There are several parts of the human genome that contribute to physical appearance. One of the most well-known is the MC1R gene, which is involved in determining hair color and skin pigmentation. Another example is the HERC2 gene, which influences eye color. While these genes are just a small part of the complex system that determines physical appearance, advances in genome editing technologies like CRISPR have made it possible to modify these genes to some extent.

Here's an example of how you might use Python code to work with the MC1R gene:

```python
# Sample Python code for working with the MC1R gene
import crispr

# Define the MC1R gene
mc1r = crispr.Gene("MC1R")

# Get the current sequence for the gene
mc1r_sequence = mc1r.get_sequence()

# Use CRISPR to modify the gene sequence
modified_sequence = crispr.crispr_edit(mc1r_sequence, "TAC", "ATC")

# Update the gene with the modified sequence
mc1r.set_sequence(modified_sequence)

# Print the updated sequence
print(mc1r.get_sequence())
```

In this example, we first define the MC1R gene using the crispr.Gene() function. We then retrieve the current sequence of the gene using the get_sequence() method. Next, we use the crispr_edit() function from the crispr module to modify the gene sequence by changing the codon "TAC" to "ATC". Finally, we update the gene with the modified sequence using the set_sequence() method and print the updated sequence using the get_sequence() method.

While this is just a simple example, it demonstrates how CRISPR can be used to modify genes like MC1R that are involved in physical appearance. However, it's important to note that genome editing technologies are still in the early stages of development and there are ethical considerations to be taken into account before making any modifications to the human genome.
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