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Mendel and the Gene Idea
PowerPoint Lectures for
Biology, Seventh Edition
Chapter 14
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Points to Ponder
•
What is the genotype and the phenotype of an individual?
•
What are the genotypes for a homozygous recessive and dominant individuals
and a heterozygote individual?
•
Be able to draw a punnett square for any cross (1-trait cross, 2-trait cross and
a sex-linked cross).
•
What are Tay-Sachs disease, Huntington disease, sickle-cell disease, and
PKU?
•
How are each of the above inherited?
•
What is polygenic inheritance?
•
What is a multifactorial trait?
•
What is sex-linked inheritance?
•
Name 3 X-linked recessive disorders.
•
What is codominance?
•
What is incomplete dominance?
•
What do you think about genetic profiling?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gregor Mendel (1822-1884)
Documented a particulate mechanism of
inheritance through his experiments with
garden peas
Figure 14.1
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“particulate” inheritance
– Mendel thought that parents pass on discrete
heritable units, “genes” (Mendel referred to
these as “factors”)
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Mendel’s experiments
• Concept 14.1: Mendel
used the scientific
approach to identify some
basic Laws of
Inheritance
• Mendel discovered the
basic principles of heredity
by breeding garden peas
in carefully planned and
executed experiments
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Some genetic vocabulary
– Gene: the basic unit of
heredity in a living
organism; a gene is a
section of a chromosome
that codes for a trait, or
characteristic.
– Alleles: alternate forms of a
specific gene at the same
position (locus) on a gene
(e.g. allele for unattached
earlobes and attached
lobes); alleles occur in pairs
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• For each character,
– An organism inherits two alleles, one from
each parent
– A genetic locus is actually represented twice,
in diploid organisms
Homologous
chromosomes, one
paternal, one maternal 
Allele for purple flowers
Locus for flower-color gene
Homologues are similar
in size, centromere
location, and in the
genes that they carry
Figure 14.4
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Allele for white flowers
Homologous
pair of
chromosomes
Dominants and Recessives
• If the two alleles at a locus differ
– Then one, the dominant allele, determines
the organism’s appearance
– The other allele, the recessive allele, has no
noticeable effect on the organism’s
appearance (is “masked”)
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Vocabulary review
Homozygous: (aka
“purebred” or “truebreeding”) have an
identical set of alleles
for a trait
Heterozygous: hybrid;
nonmatching alleles
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Understanding genotype & phenotype
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Genotype –
specific genes
for a particular
trait written with
symbols
(example: EE,
Ee, or ee)
Phenotype – the
physical or
outward
expression of
the genotype
egg
E
e
E
e
E
e
sperm
fertilization
EE
ee
Ee
growth and
development
EE
unattached earlobe
ee
attached earlobe
Allele Key
E= unattached earlobes
e= attached earlobes
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Ee
unattached earlobe
zygote
Mendel’s Experimental, Quantitative Approach
• Mendel chose to work with peas
– Because they are available in many varieties
– Because he could strictly control which plants mated
with which
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•
He started his experiments
with varieties that were
“true-breeding” (purebred
lines isolated through selfpollination) = “P”
generation
•
When Mendel crossed
contrasting, true-breeding
white and purple flowered
pea plants
– ALL of the offspring (F1
generation) were purple.
Why?
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Punnett Squares tutorial
The Punnett square is a diagram that is
used to predict an outcome of a particular
cross or breeding experiment. It is named
after Reginald C. Punnett, who devised the
approach, and is used by biologists to
determine the probability of an offspring
having a particular genotype.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s Principle of Dominance and Recessiveness
• Mendel reasoned that
– In the F1 plants, only the
purple flower “factor“ was
affecting flower color
(phenotype) in these hybrids
– Purple flower color was
dominant, and white flower
color was recessive
In such cases, the
dominant allele
“MASKS” the
recessive
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Next, when Mendel crossed the purple F1 plants:
WHAT DO YOU PREDICT THE COLOR RATIOS
WOULD BE FOR THE F2 GENERATION?
