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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 14
Mendel and the Gene Idea
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Figure 14.1
Gregor Mendel
“FATHER OF GENETICS”
• An Austrian monk
• His work was important to the
understanding of heredity
• Carried out quantitative work
with ordinary garden peas
Concept 14.1: Mendel used the scientific
approach to identify two laws of inheritance
• Mendel discovered the basic principles of
heredity by breeding garden peas in carefully
planned experiments
© 2011 Pearson Education, Inc.
Mendel’s Experimental, Quantitative
Approach
• Advantages of pea plants for genetic study
– There are many varieties with distinct heritable
features, or traits
– Mating can be controlled
– Each flower is perfect
• has sperm-producing organs (stamens) and an
egg-producing organ (carpel)
– Cross-pollination relatively simple
© 2011 Pearson Education, Inc.
Genes and Dominance
TRAIT: specific characteristic (color or height)
that varies from one individual to another
Mendel studied SEVEN different traits
Table 14.1
• In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a process
called hybridization
• In this example, the true-breeding parents are the
P generation
• The hybrid offspring of the P generation are called
the F1 generation
• When F1 individuals self-pollinate or crosspollinate with other F1 hybrids, the F2 generation
is produced
© 2011 Pearson Education, Inc.
Figure 14.3-1
EXPERIMENT: Law of Segregation
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
Figure 14.3-2
EXPERIMENT Law of Segregation
EXPERIMENT:
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
Purple
flowers
White
flowers
All plants had purple flowers
Self- or cross-pollination
Figure 14.3-3
EXPERIMENT Law of Segregation
EXPERIMENT:
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
Did the recessive alleles disappear?
Mendel crossed F1 generation with itself and traits controlled
by recessive alleles reappeared in the F2 generation!!
P Generation
Tall
Short
F2 Generation
F1 Generation
Tall
Tall
Tall
Tall
Tall
Short
Mendel’s Model (His 4 Main Ideas)
• Mendel developed a hypothesis to explain the
3:1 inheritance pattern he observed in F2
offspring
• Four related concepts make up this model
• These concepts can be related to what we now
know about genes and chromosomes
© 2011 Pearson Education, Inc.
• First: alternative versions of genes account for variations in
inherited characters
– Ex: gene for flower color in pea plants exists in two
versions, one for purple flowers and the other for white
flowers
• These alternative versions of a gene are now called alleles
• Each gene resides at a specific locus on a specific
chromosome
Allele for purple flowers
Locus for flower-color gene
Pair of
homologous
chromosomes
Allele for white flowers
© 2011 Pearson Education, Inc.
• Second: for each character, an organism
inherits two alleles, one from each parent
– Mendel made this deduction without knowing
about the role of chromosomes
• The two alleles at a particular locus may be
identical, as in the true-breeding plants of
Mendel’s P generation, or they may differ
© 2011 Pearson Education, Inc.
• Third: if the two alleles at a locus differ, then one
(the dominant allele) determines the organism’s
appearance, and the other (the recessive allele)
has no noticeable effect on appearance
• In the flower-color example, the F1 plants had
purple flowers because the allele for that trait is
dominant
© 2011 Pearson Education, Inc.
• Fourth (now known as the law of segregation):
the two alleles for a heritable character separate
(segregate) during gamete formation and end up
in different gametes
– Thus, an egg or a sperm gets only one of the two
alleles that are present in the organism
© 2011 Pearson Education, Inc.
Law of Segregation
States that allele pairs separate during gamete
formation and randomly reunite at fertilization
Person with a
dominant and
recessive
produces two
types of
gametes
because they
separate!
Person with a
both alleles
the same
produces one
type of
gamete even
though they
separate!
Figure 14.6
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
How to discover genotype of unknown
flower
• How can we tell the genotype of an individual
with the dominant phenotype?
The Testcross
• How can we tell the genotype of an individual with
the dominant phenotype?
