Northern blotting is not useful in detecting the SNPs associated with the A and S alleles. Explain ...

The DNA sequences of the A and S alleles are identical except for the SNP that distinguishes the sequences of one from the other. Upon transcription, each allele produces an mRNA molecule containing 664 nucleotides. The single-nucleotide substitution that differentiates the two alleles does not alter the length of the mRNA transcript. Considering that the molecular attribute producing electrophoretic mobility differences among mRNAs is the total length of the molecule, it is not surprising that in this instance there is no difference in the electrophoretic mobilities of the mRNA transcripts of these two alleles, because the lengths of their mRNAs are identical. A northern blot analysis performed on mRNA from individuals each having one of the three -globin genotypes detects the same single mRNA band for each genotype. Consequently, northern analysis is not useful in detecting variation in this case.