– Mendel found that
most of the F2 plants
had purple flowers,
but some had white
flowers
– He found a
repeatable ratio of
about 3:1, purple to
white flowers, in the
F2 generation
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• Mendel observed the
same pattern in many
other pea plant
characters 
• Mendel developed a
hypothesis to
explain the 3:1
inheritance pattern
that he observed
among the F2
offspring
Table 14.1
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The Law of Segregation
• The two alleles for a heritable character
separate (segregate) during gamete formation
and end up in different gametes
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Segregation of Alleles (meiosis)
• Does Mendel’s segregation model account for
the 3:1 ratio he observed in the F2 generation
of his numerous crosses?
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The Law of Independent Assortment
• Independent assortment
– Each pair of alleles segregates independently
during gamete formation
Animated version:
http://www.sumanasinc.com/webcon
tent/animations/content/independent
assortment.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Possible gametes for TWO traits
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Sex and variation
• Crossing over prior to gamete formation
increases variables
• Independent assortment
http://www.csuchico.edu/~jbell/Biol207/animations/assortment.html
• Crossover animation
http://www.csuchico.edu/~jbell/Biol207/animations/recombination.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The extent of variation
• Not only does meiosis guarantee segregation and
independent assortment of alleles, but crossing over also
mixes up alleles on homologous chromosomes before
distribution
“Therefore, in humans with 23 pairs of chromosomes, a gamete (egg or
sperm) could have 223 or 8,388,604 possible combinations of
chromosomes from that parent. Any couple could have 223 × 223 or
70,368,744,177,644 (70 trillion) different possible children, based just on
the number of chromosomes, not considering the actual genes on those
chromosomes.
Thus, the chance of two siblings being exactly identical would be 1 in 70
trillion.
In addition, something called crossing over, in which the two
homologous chromosomes of a pair exchange equal segments during
synapsis in Meiosis I, can add further variation to an individual’s genetic
make-up.”
http://www.biology.clc.uc.edu/Courses/bio104/meiosis.htm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dihybrid Crosses & Independent Assortment of alleles
• Illustrates the inheritance of two characters
• Produces four phenotypes in the F2 generation
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The reality of inheritance
• Concept 14.3: Inheritance patterns are often
more complex than predicted by simple
Mendelian genetics
• The relationship between genotype and
phenotype is rarely simple
What about
green eyes?
Hazel eyes?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The genetics of eye color
•
At one locus (site=gene) there are two different alleles segregating: the B allele confers
brown eye color and the recessive b allele gives rise to blue eye color.
•
At the other locus (gene) there are also two alleles: G for green or hazel eyes and g for
lighter colored eyes.
•
The B allele will always make brown eyes regardless of what allele is present at the
other locus. In other words, B is dominant over G. In order to have true blue eyes your
genotype must be bbgg. If you are homozygous for the B alleles, your eyes will be
darker than if you are heterozygous and if you are homozygous for the G allele, in the
absence of B, then your eyes will be darker (more hazel) that if you have one G allele.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Extending Mendelian Genetics for a Single Gene
• The inheritance of characters by a single gene
– May deviate from simple Mendelian patterns
– Exceptions to the “rules” are listed in the
following slides
Need practice? Try these “drag and drop” Punnett squares
http://www.execulink.com/~ekimmel/mendel1a.htm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Spectrum of Dominance
• Complete dominance
– Occurs when the phenotypes of the
heterozygote and dominant homozygote are
identical
PP
pp
Pp
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Incomplete dominance (aka “blended inheritance)
• The phenotype of F1 hybrids is somewhere between
the phenotypes of the two parental varieties
Example: Flower
colors
RR = red
R’R’ = white
RR’ = pink
(intermediate
pigmentation)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Spectrum of Dominance
• In codominance
– Two dominant alleles both affect the
phenotype in separate,
distinguishable ways
• IAIB = The human blood group type AB
is an example of codominance
• RW = roan coat color in cattle or horses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Multiple Alleles and Codominance
• Most genes exist in populations
– In more than two allelic forms
• The ABO blood
group in humans
– Is determined by
multiple alleles
Table 14.2
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Frequency of Dominant Alleles
• Dominant alleles
– Are not necessarily more common in
populations than recessive alleles
– It really depends on whether a trait gives an
individual an adaptive advantage, and is
naturally selected.
