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More Wrinkles: Beyond Mendelian Genetics
• In codominance, a heterozygote expresses the
traits of both alleles.
• Example: human blood type
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Beyond Dominant and
Recessive Alleles
Codominance
In codominance, both alleles contribute to the
phenotype.
In certain varieties of chicken, the allele for black
feathers is codominant with the allele for white
feathers.
Heterozygous chickens are speckled with both black
and white feathers. The black and white colors do
not blend to form a new color, but appear
separately.
More Wrinkles: Beyond Mendelian Genetics
• Polygenic traits are determined by more than
one gene. They tend to show more of a
continuum than traits determined by a single
gene.
• Examples: human eye color, skin color, and
height
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Beyond Dominant and
Recessive Alleles
Multiple Alleles
Genes that are controlled by more than two alleles
are said to have multiple alleles.
An individual can’t have more than two alleles.
However, more than two possible alleles can exist
in a population.
A rabbit's coat color is determined by a single gene
that has at least four different alleles.
Beyond Dominant and
Recessive Alleles
Different combinations of alleles result in the colors
shown here.
KEY
C=
full color; dominant
to all other alleles
cch = chinchilla; partial
defect in pigmentation;
dominant to
ch and c alleles
ch = Himalayan; color in
certain parts of the
body; dominant to
c allele
ch
hCc
ch
hc
chc
h, or
AIbino:
Chinchilla:
Himalayan:
cc cCC,
chcc,
, cor
, hor
cchCc
c
Full color:
,cch
Cc
c = albino; no color;
recessive to all other
alleles
Beyond Dominant and
Recessive Alleles
Polygenic Traits
Traits controlled by two or more genes are said to be
polygenic traits.
Skin color in humans is a polygenic trait controlled
by more than four different genes.
LE 14-12
AaBbCc
aabbcc
20/64
Fraction of progeny
15/64
6/64
1/64
Aabbcc
AaBbcc
AaBbCc
AaBbCc AABbCc
AABBCc AABBCC
More Wrinkles: Beyond Mendelian Genetics
• Pleiotropy occurs when a single
gene affects more than one trait.
• Example: sickle cell anemia in
humans
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More Wrinkles: Beyond Mendelian Genetics
• Linked genes are often inherited together. The
closer two genes are to each other on a
chromosome, the more likely they are to be
inherited together.
• Example: body color and wing size in fruit flies
are linked
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More Wrinkles: Beyond Mendelian Genetics
• Sex-linked traits are determined by genes found
on the X chromosome. Men, who have only one
X chromosome, need only one recessive allele
to express a recessive sex-linked trait. These
traits are more common in males than females.
• Examples: red-green color-blindness,
hemophilia
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The Human Genome
• A genome is the total genetic material of an
organism.
• The Human Genome Project determined the
DNA sequence of the entire human genome.
• Over 99.9% of the 3.2 billion nucleotide pairs in
the human genome are identical in all humans.
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The Human Genome
• Humans have about 22,000 genes.
• Many human genes give rise to RNA transcripts
that are processed in different ways. So, one
gene can provide the instructions for building
multiple proteins.
• The function of more than half of our genes is
still unknown.
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DNA and Chromosomes
DNA and Chromosomes
In prokaryotic cells, DNA is located in the cytoplasm.
Most prokaryotes have a single DNA molecule
containing nearly all of the cell’s genetic
information.
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DNA and Chromosomes
Chromosome
E. Coli Bacterium
Bases on the
Chromosomes
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DNA and Chromosomes
Many eukaryotes have 1000 times the amount of
DNA as prokaryotes.
Eukaryotic DNA is located in the cell nucleus
inside chromosomes.
The number of chromosomes varies widely from
one species to the next.
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DNA and Chromosomes
Chromosome Structure
Eukaryotic chromosomes contain DNA and protein,
tightly packed together to form chromatin.
Chromatin consists of DNA tightly coiled around
proteins called histones.
DNA and histone molecules form nucleosomes.
