Final answer:
Antisense oligonucleotides (ASOs) can specifically target the abnormal Huntington allele due to their design that matches its unique expanded sequence of CAG repeats, enabling selective binding to the mutant RNA and preventing translation into the harmful protein.
Step-by-step explanation:
The reason why an antisense oligonucleotide (ASO) can target only the abnormal Huntington allele is due to the unique sequence of DNA present in this allele, which includes an expanded sequence of CAG repeats causing an abnormal protein. ASOs are designed to bind to the RNA transcribed from the mutant allele, blocking it from being translated into the faulty protein that leads to Huntington's disease symptoms. This specificity comes from the precise molecular design of ASOs to match the nucleotide sequence of the aberrant RNA produced by the mutant allele, without interfering with the normal allele.