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Mutations
• Mutations – heritable changes in genetic information (changes to the
DNA sequence)
• Two types - gene and chromosomal mutations
• Remember mutations are not always negative – catalysts for evolution
• Mutations can be caused by chemical or physical agents (mutagens)
– Chemical – pesticides, tobacco smoke, environmental pollutants
– Physical – X-rays and ultraviolet light
17.5 Gene Mutations
• Point Mutations
– Substitutions: mutations that affect a single nucleotide
– Insertions or Deletions: can cause frameshifts that shift the
reading frame of the genetic message.
• Can change the entire protein so it does not work
• Disastrous effects whenever the number of nucleotides
deleted or inserted is not a multiple of three (size of a
codon).
17.5 Gene Mutations
Silent mutations
• Silent: a substitution in the DNA sequence that does not
result in an amino acid change because many codons code
for the same amino acid. For instance: GAA and GAG
both code for amino acid GLU
17.5 Missense mutations
• Missense mutation: a substitution that changes one amino
acid to another one. These mutations may have little
effect on the protein: the new amino acid may have similar
properties to the one it replaces or be physically located in
a non-critical region of the protein. Some missense are
serious like sickle cell anemia.
17.5 Nonsense mutations
• Nonsense mutation: substitution of one base in the DNA
code results in a “stop” codon therefore shortening the
protein. Translation is stopped prematurely; the resulting
polypeptide will be shorter. Nearly all nonsense mutations
lead to nonfunctional protein.
15.4 Chromosomal Mutations
• Chromosomal mutation: mutation
that changes the number or structure
of chromosomes.
15.4 Chromosomal Mutations
• Types of chromosomal mutations:
– Deletion: The loss of all or part of
a chromosome
– Duplication: A segment is
repeated
– Inversion: part of the
chromosome is reverse from its
usual direction.
– Translocation: one chromosome
breaks off an attaches to another
chromosome.
Nondisjunction
Karyotype of a Patau’s male
(notice chromosome #13 has three Karyotype of a normal
chromosomes instead of two)
male
15.1 and 15.2 Basis for sex linkage (Thomas Hunt Morgan)
Thomas Hunt Morgan: experimental embryologist from Columbia
University – famous for fruit fly (Drosophila melanogaster)
experiments early 20th century.
Drosophila melanogaster: only four pairs of chromosomes 3 are
autosomes and 1 is the sex chromosomes (XX or XY).
Wild type: Red eyes (w+)
Mutant: White eyes (w)
15.1 and 15.2 Basis for sex linkage (Thomas Hunt Morgan)
15.1 and 15.2 Basis for sex linkage (Thomas Hunt Morgan)
Significance of these
findings: a correlation
between a particular trait
and an individual’s sex
suggested that a specific
gene is carried on a specific
chromosome
15.1 and 15.2 Basis for sex linkage (Thomas Hunt Morgan)
Significance of these
findings: a correlation
between a particular trait
and an individual’s sex
suggested that a specific
gene is carried on a specific
chromosome
The Chromosomal Basis of Sex
• In humans and other mammals, there are two
varieties of sex chromosomes: a larger X
chromosome and a smaller Y chromosome
• Only the ends of the Y chromosome have regions
that are homologous with the X chromosome
• The SRY gene on the Y chromosome codes for the
development of testes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-5
X
Y
• Some disorders caused by recessive alleles on the X
chromosome in humans:
– Color blindness
– Duchenne muscular dystrophy
– Hemophilia
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
X Inactivation in Female Mammals
• In mammalian females, one of the two X
chromosomes in each cell is randomly inactivated
during embryonic development
• The inactive X condenses into a Barr body
• If a female is heterozygous for a particular gene
located on the X chromosome, she will be a mosaic
for that character
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
15.2 - X Inactivation in Females
Sugar
(girl)
Spice
(girl)
Concept 15.3: Linked genes tend to be inherited together
because they are located near each other on the same
chromosome
• Each chromosome has hundreds or thousands of
genes
• Genes located on the same chromosome that tend to
be inherited together are called linked genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
b vg
b+ vg+
Parents
in testcross
Most
offspring

b vg
b vg
b+ vg+
b vg
or
b vg
b vg
Fig. 15-10
Gray body, normal wings
(F1 dihybrid)
Testcross
parents
Replication
of chromosomes
Meiosis I
Black body, vestigial wings
(double mutant)
b+ vg+
b vg
b vg
b vg
Replication
of chromosomes
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
b+ vg+
Meiosis I and II
b+ vg
b vg+
b vg
Meiosis II
Recombinant
chromosomes
Eggs
Testcross
offspring
b vg
b+ vg+
b+ vg
b vg+
965
944
206
185
Wild type
(gray-normal)
Blackvestigial
Grayvestigial
Blacknormal
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Parental-type offspring
Recombination
frequency
=
Recombinant offspring
391 recombinants
2,300 total offspring
 100 = 17%
b vg
Sperm
• Morgan found that body color and wing size are usually
inherited together in specific combinations (parental
phenotypes)
• He noted that these genes do not assort independently,
and reasoned that they were on the same chromosome
• However, nonparental phenotypes were also produced
• Understanding this result involves exploring genetic
recombination, the production of offspring with
combinations of traits differing from either parent
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Recombination of Unlinked Genes: Independent
Assortment of Chromosomes
• Mendel observed that combinations of traits in
some offspring differ from either parent
• Offspring with a phenotype matching one of the
parental phenotypes are called parental types
• Offspring with nonparental phenotypes (new
combinations of traits) are called recombinant
types, or recombinants
• A 50% frequency of recombination is observed for
any two genes on different chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Recombination of Linked Genes: Crossing Over
• Morgan discovered that genes can be linked, but the
linkage was incomplete, as evident from
recombinant phenotypes
• Morgan proposed that some process must
sometimes break the physical connection between
genes on the same chromosome
• That mechanism was the crossing over of
homologous chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mapping the Distance Between Genes Using
Recombination Data
• Alfred Sturtevant, one of Morgan’s students,
constructed a genetic map, an ordered list of the
genetic loci along a particular chromosome
• Sturtevant predicted that the farther apart two genes
are, the higher the probability that a crossover will
occur between them and therefore the higher the
recombination frequency
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mapping the Distance Between Genes Using
Recombination Data
• A linkage map is a genetic map of a chromosome
based on recombination frequencies
• Distances between genes can be expressed as map
units; one map unit, or centimorgan, represents a
1% recombination frequency
• Map units indicate relative distance and order, not
precise locations of genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-11
RESULTS
Recombination
frequencies
9%
Chromosome
9.5%
17%
b
cn
vg
Mapping the Distance Between Genes Using
Recombination Data
• Genes that are far apart on the same chromosome
can have a recombination frequency near 50%
• Such genes are physically linked, but genetically
unlinked, and behave as if found on different
chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings