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In Chapter 12 we defined mutations as stable, inherited changes in genes, and we saw that different alleles may produce different phenotypes (short pea plants versus tall, for example). In the two chapters that followed we described the chemical nature of genes as DNA sequences, and how they are expressed as phenotypes (in particular, proteins). To reiterate, we can now define a mutation as a change in the nucleotide sequence of DNA that can be passed on from one cell, or organism, to another.
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Mutations that occur at the nucleotide sequence level can be classified by the specific changes they cause in DNA and phenotypic effects in proteins.
Mutations that occur at the chromosomal level can be classified by the specific changes they cause in chromosomes.
Induced mutations are caused by mutagens, which are environmental factors such as chemicals and radiation that alter DNA structure.
Mutation hot spots occur where bases are more susceptible to mutation.