted in this collection of 750 mutants and found that about 150 genes had been mutated. They reached this conclusion by crossing mutants with similar defects to each other and asking whether the progeny had defects similar to their parents. As discussed above, such crosses can resolve whether two mutants with similar appearances have disruptions within the same gene or within two different genes involved in the same biological process. The ratio of 5 mutants per patterning gene obtained in the screen meant that each of the 150 patterning genes had been mutated 5 times on average (i.e., 750 patterning mutants divided by 150 patterning genes = 5 mutants per gene).
One can think of a gene as a target in a mutant hunt. The goal of a comprehensive mutant hunt is to hit every target at least once. If you fire enough rounds at the fly's DNA to hit an average target gene five times, you will have missed very few genes entirely. Because only 150 genes were found that could be mutated to give defects in embryonic pattern formation, their collection of 750 patterning mutants contained an average of five different lesions in each patterning gene. If the gene team had doubled their heroic efforts by screening 60,000 initial mutants instead a mere 30,000, they would have recovered twice as many total patterning mutants (i.e., 1,500 versus 750). However, these additional mutants would have included few additional disrupted genes beyond the 150 genes identified in the first group of 750 mutants. Thus, in this larger screen they would have ended up with an average of 10 distinct lesions per gene instead of 5. Because extending a mutant hunt beyond the point of recovering 5 independent mutations in an average gene predominantly generates more hits in the same small set of genes, the screen is said to be saturating. The degree of saturation can be quantified statistically. When you have examined a sufficient number of mutants to have identified an average of 5 mutations in each gene within your collection, you will have found more than 95% of all genes that could be identified using screening methods. In other words, the odds are better than 20 to 1 that any given gene important for pattern formation will have been hit at least once in such a large collection of mutants.
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