– Examples: type O blood (ii) recessive present
in majority of population
http://www.bio-medicine.org/biology-definition/Blood_type/#Frequency
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pleiotropy
• Pleiotropy describes the genetic effect of a single
gene on multiple phenotypic traits.
•
The underlying mechanism is that the gene codes for a product that is, for
example, used by various cells, or has a signaling function on various
targets.
•
A classic example of pleiotropy is the human disease PKU
(phenylketonuria).
•
This disease can cause mental retardation and reduced hair and skin
pigmentation, and can be caused by any of a large number of mutations in
a single gene that codes for the enzyme (phenylalanine hydroxylase), which
converts the amino acid phenylalanine to tyrosine, another amino acid.
•
Depending on the mutation involved, this results in reduced or zero
conversion of phenylalanine to tyrosine, and phenylalanine concentrations
increase to toxic levels, causing damage at several locations in the body.
•
PKU is totally benign if a diet free from phenylalanine is maintained.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gene Linkage
• Sometimes, when genes are located close
together on the same chromosome, they tend
to be inherited together.
• Which of Mendel’s Laws does this defy?
• See linkage animation
http://www.csuchico.edu/~jbell/Biol207/animations/linkage.html
• A “classic” example of this is why cats with white
fur and blue eyes are (more often than not) also
deaf.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Extending Mendelian Genetics for Two or More Genes
• Many traits (especially in complex organisms)
– May be determined by two or more genes
(polygenic)
• In EPISTASIS
“standing upon”
– the phenomenon where the effects of one
gene are modified by one or several other
genes (which are sometimes called modifier
genes).
– A gene at one locus alters the phenotypic
expression of a gene at a second locus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
An example of epistasis in mice
Here we see the effect
the bb gene combo
has on the C gene for
the agouti (brownish)
fur color in mice

BbCc
BbCc
Sperm
1⁄
BC
4
1⁄
4
bC
1⁄
4
1⁄
Bc
4
bc
Eggs
1⁄
4
BC
BBCC
BbCC
BBCc
BbCc
4
bC
BbCC
bbCC
BbCc
bbCc
cc combo = white fur
1⁄
C_ combo = black fur,
unless bb “stands
upon” it. Then you
get agouti color.
1⁄
1⁄
4
Bc
BBCc
BbCc
BBcc
4
bc
BbCc
bbCc
Bbcc
9⁄
Figure 14.11
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
16
3⁄
16
Bbcc
4⁄
bbcc
16
Other variations in gene expression
• Incomplete Expressivity is seen in cases where
individuals with the same genotypes may have, often for
unknown reasons, variability in their phenotypes.
–
This is often seen in genetic diseases where one person
with a disease such as diabetes may be very severely
effected while another with the same allele may have a
milder form of the disease.
• Incomplete Penetrance is seen when an individual with a
particular genotype does not express the phenotype. Again
the reasons for this are not clearly understood.
–
For example, known mutations in the gene responsible for
Huntington disease have “95% penetrance”, because 5%
of those with the dominant allele for Huntington disease
don't develop the disease and 95% do.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Beyond simple inheritance
• Polygenic traits - two or more
sets of alleles govern one trait
Note that:
Environmental
effects can
cause intervening
phenotypes!
– Each dominant allele codes for a
product so these effects are additive
– Results in a continuous variation of
phenotypes
– e.g. skin color ranges from very dark
to very light

AaBbCc
aabbccAabbccAaBbccAaBbCc
AABBCc
AABBCC
AABbCc
20⁄
15⁄
6⁄
64
64
64
1⁄
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
AaBbCc
64
Polygenic inheritance
Number of Men
Multifactorial trait – a trait that is
influenced by both genetic and
environmental factors
e.g. skin color is influenced by sun
exposure
e.g. height can be affected by nutrition
most
are
this
height
few
62
short
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64
Courtesy University of Connecticut/Peter Morenus, photographer;
few
66
68
Height in Inches
70
72
74
tall
Genes are affected by Environment
• While there is a strong genetic component in human
height, the average height has increased over the past
50 years in developed countries. This is considered to
be due to improved nutrition.
