Mutations are changes in an organism's DNA that may or may not affect phenotype. Most mutations occur during DNA replication and are repaired by the organism's enzymes, having no effect. If not repaired, the altered DNA is passed to daughter cells. Mutations in somatic cells only affect the individual, while mutations in gametes can be transmitted to offspring and between generations, potentially causing genetic disorders like Tay-Sachs disease or Down syndrome from nondisjunction. Mutations can arise from replication errors, mutagens like UV radiation, or cancer treatments and may be removed by natural selection or provide benefits in changing environments.

1. 8.7 Mutations
KEY CONCEPT
Mutations are changes in DNA that may or may not
affect phenotype.

2. 8.7 Mutations
• A mutation is a change in an organism’s DNA.
• Many kinds of mutations can occur, especially during
replication.
• Most mutations are repaired by organism’s enzymes
and have no effect.
• If not repaired, the altered gene/chromosome will be
passed to daughter cells.

3. 8.7 Mutations
Mutations in body (somatic) cells verses gametes.
• In a somatic cell, daughter cells are affected by the DNA
but not the offspring of the organism.
– Body cell mutations contribute to aging or the
development of cancer.
• If the mutant is a gamete (sex cell), the altered DNA will
be transmitted to the embryo and may be passed on from
generation to generation. These cause genetic
disorders.
• If the mutation affects a gene, then it’s a gene mutation.
• If the mutation affects a chromosome or multiple genes
it’s a chromosomal mutation.

4. 8.7 Mutations
Gene Mutation Examples
• A point mutation substitutes one nucleotide for another.
• Example disorders: Tay-Sachs disease, Sickle Cell
mutated
base

5. 8.7 Mutations
– A frameshift mutation inserts (insertion) or deletes
(deletion) a nucleotide in the DNA sequence.
Frameshift Mutation

6. 8.7 Mutations
– Gene duplication results from unequal crossing over.
• Chromosomal mutations may occur during crossing over
Chromosomal Mutations

7. 8.7 Mutations
• Translocation results from the exchange of DNA segments
between nonhomologous chromosomes.

8. 8.7 Mutations
Deletion
Duplication
Inversion
Translocation
Figure 12–20 Chromosomal Mutations
Section 12-4

9. 8.7 Mutations
Nondisjunction
• Usually occurs during meiosis and can cause gametes to
lack a chromosome or have an extra chromosome.
– Example: An egg may have 22 or 24 chromosomes.
• If a zygote has only 45 chromosomes then one pair has only
one chromosome (monosomy).
• If a zygote has 47 chromosomes then there are 3 copies of
one chromosome (trisomy).
– Example disorders: Down syndrome, klinefelter’s
syndrome and Turner’s syndrome

10. 8.7 Mutations Turner Syndrome
Nondisjunction

11. 8.7 Mutations
Mutations can be caused by several factors.
• Replication errors can cause
mutations.
• Mutagens, such as UV ray and
chemicals (carcinogens), can
cause mutations.
• Some cancer drugs use
mutagenic properties to kill
cancer cells.

12. 8.7 Mutations
• Mutations in body cells do not affect offspring.
• Mutations in sex cells
can be harmful or
beneficial to offspring.
• Natural selection often
removes mutant alleles
from a population when
they are less adaptive.
• Beneficial mutations
may be useful to
organisms in changing
environments.
These mutations
may be favored by
natural selection.