Make and defend a claim, based on evidence, that mutations and new genetic combinations are sources of inheritable variation (Louisiana Student Standards for Science, High School Biology, HS-LS3-2).

What this topic is asking

Louisiana's LS3 standards (HS-LS3-2) ask you to make and defend a claim that mutations and new genetic combinations are sources of inheritable variation. For LEAP 2025 Biology you should know what a mutation is, the types (substitution, insertion, deletion), the frameshift effect, how a mutation can change a protein, and why mutations matter for evolution. Because this is an argument standard, the test rewards explaining how a mutation produces variation and linking it to natural selection.

What a mutation is

Mutations happen during DNA replication (copying errors) or are caused by environmental factors such as radiation or certain chemicals. They are the original source of the differences between alleles, which links this topic to the whole of genetics and to evolution.

Types of point mutation

There are three common types of mutation at the level of a single base:

• Substitution. One base is replaced by another. This changes only the one codon it falls in, so it may swap a single amino acid (or none at all).
• Insertion. An extra base is added to the sequence.
• Deletion. A base is removed from the sequence.

The frameshift effect

This is a favorite test point: a one-base substitution affects at most one codon, but a one-base insertion or deletion can ruin everything after it.

Effects: harmful, beneficial, or neutral

A mutation is not automatically bad. Its effect can be:

• Harmful. The protein works less well or not at all (many genetic disorders arise this way).
• Beneficial. The change gives a useful new trait (such as resistance to a disease).
• Neutral. No noticeable effect on the organism (for example, a substitution that does not change the amino acid).

Whether a given mutation helps or harms can depend on the environment: a trait that is useless in one setting may be advantageous in another.

Why mutations matter for evolution

Mutations are the only source of genuinely new alleles. Meiosis (crossing over and independent assortment) shuffles existing alleles into new combinations, but it cannot create an allele that did not already exist. Mutation can. This new variation is the raw material that natural selection acts on, so without mutation, populations could not evolve new traits over time. Note that only mutations in gametes (eggs and sperm) are inherited; a mutation in a body cell affects only that individual.

Try this

Q1. Name the three types of point mutation and state which cause a frameshift. [2]

• Cue. Substitution, insertion, and deletion; insertion and deletion cause a frameshift.

Q2. Explain why mutations are important for evolution. [2]

• Cue. Mutations are the only source of new alleles, so they create the new genetic variation that natural selection acts on.