Sentence:  <WSD>Tinman</WSD> belongs to a special class of genes. [...] But some genes make proteins that control other genes. Some act like master switches, triggering many different genes to work together to build a structure. <WSD>Tinman</WSD> is one such “master gene” for the fruit fly. 
Meaning of the word marked with <WSD> tags:  A master gene in the fruit fly "Drosophila melanogaster", whose absence causes the fly to develop without a heart.