Final answer:
It is false that there is no reason to perform a BLASTX search if a DNA sequence shows less than 20% identity in BLASTN searches; BLASTX can detect conserved protein domains not apparent in nucleotide sequences.
Step-by-step explanation:
False. A DNA sequence showing less than 20% identity in BLASTN searches may still have significant similarities at the protein level that could be detected using a BLASTX search. BLASTN is designed for nucleotide-to-nucleotide comparisons, while BLASTX compares the nucleotide sequence against a protein database by first translating the nucleotide sequence into a protein sequence in all six possible reading frames. This process can reveal conserved protein domains that are not apparent in the nucleotide sequence due to codon redundancy or differences in genetic codes.