• Carry out a testcross: breeding the mystery
individual with a homozygous recessive individual
• If any offspring display the recessive phenotype,
the mystery parent must be heterozygous
© 2011 Pearson Education, Inc.
• The testcross
APPLICATION An organism that exhibits a dominant trait,
such as purple flowers in pea plants, can be either homozygous for
the dominant allele or heterozygous. To determine the organism’s
genotype, geneticists can perform a testcross.
TECHNIQUE In a testcross, the individual with the
unknown genotype is crossed with a homozygous individual
expressing the recessive trait (white flowers in this example).
By observing the phenotypes of the offspring resulting from this
cross, we can deduce the genotype of the purple-flowered
parent.
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
If PP,
then all offspring
purple:
If Pp,
then 2 offspring purple
and 1⁄2 offspring white:
p
1⁄
p
p
p
Pp
Pp
pp
pp
RESULTS
P
P
Pp
Pp
P
p
Pp
Figure 14.7
Pp
Probability
Likelihood that a particular event will occur
PRINCIPLES OF PROBABILITY CAN BE
USED TO PREDICT THE OUTCOME OF
GENETIC CROSSES… MENDEL WAS
ALWAYS ABLE TO PREDICT WHAT HIS
RESULTS SHOULD ROUGHLY BE!!
Figure 14.9
Rr
Segregation of
alleles into eggs

Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
Figure 14.UN01
Probability of YYRR  1/4 (probability of YY)  1/4 (RR)  1/16
Probability of YyRR  1/2 (Yy)
 1/4 (RR)  1/8
Figure 14.UN02
ppyyRr
ppYyrr
Ppyyrr
PPyyrr
ppyyrr
1/ (yy)  1/ (Rr)
(probability
of
pp)

4
2
2
1/  1/  1/
4
2
2
1/  1/  1/
2
2
2
1/  1/  1/
4
2
2
1/  1/  1/
4
2
2
1/
Chance of at least two recessive traits
 1/16
 1/16
 2/16
 1/16
 1/16
 6/16 or 3/8
Punnett Squares
Diagram used to show the gene combinations
that might result from a genetic cross
Parent alleles shown
on the TOP and
LEFT…
Letters are used to
represent different
ALLELES
CAPITAL letters =
DOMINANT alleles
lowercase letters =
recessive alleles
R
R
r
r
POSSIBLE gene
combinations of the
offspring appear in the four
boxes
Punnett Squares
ALLELES OF GENES:
• HOMOZYGOUS  Two identical alleles for a particular trait
EX: TT, tt, BB, bb, etc.
(TRUE BREEDING)
• HETEROZYGOUS  Two different alleles for a trait
EX: Tt, Bb, etc.
(HYBRID)
OUTCOMES:
• PHENOTYPE  physical characteristics or what you see in
the offspring of a cross
• GENOTYPE  genetic makeup or what combinations of alleles
form from a cross
Punnett Squares
Choose letters
that look
different upper /
lowercase
T = Tall
t = Short
1) Always use a trait KEY to
indicate what the letters you
are using stand for
2) Determine the genotypes
of the parents from the
problem
3) Offspring can be predicted
by doing the cross and filling
in the boxes!
4) List PHENOTYPIC ratio
____:____:____
T
T
T
TT
TT
t
Tt
Tt
100% Tall
50% TT 50% Tt
Monohybrid Crosses
Crosses involving only ONE gene
In sheep:
•The allele for white wool (A) is dominant
•The allele for black wool (a)
• A homozygous recessive mates with a heterozygous.
•Phenotypes/genotypes of the offspring?
A = White
a = Black
A
a
a
Aa
aa
50% White
50% Black
a
Aa
aa
50% Aa
50% aa
Monohybrid Crosses Practice
In guinea pigs, the allele for rough coat is dominant over
the allele for smooth coat. Cross a hybrid guinea pig with
a homozygous rough coated individual. (Show the key,
Punnett square, and phenotypes / genotypes)
Monohybrid Crosses Practice
In tomato plants, red fruit is dominant over yellow fruit.
