if the protein chain was altered by exactly one amino acid, then its gene had to be different by precisely one triplet (“one triplet encodes one amino acid”). Indeed, as predicted, when the gene encoding the hemoglobin B chain was later identified and sequenced in sickle-cell patients, there was a single change: one triplet in DNA—GAG—had changed to another—GTG. This resulted in the substitution of one amino acid for another: glutamate was switched to valine. That switch altered the folding of the hemoglobin
