They can range in effect depending on the particular codon change. A change from UAC to CAC incorporates histidine instead of tyrosine, a missense mutation. Mutations due to DNA replication errors are rare. DNA polymerase constantly monitors, proofreads new strand against old, and repairs any irregularities, reducing mistakes to one out of every one billion nucleotide pairs replicated. Mutagens are environmental substances that cause mutations.
Common mutagens are radiation and organic chemicals such as some pesticides and smoke.
If mutagens cause mutation in gametes, then offspring may be affected. If the mutation occurs in the body cells, then cancer may result. Where two thymine molecules are near each other, UV bonds them together as thymine dimers. Lack of repair enzymes produces xeroderma pigmentosum and higher incidence of skin cancer.