• Likewise, a cotton plant may have the alleles
necessary for high yields but if it doesn't receive
enough water or fertilizer it cannot reach its genetic
potential.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Example:
– A phenotypic range of a particular genotype
may be influenced by the environment
Figure 14.13
Exact color often mirrors the pH of the soil; acidic soils produce
blue flowers, neutral soils produce very pale cream petals, and
alkaline soils results in pink or purple. This is caused by a color
change of the flower pigments in the presence of aluminum ions
which can be taken up into the flower.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Beyond simple inheritance
Demonstrating environmental influences
on phenotype
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
•
Himalayan rabbit/Siamese cat
coat color influenced by
temperature
•
There is an allele responsible
for melanin production that
appears to be active only at
lower temperatures
•
The extremities have a lower
temperature and thus the ears,
nose paws and tail are dark in
color
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
© H. Reinhard/Arco Images/ Peter Arnold
• Many human characters
– Vary in the population along a continuum and
are called “quantitative characters”
• Humans are not convenient
subjects for genetic research.
Why?
– However, the study of human
genetics continues to advance
– Advances in personal
genomic testing
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A pedigree
–
•
Is a family tree that describes the interrelationships of parents and
children across generations
Inheritance patterns of particular traits can be traced and described
using pedigrees
Ww
ww
Ww ww ww Ww
WW
or
Ww
Widow’s peak
ww
Ww
Ww
ww
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
FF or Ff
Ff
ff
Third
generation
(two sisters)
ww
No Widow’s peak
(a) Dominant trait (widow’s peak)
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Attached earlobe
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Free earlobe
(b) Recessive trait (attached earlobe)
Genetic mutations can lead to familial diseases
• Somatic mutations — such as in skin cells as a
result of sun exposure — tend to accumulate
over the course of our lives, but are not
typically passed on to children.
• But other errors can occur in the DNA of
cells that produce the eggs and sperm. These
are called germline mutations and can be
passed from parent to child.
• If a child inherits a germline mutation from
their parents, every cell in their body will have
this error in their DNA.
• Germline mutations are what cause diseases to
run in families, and are responsible for
hereditary diseases.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
How does it work?
• A gene is essentially a sentence made up of the bases A, T, G,
and C that describes how to make a protein.
• Any changes to those instructions can alter the gene's meaning
and change the protein that is made, or how or when a cell
makes that protein.
• There are many different ways to alter a gene, just as there are
many different ways to introduce typos into a sentence. In the
following examples of some types of mutations, we use the
sentence to represent the sample gene:
THE FAT CAT ATE THE RAT
(this is how the code should read)
Here’s a handy table listing all the types of mutations: http://www.uvm.edu/~cgep/Education/Mutations.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Point mutation: a change in a single
nucleotide
Example 1:
• A SUBSTITUTION mutation
• occurs where one nucleotide base is
replaced by another.
– example
• THE FAT CAT ATE THE RAT
• THE FAT HAT ATE THE RAT
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Point mutations: types
• Base (A,T,C,G) substitutions can lead to
“Missense” or “Nonsense” mutations:
Missense: a change in DNA sequence that
changes the codon to a different amino acid.
This can alter the protein enough to render it
nonfunctional.
Not all missense mutations are deleterious,
some changes can have no effect.
Because of the ambiguity of missense
mutations, it is often difficult to interpret the
consequences.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Nonsense: a change in the
genetic code that results in the
coding for a stop codon rather
than an amino acid.
The shortened protein is
generally non-function or its
function is impeded.
Point mutations can sometimes be “SILENT”.
How does this change the protein that this gene
codes for?
More often than not,
“third-base mutations”
are silent. Why?
Another example:
Amino acid: leucine
GAA
GAG
GAT
GAC
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Sickle-Cell Disease
– Affects one out of 400 African-Americans
– Is caused by the substitution of a single amino acid in
the hemoglobin protein in red blood cells
(shows seven out of the 146 amino acid units of normal hemoglobin)
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Sickle Cell
• Symptoms include physical weakness, pain,
organ damage, and even paralysis
• Sickle-cell disease occurs more commonly in people (or their descendants) from parts of
the world such as sub-Saharan Africa, where malaria is or was common, but it also occurs in
people of other ethnicities.
• This is because those with one or two alleles of the sickle cell disease are resistant to
malaria since the red blood cells are not conducive to the parasites.
• In areas where malaria is common there is a survival value in carrying the sickle cell
genes.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sickle Cell Disease
• Homozygous recessive individuals have sickle cell disease.
• Heterozygous individuals are phenotypically normal, but exhibit a mild
version of the disease; said to “carry” the trait for SC, but not actually have
the disease (symptoms usually only apparent @ altitude or heavy exercise).
• Homozygous dominant individuals are phenotypically normal.
Genotypes:
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Electrophoresis of the hemoglobin protein shows:
We see that homozygous normal
people have one type of
hemoglobin (A) and anemics
have type S, which moves
more slowly in the electric
field.
The heterozygotes have both
types, A and S. In other
words, there is codominance
at the molecular level.
Try this pedigree of familial inheritance of SC disease
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Point mutation: a change in a single
nucleotide in a gene
•
Examples 2 & 3:
– Insertion example
• THE FAT CAT ATE THE RAT (original gene)
• THE FAT CAT LAT ETH ERA T
– Deletion example
• THE FAT CAT ATE THE RAT (original gene)
• THE FAT ATA TET HER AT
These substitutions are known as “Frameshift Mutations” because they
shift the reading frame of the genetic message (triplets) so that the
protein may not be able to perform its function.
Usually more serious than a substitution mutation. Why?
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Follow this summary:
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Remember: not all mutations are bad!
•
A harmful mutation is a mutation that
decreases the fitness of the organism.
•
Sometimes, mutations can change a gene form in a
nonharmful, or even beneficial, way.
•
“Polymorphisms”: slight variations in a gene that
make us different, ex: eye colors, blood types, etc.
Are these good or bad?
•
A mutation may enable the mutant organism to
withstand particular environmental stresses better
than non-mutant organisms, or reproduce more
quickly. In these cases a mutation will tend to
become more common in a population through
natural selection. This is how populations
EVOLVE over time.
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Many mutations are neutral
• A neutral mutation has no harmful or beneficial
effect on the organism.
• Many of these mutations occur in the noncoding
DNA describes components of an organism's
DNA sequences that do not encode for protein
sequences.
• More than 98% of the human genome does not
encode protein sequences. We often call this
“junk DNA” (extra).
– Why are these “junk” sequences more likely to
vary between individuals?
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Chromosome Mutations
• A chromosome mutation is an unpredictable
(spontaneous) change that occurs in a chromosome.
• These changes are most often brought on by problems
that occur during meiosis (cell division that makes
gametes) or by mutagens (chemicals, radiation, etc.).
• Chromosome mutations can result in changes in the
number of chromosomes in a cell or changes in the
structure of a chromosome.
• Usually MUCH more serious than a gene mutation.
Why?
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Changes in chromosome number or structure
(a) A deletion removes a
chromosomal segment.
A B C D E
F G H
(b) A duplication repeats a
segment.
A B C D E
F G H
(c) An inversion reverses a
segment within a chromosome.
A B C D E
F G H
d) A translocation moves a
segment from one chromosome
to another, nonhomologous one.
A B C D E
F G H
•
In a “reciprocal
translocation", the most
common type, nonhomologous
chromosomes exchange
fragments.
Deletion
Duplication
Inversion
A B C E
F G H
A B C B C D E
A D C B E
F G H
F G H
M N O C D E
F G H
Reciprocal
translocation
M N O P Q
R
A B P
Q
R
What occurs in a “nonreciprocal translocation”?
•
58
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Test your knowledge
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Changes in Chromosomal Structure
(animation)
• http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter24/chang
es_in_chromosome_structure.html
• The Consequence of Inversion (animation)
• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/
free/0072437316/120082/bio33.swf::The%20Consequence
%20of%20Inversion
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Recessively Inherited Autosomal Disorders
• Many genetic disorders
– Are inherited in a recessive manner (aa)
– So, most affected individuals are homozygous (dual
inheritance)
– AA, Aa are normal phenotype, in this case
•
Recessively inherited disorders
–
•
Show up only in individuals homozygous
for the allele
Carriers
–
Are heterozygous individuals who carry
the recessive allele but are phenotypically
normal
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Cystic Fibrosis (recessive mutation in CFTR gene)
• Symptoms of cystic fibrosis include
– Mucus buildup in the some internal organs
– Abnormal absorption of nutrients in the small
Lung tissue from a cystic fibrosis
intestine
patient, showing extensive
destruction as a result of
obstruction and infection.
