Using this logic, Morgan concluded that whenever a mutation showed up overwhelmingly in males, it was probably carried on the X chromosome. This allowed him to calculate how often different mutants arose, how often they were associated with other mutations in the newly discovered phenomenon called linkage, and where exactly along the chromosome “string” each genetic “bead” was located. Within a few years of finding the white-eyed fly, Morgan and his associates had created a map for the X chromosome—a map that was the precursor for all subsequent genetic maps, including the gargantuan map now
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