A single base pair change in DNA is least observable when it affects an unexpressed recessive trait, as the redundancy of the genetic code can render silent mutations phenotypically undetectable, especially when masked by a dominant allele.
A change within a single base pair in DNA is least likely to be observable if the change affects an unexpressed recessive trait.
A point mutation affecting a single base pair could potentially lead to several types of mutations such as missense, nonsense, or silent mutations. A silent mutation changes a codon to another codon that encodes for the same amino acid, and thus often has no observable effect on the phenotype. This is due to the redundancy of the genetic code, where several codons can code for the same amino acid. When the mutation is in a portion of the DNA that codes for an unexpressed recessive trait, it is even less likely to have an observable effect because the dominant allele masks the expression of the recessive trait, making the change essentially undetectable unless DNA sequencing is conducted.
Therefore, the least likely scenario for a single base pair change to be observable is one that affects an unexpressed recessive trait due to the masking by a dominant allele and the redundancy of the genetic code in case of silent mutations.