In the source text about rare blue lobsters ("Only 1 in about 2 million lobsters is blue"), how can a change to just one gene cause a lobster to be blue?

What the question is really asking

You are connecting genotype (a gene) to phenotype (a visible trait, shell color). The source says blue lobsters are rare and caused by a genetic mutation, so we need to explain how one mutation can affect color.

From gene to protein to trait

A gene is a set of DNA instructions for making a protein.

• DNA is transcribed into mRNA.
• mRNA is translated into a protein.
• Proteins do jobs in the body, including making pigments, moving pigments, or building shell structures that affect how pigments look.

So, if a mutation changes the DNA sequence of one gene, it can change the protein that gene makes.

Why one changed protein can change shell color

Shell color often depends on pigments (color chemicals) and on how those pigments are packaged in the shell.

A mutation in a single gene might:

• change an enzyme that makes or breaks down a pigment,
• change a protein that binds to a pigment (changing the pigment’s apparent color),
• change a transport protein that moves pigment to the shell,
• change a structural protein in the shell that alters how light reflects.

Any of these can shift the final color you see. Even one amino-acid change in a protein (caused by a small DNA change) can alter how that protein works.

Connecting back to “rare genetic mutation”

Because the blue color is caused by a rare mutation, it shows that most lobsters have the usual version (allele) of the gene, which produces the typical color. A lobster that inherits the rare mutated allele can produce a different pigment setup, leading to a blue shell.

Quick summary

One gene can have a big effect if it controls a key protein in pigment production or pigment binding. Change the gene, change the protein, and the lobster’s shell color can change to blue.