Nucleosomes pack together, forming a thick fiber.
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DNA and Chromosomes
Eukaryotic Chromosome Structure
Chromosome
Nucleosome
DNA
double
helix
Coils
Supercoils
Histones
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DNA Replication
DNA Replication
Each strand of the DNA double helix has all the
information needed to reconstruct the other half by
the mechanism of base pairing.
In most prokaryotes, DNA replication begins at a
single point and continues in two directions.
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DNA Replication
In eukaryotic chromosomes, DNA replication
occurs at hundreds of places. Replication
proceeds in both directions until each chromosome
is completely copied.
The sites where separation and replication occur
are called replication forks.
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DNA Replication
Duplicating DNA
Before a cell divides, it duplicates its DNA in a
process called replication.
Replication ensures that each resulting cell will have
a complete set of DNA.
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DNA Replication
During DNA replication, the DNA molecule
separates into two strands, then produces two
new complementary strands following the
rules of base pairing. Each strand of the
double helix of DNA serves as a template for
the new strand.
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DNA Replication
New Strand
Original strand
Nitrogen Bases
Growth
Growth
Replication Fork
Replication Fork
DNA Polymerase
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DNA Replication
How Replication Occurs
DNA replication is carried out by enzymes that
“unzip” a molecule of DNA.
Hydrogen bonds between base pairs are broken and
the two strands of DNA unwind.
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DNA Replication
The principal enzyme involved in DNA replication
is DNA polymerase.
DNA polymerase joins individual nucleotides to
produce a DNA molecule and then “proofreads”
each new DNA strand.
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The Human Genome
• Single-nucleotide polymorphisms (SNPs) are
locations in the genome where the nucleotide
sequence differs among humans.
• More than 3 million SNPs are known.
• SNPs may help scientists identify genes related
to human diseases.
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Cancer: Genes Gone Awry
• Cancer occurs when cells in the body divide out
of control.
• Mutations in the genes that control cell division
result in cancer.
• A mutation in a single gene is not enough to
cause cancer—mutations in many key genes are
required.
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Mutations
Mutations are changes in the genetic material.
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Kinds of Mutations
Kinds of Mutations
Mutations that produce changes in a single gene are
known as gene mutations.
Mutations that produce changes in whole
chromosomes are known as chromosomal
mutations.
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Kinds of Mutations
Gene Mutations
Gene mutations involving a change in one or a few
nucleotides are known as point mutations
because they occur at a single point in the DNA
sequence.
Point mutations include substitutions, insertions,
and deletions.
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Kinds of Mutations
Substitutions
usually affect no
more than a single
amino acid.
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Kinds of Mutations
The effects of insertions or deletions are more
dramatic.
The addition or deletion of a nucleotide causes a
shift in the grouping of codons.
Changes like these are called frameshift
mutations.
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Kinds of Mutations
In an insertion, an
extra base is inserted
into a base sequence.
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Kinds of Mutations
In a deletion, the loss of a single base is deleted
and the reading frame is shifted.
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Kinds of Mutations
Chromosomal Mutations
Chromosomal mutations involve changes in the
number or structure of chromosomes.
Chromosomal mutations include deletions,
duplications, inversions, and translocations.
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Kinds of Mutations
Deletions involve the loss of all or part of a
chromosome.
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Kinds of Mutations
Duplications produce extra copies of parts of a
chromosome.
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Kinds of Mutations
Inversions reverse the direction of parts of
chromosomes.
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Kinds of Mutations
Translocations occurs when part of one
chromosome breaks off and attaches to another.
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Significance of Mutations
Significance of Mutations
Many mutations have little or no effect on gene
expression.
Some mutations are the cause of genetic disorders.
Polyploidy is the condition in which an organism
has extra sets of chromosomes.
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Cancer: Genes Gone Awry
• Over a lifetime, mutations build up until a
combination of mutations in a single cell allows
uncontrolled cell division.
• Further mutations expand the tumor cells' ability
to divide and spread.