Cl -
H2O
nebulizer
defective
channel
percussion
vest
thick mucus
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Phenyketonuria
PKU is an autosomal recessive genetic
disorder that is characterized by an inability
of the body to utilize the essential amino
acid, phenylalanine. Amino acids are the
building blocks for body proteins.
• In 'classic PKU', the enzyme (phenylalanine hydroxylase), that
breaks down phenylalanine is completely or nearly completely
deficient. (PAH gene, 12q23.2 )
• This enzyme normally converts phenylalanine to another amino
acid, tyrosine. Without this enzyme, phenylalanine and its'
breakdown chemicals from other enzyme routes, accumulate in
the blood and body tissues.
• Infants are born “normal” but if not treated, severe brain problems,
such as mental retardation and seizures, will occur.
• Thus, this disease is GREATLY influenced by environmental
factors: “multifactorial”
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Other autosomal recessive disorders
• Tay-Sachs disease
• Albinism
• Hemochromatosis types 1-3: the most
common genetic disease in Europe.
You can watch this animation on inheritance
of the HFE allele: https://www.koshland-sciencemuseum.org/sites/all/exhibits/exhibitdna/inh05.jsp
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dominantly Inherited Disorders
• Some human disorders
– Are due to dominant alleles
One example is
achondroplasia
– A form of dwarfism that is
lethal when homozygous for
the dominant allele (DD)
Figure 14.15
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Dominant allele
• Huntington’s disease
– Is a degenerative disease of the nervous system
– Has no obvious phenotypic effects until about 35
to 40 years of age
Lake Maracaibo, Venezuela
Village pedigree
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sex-linked inheritance
X-linked disorders
• More often found in males than females
because recessive alleles are always expressed
• Most X-linked disorders are recessive:
– Color blindness: most often characterized by redgreen color blindness
– Duchenne’s muscular dystrophy (DMD):
characterized by wasting of muscles and death by
age 20
– Fragile X syndrome: most common cause of
inherited mental impairment
– Hemophilia: characterized by the absence of
particular clotting factors that causes blood to clot
very slowly or not at all
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sex-linked inheritance
X-linked
disorders
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B
X X
B
B
b
X Y grandfather
B b
X X daughter
X Y
B
b
X Y
X X
b
X Y
B
X Y
B
X X
B
B
X X
b
b
X Y grandson
B
B
X X
B b
X X
b b
X X
B
X Y
b
X Y
Key
=Unaffected female
=Carrier female
=Color-blind female
=Unaffected male
=Color-blind male
X-linked Recessive
Disorders
• More males than females are affected.
• An affected son can have parents who have the normal
phenotype.
• For a female to have the characteristic, her father must
also have it. Her mother must have it or be a carrier.
• The characteristic often skips a generation from the
grandfather to the grandson.
• If a woman has the characteristic, all of her sons will
have it.
