In the standard genetic code (above), tryptophan and methionine are coded for the single codon, other amino acids such as phenylalanine by two codons, isoleucine by three codons, and some by four or even six codons.

Is there any explanation for this difference in number?

One possibility that occurred to me is whether, amino acids which are more crucial to a protein’s structure tend to have more synonymous codons?
For example, Tryptophan is coded by a single codon, TGG, whereas Proline is coded for by the four codons, CCA, CCG, CCC, and CCT. A single mutation would always change tryptophan, but would have a 1 in 3 chance of leaving proline the same. Is this because a change in proline would be more likely to have a deleterious affect on a protein than a change in tryptophan, and for this reason there has been evolutionary selection to provide proline with more codons?

N.B. This is a different question from the previous general one on the evolution of the genetic code.

In the standard genetic code (above), tryptophan and methionine are coded for by a single codon, other amino acids such as phenylalanine by two codons, isoleucine by three codons, and some by four or even six codons.

Is there any explanation for this difference in number?

One possibility that occurred to me is whether, amino acids which are more crucial to a protein’s structure tend to have more synonymous codons?
For example, Tryptophan is coded by a single codon, TGG, whereas Proline is coded for by the four codons, CCA, CCG, CCC, and CCT. A single mutation would always change tryptophan, but would have a 1 in 3 chance of leaving proline the same. Is this because a change in proline would be more likely to have a deleterious affect on a protein than a change in tryptophan, and for this reason there has been evolutionary selection to provide proline with more codons?

N.B. This is a different question from the previous general one on the evolution of the genetic code.

Tryptophan is coded for by TGG. Proline is coded for by CCA, CCG, CCC, and CCT. A single mutation would always change tryptophan, but would have a 1 in 3 chance of leaving proline the same. That would make sense if, somehow, changing tryptophan was not as bad as changing proline.

Do amino acids which are more crucial to a protein's structure tend to have more redundant codons?

This question addresses the "why did codons evolve the way they did" question, but none of the answers directly address this question.

In the standard genetic code (above), tryptophan and methionine are coded for the single codon, other amino acids such as phenylalanine by two codons, isoleucine by three codons, and some by four or even six codons.

Is there any explanation for this difference in number?

One possibility that occurred to me is whether, amino acids which are more crucial to a protein’s structure tend to have more synonymous codons?
For example, Tryptophan is coded by a single codon, TGG, whereas Proline is coded for by the four codons, CCA, CCG, CCC, and CCT. A single mutation would always change tryptophan, but would have a 1 in 3 chance of leaving proline the same. Is this because a change in proline would be more likely to have a deleterious affect on a protein than a change in tryptophan, and for this reason there has been evolutionary selection to provide proline with more codons?

N.B. This is a different question from the previous general one on the evolution of the genetic code.

Tryptophan is coded for by TGG. Proline is coded for by CCA, CCG, CCC, and CCT. A single mutation would always change Tryptophan, but would have a 1 in 3 chance of leaving Proline the same. That would make sense if, somehow, changing Tryptophan was not as bad as changing Proline.

Do amino acids which are more crucial to a protein's structure tend to have more redundant codons?

This question addresses the "why did codons evolve the way they did" question, but none of the answers directly address this question.

Tryptophan is coded for by TGG. Proline is coded for by CCA, CCG, CCC, and CCT. A single mutation would always change tryptophan, but would have a 1 in 3 chance of leaving proline the same. That would make sense if, somehow, changing tryptophan was not as bad as changing proline.

Do amino acids which are more crucial to a protein's structure tend to have more redundant codons?

This question addresses the "why did codons evolve the way they did" question, but none of the answers directly address this question.