Final answer:
The underlined portion should be corrected by removing the word 'more' to avoid redundancy. The sentence should read 'the dogs in the Yukon were fiercer.'
Step-by-step explanation:
The underlined portion of the sentence 'The dogs at Judge Miller's house were calm and domesticated but the dogs in the Yukon were more fiercer.' should be rewritten to correct the grammatical error. The issue here is the use of the comparative 'more' with the superlative form 'fiercer,' which is redundant. Therefore, the correct sentence should be 'The dogs at Judge Miller's house were calm and domesticated, but the dogs in the Yukon were fiercer.'