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b
Mating of Close Relatives
• Matings between relatives are called
“consanguineous” matings
• Can increase the probability of the appearance of
a genetic disease (especially harmful recessive
homozygotes)
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Sex-linked inheritance
X-linked disorders: Hemophilia
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Key
Unaffected male
Hemophiliac
Unaffected female
Carrier
Queen Victoria
Prince Albert
4 children of 9
are shown
Victoria
Frederick III
(Germany)
Alice
Louis IV
(Hesse)
Princess
Helena of
Waldeck
Leopold
(died at 31)
Beatrice
Prince Henry of
Battenberg
12 children of 26
are shown
Henry
Irene
Frederick Alexandra
(died at 3)
Nicholas II
(Russia)
Alice
Alexander Alfonso XII Victoria
(Earl of
(Spain)
Athlone)
6 children of 34
are shown
Waldemar
(died at 56)
Henry
(died at 4)
Alexei
(murdered)
Rupert
(died at 21)
Alfonso
Gonzalo
(died at 31) (died at 20)
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as Benjamin
Cummings
(queen):
© Stapleton
Collection/Corbis; (prince): © Huton Archive/Getty Images
Leopold
(died at 32)
Multifactorial Disorders
• Many human diseases
– Have both genetic and environment
components
• Examples include
– Heart disease and cancer
– Try this interactive family pedigree (nicotine
addiction)
http://learn.genetics.utah.edu/units/addiction/genetics/pi.cfm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Genetic Testing and Counseling
• Genetic counselors
– Can provide information to prospective parents
concerned about a family history for a specific
disease
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Counseling Based on Mendelian Genetics and
Probability Rules
• Using family histories
– Genetic counselors help couples determine the
odds that their children will have genetic
disorders
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Tests for Identifying Carriers
• For a growing number of diseases
– Tests are available that identify carriers and
help define the odds more accurately
Fetal Testing
• In amniocentesis
– The liquid that bathes the fetus is removed and
tested
• In chorionic villus sampling (CVS)
– A sample of the placenta is removed and
tested
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Prenatal Genetic Screening
Genetic Screening (karyotyping) is possible, prenatally
Amniocentesis, @ 14-16 weeks
Chorionic villus sampling
(CVS), @ 9-14 weeks
75
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Concept: Some inheritance patterns are
exceptions to the standard chromosome theory
• Epigenetics
– Functional modifications to the genome that do not
involve a change in the nucleotide sequence.
– Environmental factors can alter the way our genes are
expressed, making even identical twins different.
– Examples of such modifications are DNA methylation
and histone modification, both of which serve to
regulate gene expression without altering the
underlying DNA sequence.
http://learn.genetics.utah.edu
/content/epigenetics/
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Genomic Imprinting
What Is Imprinting?
• For most genes, we inherit two working copies -- one from mom
and one from dad. But with imprinted genes, we inherit only one
working copy. Depending on the gene, either the copy from
mom or the copy from dad is epigenetically silenced. Silencing
usually happens through the addition of methyl groups during
egg or sperm formation.
The epigenetic tags on imprinted genes usually stay put for the
life of the organism. But they are reset during egg and sperm
formation. Regardless of whether they came from mom or dad,
certain genes are always silenced in the egg, and others are
always silenced in the sperm.
http://learn.genetics.utah.edu/content/epigenetics/imprinting/
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Genomic imprinting
Involves the silencing of certain genes that are
“stamped” with an imprint during gamete
production
Normal Igf2 allele
(expressed)
Paternal
chromosome
When a normal Igf2
allele is inherited from
the father,
heterozygous mice
grow to normal size.
But when a
mutant allele is
inherited from the
father, heterozygous
mice have the dwarf
phenotype.
Maternal
chromosome
Normal Igf2 allele
Wild-type mouse
with imprint
(normal size)
(not expressed)
(a) A wild-type mouse is homozygous for the normal igf2 allele.
Normal Igf2 allele
Paternal
Maternal
Mutant
lgf2 allele
Normal size mouse
Mutant
lgf2 allele
Paternal
Maternal
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Normal Igf2 allele
with imprint
Dwarf mouse
In mammals the
phenotypic
effects of certain
genes depend on
which allele is
inherited from the
mother and which
is inherited from
the father
Genes located outside the nucleus also have influence
•
Extranuclear genes
–
•
Are genes found in organelles in the cytoplasm
The inheritance of traits controlled by genes present in the chloroplasts
(ctDNA) or mitochondria (mtDNA)
–
Depends solely on the maternal parent because the zygote’s cytoplasm
comes from the egg
Figure 15.18
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In Humans:
• Some diseases affecting the muscular and nervous
systems are caused by defects in mitochondrial genes
that prevent cells from making enough ATP
–
Examples of mitochondrial DNA diseases:
• Leber's hereditary optic atrophy - causes progressive visual
impairment
• Kearns-Sayre disease
• Progressive external ophthalmoplegia
• Myoclonus epilepsy
• MELAS (mitochondrial myopathy, encephalopathy, lactic
acidosis, and stroke-like episodes)
– Mitomap - A human mitochondrial genome database
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings