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Which of the above mutations caused a change in the phenotype? Explain how the mutations for each gene changed the
DNA sequence and the resulting protein.
TYR:
OCA2:
SLCA5A:
How did these changes occur?
Which mutation(s) did not result in a change in the phenotype?
Whese mutations not result in a change in phenotype?

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**TYR Gene**

The mutation in the $$ \textbf{TYR} $$ gene altered the DNA coding sequence so that a different amino acid is incorporated into the tyrosinase enzyme. For example, if a point mutation changed a codon like  
$$
\texttt{TAC} \rightarrow \texttt{TGC},
$$
this nucleotide substitution leads to replacement of one amino acid with another at a critical position in the enzyme. Because tyrosinase is essential for melanin synthesis, even a single amino acid change can impair its catalytic activity, ultimately altering the phenotype (i.e. pigmentation).

**OCA2 Gene**

For the $$ \textbf{OCA2} $$ gene the mutation may have created a shift in the reading frame or disrupted an important splice site. For instance, if a deletion removed a nucleotide from the coding region, the reading frame would shift:
$$
\texttt{AGT ACC GGC} \rightarrow \texttt{AGT CCG GC\ldots}
$$
A frameshift like this changes every amino acid downstream, often introducing a premature stop codon that truncates the protein. As a result, the dysfunctional OCA2 protein cannot carry out its role in pigment production, which leads to a visible change in phenotype.

**SLC45A2 Gene**

The $$ \textbf{SLC45A2} $$ gene exhibited a mutation that did not result in a phenotypic change. This can occur if the mutation is "silent". In a silent mutation, a nucleotide alteration occurs without changing the encoded amino acid. For example, a codon such as 
$$
\texttt{GAA}
$$
(which codes for glutamic acid) might be mutated to 
$$
\texttt{GAG},
$$
but both codons code for the same amino acid. Because the protein’s primary structure remains unchanged, its function is preserved, and thus there is no change in phenotype.

**How These Changes Occurred**

Mutations occur as errors during DNA replication or as the result of external factors such as exposure to chemicals or radiation. In the cases discussed:  
- The $$ \textbf{TYR} $$ mutation likely arose from a point mutation that changed a single nucleotide, leading to an amino acid substitution in a key functional domain.  
- The $$ \textbf{OCA2} $$ mutation may have been caused by a deletion (or insertion) that shifted the reading frame, altering the entire downstream amino acid sequence and introducing a premature stop codon.  
- The $$ \textbf{SLC45A2} $$ mutation was a nucleotide substitution that, by chance, did not change the encoded amino acid (i.e. a silent mutation).

**Overall Summary**

- The mutations in the $$ \textbf{TYR} $$ and $$ \textbf{OCA2} $$ genes caused a change in the phenotype because they produced altered proteins (either through a missense change or a frameshift disruption) that affected the protein’s function in pigmentation.  
- In contrast, the $$ \textbf{SLC45A2} $$ mutation was silent, leaving the protein product functionally unchanged and, therefore, did not result in a phenotypic change.
**TYR Gene Mutation:** A change in the DNA sequence altered the tyrosinase enzyme, affecting melanin production, which changed the phenotype. **OCA2 Gene Mutation:** A mutation disrupted the OCA2 protein, impairing pigment production and changing the phenotype. **SLC45A2 Gene Mutation:** A silent mutation did not change the DNA sequence, leaving the protein unchanged and resulting in no phenotypic change. **Summary:** Mutations in TYR and OCA2 genes changed the phenotype by altering protein functions, while the SLC45A2 mutation did not affect the phenotype.

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