Construct an argument that mutations (changes in DNA sequence and chromosomal alterations) may result in phenotypic variation, and classify gene mutations as beneficial, harmful, or neutral (GSE SB2.b).

What this topic is asking

Standard SB2.b asks you to argue, with evidence, that mutations may result in phenotypic variation. For the Georgia Milestones Biology EOC you must know the types of mutation (substitution, insertion, deletion, and chromosomal changes), explain the chain from a DNA change to a changed protein to a changed trait, and recognize that a mutation can be beneficial, harmful, or neutral. The link to variation sets up natural selection and evolution.

Types of mutation

The gene (point) mutations the EOC tests are:

• Substitution. One base is replaced by another. It changes at most one codon, so its effect ranges from none (a silent mutation, if the codon still codes for the same amino acid) to a single changed amino acid.
• Insertion. A base is added. This shifts the reading frame, changing every codon after the insertion (a frameshift).
• Deletion. A base is removed. This also shifts the reading frame (a frameshift), changing every codon after the deletion.

Chromosomal mutations are larger: a whole segment may be deleted, duplicated, inverted, or moved, or the chromosome number may change (for example, an extra chromosome). These tend to have bigger effects because they involve many genes.

Why insertions and deletions are usually worse

From DNA change to phenotype

The argument SB2.b wants is a clear cause-and-effect chain:

1. A gene's DNA sequence specifies the order of amino acids in a protein.
2. A mutation changes the DNA sequence.
3. The changed sequence can change the amino acid sequence.
4. A changed amino acid sequence can change the protein's shape and function.
5. A changed protein can change the trait (phenotype) it controls.

Not every mutation changes the phenotype (silent mutations, or mutations in non-coding DNA, may have no effect), but this chain explains how those that do exert their effect.

Beneficial, harmful, or neutral

A mutation is not automatically "bad." Its effect depends on the environment:

• Beneficial. The new protein gives an advantage (for example, a mutation that lets bacteria survive an antibiotic, or that improves camouflage). Beneficial mutations are the raw material for adaptation.
• Harmful. The change disrupts an important protein (for example, a mutation causing a genetic disorder).
• Neutral. The change has no real effect on survival or reproduction (for example, many silent mutations).

Crucially, mutations are random with respect to need: the environment does not create the mutation an organism needs; it merely selects among the variation that mutations (and recombination) already produce. Only mutations in gametes (sex cells) are passed to offspring; mutations in body cells are not inherited.

Try this

Q1. State which two types of point mutation cause a frameshift. [2 points]

• Cue. Insertions and deletions (they shift the reading frame); substitutions do not.

Q2. Explain why a mutation can be beneficial in one environment but not another. [2 points]

• Cue. A mutation's value depends on the environment: a change that helps survival or reproduction in one setting (for example, antibiotic resistance where the antibiotic is present) may be neutral or harmful where that pressure is absent.