• Cancer is most likely to strike older people,
those who have been exposed to mutationcausing agents, and those who have inherited
mutations in cancer-related genes.
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Cancer: Genes Gone Awry
• Genes that have been implicated in cancer:
– Proto-oncogenes: When mutated, they
become oncogenes that stimulate abnormal
cell division.
– Tumor-suppressor genes: They prevent
cancer by inhibiting cell division.
• Metastasis is the ability of tumor cells to spread
around the body and give rise to secondary
tumors. Cancer is much harder to treat once
metastasis has occurred.
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Environmental Causes of Cancer
• A person's environment is responsible for about
80%–90% of the mutations that result in cancer.
• Environmental risk factors:
– Smoking
– Diet
– Radiation
– Ultraviolet light
– Chemicals
– Infection by certain viruses and bacteria
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Transgenic Organisms and Cloning
• A transgenic organism is one that contains a
gene from another species.
• Typical process for developing transgenic
bacteria
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Transgenic Organisms and Cloning
• Examples of transgenic organisms:
– Bacteria that produce insulin and other
important products
– Plants that
• produce medicines
• have resistance to pests, diseases, or herbicides
• are drought-resistant or able to grow in salty soils
– Animals that produce products:
• Sheep with increased wool production
• Pork with higher levels of omega-3 fatty acids
• Salmon that grow faster
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Transgenic Organisms and Cloning
• Cloning is the creation of an organism that is
genetically identical to one that already exists.
• In mammals, cloning is done through the
process of nuclear transplantation.
• Potential uses of cloning:
– A routine part of agriculture
– Could generate herds of identical animals with
desirable traits
– Cloning of endangered species could help
increase their numbers
– Cloning of deceased pets
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DNA Technology – What Could Possibly Go
Wrong?
• Some bacteria and viruses are a danger to
human health or to natural habitats.
– How likely is an accidental release?
• Potential dangers of genetically modified (GM)
plants and animals:
– Is the safety of GM food adequately tested?
– Should GM foods be labeled?
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DNA Technology – What Could Possibly Go
Wrong?
• Potential dangers of GM plants and animals
(continued):
– Plants that are toxic to pests also harm
nontarget species --for example, Monarch
butterflies
– May lead to the evolution of resistant
"superweeds" that can be controlled only with
very toxic chemicals
– Contamination of natural habitats or
populations by transgenic plants and animals
or their genes
– Cost of GM seeds and products
• Effects on human societies
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History of Science: Discovery of the Double
Helix
• By 1950, scientists knew DNA was the genetic material,
but they did not know the structure of DNA.
• In 1953, Watson and Crick built a model of DNA that was
consistent with available evidence.
• Watson and Crick used X-ray photos of DNA taken by
Franklin and Wilkins as part of their research.
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Technology: Gene Therapy
• Many genetic diseases
occur when people do not
have a working gene for
making a key protein.
• Gene therapy attempts to
introduce DNA for the
normal, working gene into
a person's cells.
• Some large setbacks have
occurred in gene therapy,
but there are some recent
promising developments also.
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Science and Society: Genetic Counseling
• A pedigree is a family tree that
shows which relatives are and
are not affected by a particular
genetic disease.
• Medical tests can determine
whether a person is a carrier of a
disease allele.
• Amniocentesis and chorionic villus
sampling can determine whether a
fetus has a genetic disease.
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Science and Society: DNA Forensics
• Forensic scientists use short tandem repeat (STR)
analysis to determine whether DNA samples match.
• Between 1989 and 2011, DNA evidence exonerated 272
people who were imprisoned for crimes they did not
commit.
• DNA forensics was used to identify the victims of the
2001 World Trade Center terrorist attacks.
• DNA forensics can be used to establish paternity and
trace familial relationships.
• DNA forensics can be used to identify disease-causing
microorganisms or endangered species.
• Ethical concerns – DNA contains a wealth of private
information about family relationships, susceptibility to
diseases, and so on.
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