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Unit 2B
Human Diversity & Change
Inheritance
Mutations
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A mutation
A mutation is a permanent structuralalteration in DNA.
In most cases, DNA changes either haveno effect or cause harm - occasionally amutation can improve an organism'schance of surviving and passing thebeneficial change on to its descendants.
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NIH - National HumanGenome ResearchInstitute
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Somatic mutations
A somatic mutation is a DNA change that
occurs in body cells, other than the egg orsperm (germ cells) they cannot bepassed on to offspring.
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Germ line mutations
A germ line mutation is a DNA change
that occurs in the egg or sperm.Germ line mutations can be passed on tooffspring.
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Point mutations
The simplest type of mutation is a point
mutation.This involves a change in a single basepair.
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An analogy
The following short sentence represents asequence of nucleotides in a fragment ofDNA:
THEBOYATETHEBUN-----
This is read in codons (3-letter segments)THE BOY ATE THE BUN------
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Deletion
THEBOYATETHEBUN------
THEBOYTETHEBUN------
THE BOY TET HEB UN------
A
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Duplication
THEBOYATETHEBUN
THEBOYATETOYATETHEBUN
THE BOY ATE TOY ATE THE BUN
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Inversion
THEBOYATETHEBUN
THEBTETAYOHEBUN
THE BTE TAY OHE BUN
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Insertion
THEBOYATETHEBUN------
THEBOYAXTETHEBUN-----
THE BOY AXT ETH EBU N-----
In the above cases part, or all, of the sentence is nolonger readable.
X
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Substitution
THEBOYATETHEBUN------
THEBOYATETHEBUG------
THE BOY ATE THE BUG------
In this case, the meaning is changed but sentence is stillreadable.
G
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Non-disjunction
A non-disjunction occurs if homologouschromosomes fail to separate correctlyand migrate to opposite poles during
meiosis.This results in too many chromosomes insome daughter cells and too few in others.The most common disease resulting from
a non-disjunction is Downs syndrome, ortrisomy 21, where there are three copiesof chromosome 21.
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Cytogenic map
This is how chromosomeslook when stained andexamined under amicroscope. The light and
dark bands give eachchromosome a uniqueappearance. This feature isuseful for the study of
karyotypes, which allowsscientists to look forchromosomal alterations.
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Normal karyotypes
Normal female - XX Normal male - XY
Wessex Reg, Genetics Centre, Wellcome Images
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Autosomal mutations
Downs syndrome trisomy 21 Edwards syndrome trisomy 18
Wessex Reg, Genetics Centre, Wellcome Images
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Downs syndrome trisomy 21
People with Downs
syndrome have anextra copy of
chromosome 21. Affected people have
varying degrees ofmental retardation,
short stature, and aflattened facial profile.
Wellcome Photo Library
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Sex chromosome mutations
Klinefelters syndrome - XXY Turners syndrome - XO
Wessex Reg, Genetics Centre, Wellcome Images
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Turners syndrome - XO
The Turners syndromekaryotype has only one Xchromosome and no Ychromosome.
Women with Turnerssyndrome are usuallyshort and have a numberof abnormal physicalfeatures. Their ovariesnever develop so theyare infertile, and nevermenstruate.
Wellcome Photo Library
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Klinefelters syndrome - XXY
The Klinefelters syndromekaryotype has an extra Xchromosome (XXY).
Men with Klinefelters
syndrome do not produceenough testosterone. Thisresults in underdevelopedsecondary sex
characteristics, breastenlargement, small testesand infertility due to lack ofsperm.
Wellcome Photo Library
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Factors affecting the outcome of amutation
The type of cell affected (somatic or germline)
The type of mutation (e.g. substitution,insertion)
The extent of the mutation (e.g. point, non-disjunction)
The area of the gene affected (coding ornon-coding)
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Mutagens
A mutagen is anything that causes amutation (i.e. causes a permanent changein a cells DNA).
Most mutagens are either chemicals orphysical agents such as radiation.
Some mutations are spontaneous,occurring as a result of errors during DNAreplication or repair.