What is a mutation, and why can the same kind of change be harmful, helpful, or have no effect?
Recognize the types of gene mutations and explain how a change in the DNA base sequence may be harmful, beneficial, or neutral and how it can be inherited (TEKS Biology, Reporting Category 2; cause and effect; stability and change).
A TEKS-level answer on mutations for the Texas STAAR Biology EOC: what a mutation is, substitution, insertion, and deletion, why an effect can be harmful, beneficial, or neutral, and how mutations in gametes are inherited and supply variation.
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What this topic is asking
The Biology TEKS ask you to recognize types of gene mutations and explain how a change in the DNA base sequence can affect a trait. For STAAR Reporting Category 2 you need a clear definition, the idea that an effect can be harmful, beneficial, or neutral, and the difference between mutations in body cells and in gametes. This is a cause-and-effect and stability and change topic, and it connects mutations to the source of variation that drives evolution.
What a mutation is
Because the order of bases determines the protein a gene makes (see protein synthesis), changing the bases can change the protein and therefore the trait.
Types of mutation
The simplest gene mutations change one or a few bases:
- Substitution. One base is replaced by another (for example, A becomes G). This may change one codon and so one amino acid, or it may have no effect if the codon still codes for the same amino acid.
- Insertion. An extra base is added to the sequence.
- Deletion. A base is removed from the sequence.
Insertions and deletions can be more disruptive than substitutions, because adding or removing a base shifts how all the following codons are read.
Harmful, beneficial, or neutral
This is why "type of mutation" and "effect of mutation" are different questions. A substitution is a type; harmful, beneficial, or neutral is an effect. STAAR often pairs a described change with a request for its likely effect.
Inherited or not
Whether a mutation is passed on depends on which cell it occurs in:
- A mutation in a gamete (egg or sperm) can be inherited, because the offspring develops from that gamete and carries the change in every cell.
- A mutation in a body (somatic) cell affects only that individual and is not passed to offspring.
Mutations are the original source of new alleles. They create the genetic variation that, together with sexual reproduction, gives a population the differences that natural selection acts on. This makes mutation the ultimate raw material of evolution, linking Reporting Category 2 to natural selection and adaptation.
Try this
Q1. Define a mutation and state its three possible effects. [2]
- Cue. A change in the DNA base sequence; effects can be harmful, beneficial, or neutral.
Q2. Explain why a mutation in a sperm cell can be passed on but a mutation in a skin cell cannot. [2]
- Cue. Offspring develop from gametes, so a change in a sperm cell is inherited; a skin (body) cell change affects only that individual.
Exam-style practice questions
Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
STAAR Biology (2023 released style)1 marksA mutation changes a single base in a gene, but the protein produced is unchanged. Which term best describes the effect of this mutation? (A) Harmful. (B) Beneficial. (C) Neutral. (D) Lethal.Show worked answer →
A 1-point multiple-choice item on mutation effects.
The correct answer is C. If the protein is unchanged, the mutation has no noticeable effect, which is a neutral mutation. This can happen when the changed codon still specifies the same amino acid. A and D imply a negative effect, and B implies an improvement, neither of which fits "protein unchanged."
An effect can be harmful, beneficial, or neutral; a neutral mutation changes nothing important.
STAAR Biology (2024 SCR style)2 marksA mutation occurs in a skin cell and a different mutation occurs in a sperm cell of the same animal. Explain which mutation can be passed to offspring and why. Support your answer with reasoning.Show worked answer →
A 2-point short constructed response on inherited versus non-inherited mutations.
Full credit (2 points): only the mutation in the sperm cell (a gamete) can be passed to offspring, because offspring develop from gametes, so a change in a gamete's DNA is inherited by the next generation. The mutation in the skin cell is a body (somatic) cell change that affects only that individual and is not passed on.
Partial credit (1 point): identifies the gamete mutation as heritable without explaining why. The science is scored.
Related dot points
- Identify the components of DNA, describe the structure of the double helix and base pairing, and explain how DNA is replicated accurately before cell division (TEKS Biology, Reporting Category 2; structure and function; patterns).
A TEKS-level answer on DNA for the Texas STAAR Biology EOC: the components of a nucleotide, the double helix and complementary base pairing, and how DNA replication produces two identical copies before a cell divides.
- Describe how the information in DNA is used to build proteins through transcription and translation, and explain how the order of bases determines the order of amino acids (TEKS Biology, Reporting Category 2; cause and effect; structure and function).
A TEKS-level answer on protein synthesis for the Texas STAAR Biology EOC: transcription of DNA into mRNA, translation of codons into amino acids at the ribosome, and how the base sequence determines the protein and the trait.
- Apply Mendel's laws and use Punnett squares to predict the genotype and phenotype ratios of monohybrid crosses, and identify patterns of inheritance including dominant, recessive, codominant, and incomplete dominance (TEKS Biology, Reporting Category 2; patterns; using mathematics).
A TEKS-level answer on inheritance for the Texas STAAR Biology EOC: alleles, genotype and phenotype, dominant and recessive traits, using Punnett squares to predict ratios and probabilities, and codominance and incomplete dominance.
- Explain how natural selection acts on heritable variation to produce adaptation in populations over time, and identify the conditions required for it to occur (TEKS Biology, Reporting Category 3; cause and effect; stability and change).
A TEKS-level answer on natural selection for the Texas STAAR Biology EOC: variation, overproduction, the struggle to survive, differential survival and reproduction, and how this leads to adaptation and change in populations over time.
- Recognize the factors that influence the genetic makeup of populations and lead to speciation, including mutation, gene flow, genetic drift, and reproductive isolation (TEKS Biology, Reporting Category 3; cause and effect; patterns).
A TEKS-level answer on the mechanisms of genetic change for the Texas STAAR Biology EOC: mutation, gene flow, and genetic drift as sources of change in a population, and how reproductive isolation leads to speciation.
Sources & how we know this
- Texas Essential Knowledge and Skills for Science (Biology) — Texas Education Agency (2024)
- STAAR Biology Assessed Curriculum — Texas Education Agency (2024)