If a gene encoding a protein whose third amino acid is tyrosine is affected by a single nucleotide substitution in that tyrosine codon, how many of the other 19 amino acids could possibly end up replacing tyrosine as the third amino acid?

6 different amino acids (no including Stop codons)

Explanation:

First of all, it is necessary to know that tyrosine is an amino acid coded by two different codons: UAU and UAC.

If there's a single nucleotide substitution, we need to redefine the new possible codons:

For UAU:

- Substitution in the first nucleotide:

* CAU: His

* GAU: Asp

* AAU: Asn

- Substitution in the second nucleotide:

* UCU: Ser

* UGU: Cys

* UUU: Phe

- Substitution in the third nucleotide:

* UAG: Stop

* UAC: Tyr

* UAA: Stop

For UAC:

- Substitution in the first nucleotide:

* CAC: His

* GAC: Asp

* AAC: Asn

- Substitution in the second nucleotide:

* UUC: Phe

* UGC: Cys

* UCC: Ser

- Substitution in the third nucleotide:

* UAG: Stop

* UAA: Stop

* UAC: Tyr

Possible amino acids replacing tyrosine (His, Asp, Asn, Phe, Cys, Ser)