The wobble hypothesis predicts that codons coding for the same amino acid may differ at the third position.
Francis Crick defined the wobble hypothesis, which states that, in mRNA codons, the third base can pair "incorrectly" with the tRNA's anticodon's first base. What this means is that there is variability in the pairing of the anticodons and codons when it comes to the third base of the mRNA codon.
This hypothesis explains why the only variability that exists between different codons, that code for the same amino acid, is in the third base. The first two bases of the mRNA codon normally form hydrogen bonds with the bases on the tRNA codon in the process of translation (the process of forming proteins out of mRNA strands), so they pair in a usual Watson-Crick way, meaning that they form base pairs only with complementary bases.
The hydrogen bonds between the codon's third base, and the anticodon's first base can occur in a non-complementary way, so they form base pairs different to those that are usual at this position. This wobble hypothesis can explain why there are multiple codons that code for the same amino acid, since one tRNA molecule can attach to multiple codons due to less precise base pairing between the bases that don't follow the Watson-Crick complementary base pairing rule.
This explains why there are more codons than tRNA molecules for specific amino acids.
