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One of the main strategies to map the genes regulating specific traits is called quantitative trait loci ( QTL ) analysis , which generally requires a very large number of animals ( often more than 1000 ) to reach statistical significance .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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