A homozygous dominant plant is crossed with a true
breeding red fruited plant. What is the chance that a
yellow fruited tomato plant will form? (Show the key,
Punnett square, and phenotypes / genotypes)
Monohybrid Crosses Practice
In humans, being a tongue roller is dominant over nonroller. A man who is a non-roller marries a woman who is
homozygous for tongue rolling. (Show the key, Punnett
square, and phenotypes / genotypes)
Law of Independent Assortment
States that genes for different traits can
segregate (separate) independently during the
formation of gametes
Mendel discovered this after doing a dihybrid cross
(homozygous dominant round yellow with
homozygous recessive wrinkled green) and then
crossing the offspring (heterozygous for both traits)
Law of Independent Assortment
Helps account for the many genetic variations
observed in plants, animals, and other organisms!
Dihybrid Crosses
• Chalkboard practice cross
• Two heterozygous round, yellow pea plants
are crossed.
1. Determine parent genotype
2. Determine possible gametes using FOIL
method
3. Complete Punnett square, showing work
4. Write out phenotypic ratio
Summary of Mendel’s Ideas
• Genes are passed from parents to their offspring
• If two or more forms (alleles) of the gene for a single trait exist, some
forms of the gene may be dominant and others may be recessive (Law of
Dominance)
• In most organisms, each adult has two copies of each gene and these genes
are segregated from each other when gametes are formed (Law of
Segregation)
• Alleles for different genes usually segregate independently of one another
(Law of Independent Assortment)
Even though Mendel is right, there are
SOME exceptions to his principles…
Make A Baby
• Choose a partner
• Materials:
–
–
–
–
–
Lab packet (DO NOT WRITE ON IT)
2 pennies
Lab notebook
Paper
Colored pencils, crayons, or markers
Extending Mendelian Genetics for a Single
Gene
• Inheritance of characters by a single gene may
deviate from simple Mendelian patterns in the
following situations:
– When alleles are not completely dominant or
recessive
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
© 2011 Pearson Education, Inc.
Degrees of Dominance
• Complete dominance occurs when phenotypes
of the heterozygote and dominant homozygote are
identical
• In incomplete dominance, the phenotype of F1
hybrids is somewhere between the phenotypes of
the two homozygous parents (blending)
• In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
© 2011 Pearson Education, Inc.
Figure 14.10-1
P Generation
Incomplete
Dominance
White
CWCW
Red
CRCR
Gametes
CR
CW
Figure 14.10-2
P Generation
Incomplete
Dominance
White
CWCW
Red
CRCR
Gametes
CR
CW
F1 Generation
Gametes 1/2 CR
Pink
CRCW
1/
2
CW
Figure 14.10-3
P Generation
Incomplete
Dominance
White
CWCW
Red
CRCR
CR
Gametes
CW
F1 Generation
Pink
CRCW
1/
Gametes 1/2 CR
2
CW
Sperm
F2 Generation
1/
2
CR
1/
2
CW
Eggs
1/
2
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
Incomplete Dominance
When doing crosses for these problems, you MUST include
the intermediate in your key and TWO different letters can
be used for the dominant and recessive alleles (don’t have
to)… Otherwise, the problems are the same as a
monohybrid cross.
CRCR = Red
CRCW = Pink
CWCW = White
Incomplete Dominance
A small bird species has a gene that determines beak
size. In these birds, large beaks help crush big seeds,
small beaks are used to pick seeds from pine cones, and
intermediate beaks help consume both types of seeds.
Cross a large beaked bird with an intermediate beaked
bird. What will the offspring look like?
Incomplete Dominance
The lubber grasshopper is a large grasshopper that can
have yellow and red stripes. Assume red stripes are
expressed from the homozygous SRSR genotype, yellow
stripes from the SySy genotype, and both from the
heterozygous genotype. Cross two heterozygous
grasshoppers and predict their offspring.
Codominance
Both alleles contribute to the phenotype or both
can be seen at the same time (NOT a blending)
Ex: In certain varieties of chicken, the allele for
black feathers is codominant with the allele for
white feathers… Heterozygous chickens have
BOTH black and white feathers!
Roan cattle/horse:
Codominance
Multiple Alleles
Many genes have more than two alleles in a population
(each individual only has two alleles, but more exist)
EX: Rabbit coat color is
determined by a gene that has at
least four different alleles
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
c=
albino; no color;
recessive to all other
alleles
Multiple Alleles
• Most genes exist in populations in more than two
allelic forms
• For example, the four phenotypes of the ABO
blood group in humans are determined by three
alleles for the enzyme (I) that attaches A or B
carbohydrates to red blood cells: IA, IB, and i.
• The enzyme encoded by the IA allele adds the A
carbohydrate, whereas the enzyme encoded by
the IB allele adds the B carbohydrate; the enzyme
encoded by the i allele adds neither
© 2011 Pearson Education, Inc.
Blood Group Genes
Blood genes are a mix of codominance and multiple alleles
• Humans can have A, B, AB, or O blood type
• Multiple Alleles (3 of them)  IA, IB, or i / A, B, O
• Codominance  IA and IB are codominant
Figure 14.11
(a) The three alleles for the ABO blood groups and their
carbohydrates
IA
Allele
Carbohydrate
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
Blood Group Genes
Blood Type
Genotype
Antigen on
RBC
Safe Transfusions
To
From
A
IAIA or IAi
AA or AO
A
A, AB
A, O
B
IBIB or IBi
BB or BO
B
B, AB
B, O
AB
IAIB
AB
A and B
AB
A, B, AB, O
O
ii
OO
None
A, B, AB, O
O
When transfusing blood, a person’s blood type MUST be known
and only certain types can be given… WHY?
What blood type is the UNIVERSAL DONOR?
What blood type is the UNIVERSAL ACCEPTOR?
Blood Group Genes
A woman is homozygous for type A blood and a
man has type AB blood. What is the probability
that the couple’s child will have type B blood?
Two individuals with type AB blood have a child.
What are the chances that their child will have
type O blood?
A man with type O blood marries a woman who is
heterozygous type A blood. What would be the
blood type possibilities of their offspring?
Pleiotropy
• Most genes have multiple phenotypic effects, a
property called pleiotropy
• For example, pleiotropic alleles are responsible for
the multiple symptoms of certain hereditary
diseases, such as cystic fibrosis and sickle-cell
disease
© 2011 Pearson Education, Inc.
Extending Mendelian Genetics for Two or
More Genes
• Some traits may be determined by two or more
genes
© 2011 Pearson Education, Inc.
Epistasis
• In epistasis, a gene at one locus alters the
phenotypic expression of a gene at a second
locus
• For example, in Labrador retrievers and many
other mammals, coat color depends on two
genes
• One gene determines the pigment color (with
alleles B for black and b for brown)
• The other gene (with alleles C for color and c
for no color) determines whether the pigment
will be deposited in the hair
© 2011 Pearson Education, Inc.
Figure 14.12
BbEe
Eggs
1/
4 BE
1/
4 bE
1/
4 Be
1/
4
be
Sperm
1/ BE
4
1/
BbEe
4 bE
1/
4 Be
1/
4 be
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
9
: 3
: 4
Polygenic Inheritance
• Quantitative characters are those that vary in the
population along a continuum
• Quantitative variation usually indicates polygenic
inheritance, an additive effect of two or more
genes on a single phenotype
• Skin color in humans is an example of polygenic
inheritance
© 2011 Pearson Education, Inc.
Figure 14.13
AaBbCc
AaBbCc
Sperm
1/
1/
8
8
1/
1/
Eggs
8
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
1/
8
1/
1/
8
1/
8
1/
8
1/
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
64
3
15/
64
4
6/
64
5
1/
64
6
Nature and Nurture: The Environmental
Impact on Phenotype
• Another departure from Mendelian genetics
arises when the phenotype for a character
depends on environment as well as genotype
• The norm of reaction is the phenotypic range
of a genotype influenced by the environment
• For example, hydrangea flowers of the same
genotype range from blue-violet to pink,
depending on soil acidity
© 2011 Pearson Education, Inc.
Figure 14.14
• Norms of reaction are generally broadest for
polygenic characters
• Such characters are called multifactorial
because genetic and environmental factors
collectively influence phenotype
© 2011 Pearson Education, Inc.
Concept 14.4: Many human traits follow
Mendelian patterns of inheritance
• Humans are not good subjects for genetic
research
– Generation time is too long
– Parents produce relatively few offspring
– Breeding experiments are unacceptable
• However, basic Mendelian genetics endures
as the foundation of human genetics
© 2011 Pearson Education, Inc.
How Is Sex Determined?
50 / 50
CHANCE!
All human eggs cells carry a single X chromosome,
while half of the sperm cells carry an X and the
other half carry a Y chromosome!
Sex-Linked Genes
Genes located on the sex chromosomes
Y chromosome is much
smaller than the X
chromosome and
appears to contain
only a few genes!
Why are SEX-LINKED
disorders more
common in males than
females?
Sex-Linked Genes
• Females have TWO X chromosomes, thus there must be
two copies of a recessive allele for it to be expressed!
• Males have only ONE X chromosome, so only one copy of a
recessive allele causes the expression!
EX: Male colorblindness (1 : 10) results when a defective
version of the gene is present, while female
colorblindness (1 : 100) only results when two
defective versions are present!
Sex-Linked Cross
Similar to other crosses we have
done… just remember that the
SEX of the individual is
important and that males only
need ONE recessive allele to
have a trait!
Sex-Linked Cross
Two important genes on the X chromosome help
control blood clotting. A person who has
HEMOPHILIA is lacking a protein necessary for
normal blood clotting. About 1 : 10,000 males is
born with the disease and this can be treated with
protein injections. People with this disease can
bleed to death from minor cuts or may suffer
internal bleeding!
Cross a carrier mother with a man who has
hemophilia. What are the chances of having a
daughter with the disease?
Sex-Linked Cross
In the U.S., one out of 3,000 males is born with
Duchenne Muscular Dystrophy, a disorder that
results in the weakening and loss of skeletal
muscle. This is caused by a defective muscle
protein.
Cross a normal male with a carrier female. What
are the chances that the couple will have a child
with the disease? What are the chances that dad
will give the disease to a son?
X-Chromosome Inactivation
To adjust to the extra X chromosome in females, one X
chromosome is RANDOMLY switched off to become a Barr
body (dense region by nuclear envelope)
WHY does this female cat have multiple, random
colors being expressed?
Pedigree Analysis
• 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
© 2011 Pearson Education, Inc.
Pedigree Charts
Shows the relationships within a family and indicates
the genotypes for a certain trait
A horizontal A circle
represents
line
connecting
A shaded
a female.
a
male
and
a
circle or
female
square
represents a
indicates
that marriage.
a person
expresses
Half filled
the
trait.
circle
or
square
represents
a carrier!
A square
represents
a male. A vertical
line and a
A circle or
bracket
square that
connect
is not
the
shaded
parents
to
indicates
their
that a
children.
person does
not express
the trait.
Figure 14.15
Key
Male
1st
generation
Affected
male
Female
Affected
female
Mating
1st
generation
Ww
ww
Ww
ww
2nd
generation
Ww
ww
3rd
generation
WW
or
Ww
Widow’s
peak
ff
ff
(a) Is a widow’s peak a dominant or
recessive trait?
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd
generation
ww
No widow’s
peak
ff
Ff
2nd
generation
FF or Ff
Ww ww ww Ww
Ff
Offspring
Attached
earlobe
Free
earlobe
b) Is an attached earlobe a dominant
or recessive trait?
• Pedigrees can also be used to make
predictions about future offspring
• We can use the multiplication and addition
rules to predict the probability of specific
phenotypes
© 2011 Pearson Education, Inc.
• Tay-Sachs disease is fatal; a dysfunctional
enzyme causes an accumulation of lipids in the
brain
– At the organismal level, the allele is recessive
– At the biochemical level, the phenotype (i.e.,
the enzyme activity level) is incompletely
dominant
– At the molecular level, the alleles are
codominant
© 2011 Pearson Education, Inc.
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more
common in populations than recessive alleles
• For example, one baby out of 400 in the United
States is born with extra fingers or toes
– Polydactyl
• The allele for this unusual trait is dominant to the
allele for the more common trait of five digits per
appendage
• In this example, the recessive allele is far more
prevalent than the population’s dominant allele
© 2011 Pearson Education, Inc.
Recessively Inherited Disorders
• Many genetic disorders are inherited in a
recessive manner
• These range from relatively mild to lifethreatening
© 2011 Pearson Education, Inc.
The Behavior of Recessive Alleles
• 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; most individuals with recessive
disorders are born to carrier parents
• Albinism is a recessive condition characterized
by a lack of pigmentation in skin and hair
© 2011 Pearson Education, Inc.
Figure 14.16
Parents
Normal
Aa
Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
• If a recessive allele that causes a disease is
rare, then the chance of two carriers meeting
and mating is low
• Consanguineous matings (i.e., matings
between close relatives) increase the chance
of mating between two carriers of the same
rare allele
• Most societies and cultures have laws or
taboos against marriages between close
relatives
© 2011 Pearson Education, Inc.
Common Human Disorders
• HUMAN GENOME PROJECT  Goal was to map the
entire sequence of human genes… completed in 2003!
Common Human Disorders
CYSTIC FIBROSIS
Missing three bases…
Phenylalanine is missing
Normal CFTR allows Cl- ions
to pass… In CF, CFTR
folded wrong and does not
get put in membrane
Cells cannot transport
Cl- ions, airways
clogged with mucus
Cystic Fibrosis
• Cystic fibrosis is the most common lethal
genetic disease in the United States,striking
one out of every 2,500 people of European
descent
• The cystic fibrosis allele results in defective or
absent chloride transport channels in plasma
membranes leading to a buildup of chloride
ions outside the cell
• Symptoms include mucus buildup in some
internal organs and abnormal absorption of
nutrients in the small intestine
© 2011 Pearson Education, Inc.
Common Human Disorders
SICKLE CELL
• Characterized by bent and twisted shape of red blood cells
• In normal cells, HEMOGLOBIN is the protein that carries oxygen
• In sickle cell, one base is changed, causing abnormal hemoglobin…
during low levels of oxygen, some RBC become sickle shaped and
can stick together!
Common Human Disorders
SICKLE CELL
Common Human Disorders
SICKLE CELL
Why do so many African Americans carry the
sickle cell allele?
MALARIA  Disease that infects RBC and common in
Africa....
Individuals HETEROZYGOUS for the disease are also
RESISTANT to malaria. Low oxygen levels cause some
RBC to become sickle shaped… body destroys these cells
AND the parasite in the process!
BEING HETEROZYGOUS FOR THE DISEASE IS
ACTUALLY BENEFICIAL!!!!!
Common Human Disorders
SICKLE CELL
Why do so many African Americans carry the
sickle cell allele?
Regions where malaria is
common
Regions where the sickle cell
allele is common
Sickle-Cell Disease: A Genetic Disorder with
Evolutionary Implications
• Sickle-cell disease affects one out of 400
African-Americans
• The disease is caused by the substitution of a
single amino acid in the hemoglobin protein in
red blood cells
• In homozygous individuals, all hemoglobin is
abnormal (sickle-cell)
• Symptoms include physical weakness, pain,
organ damage, and even paralysis
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Fig. 14-UN1
• Heterozygotes (said to have sickle-cell trait) are
usually healthy but may suffer some symptoms
• About one out of ten African Americans has
sickle cell trait, an unusually high frequency of
an allele with detrimental effects in
homozygotes
• Heterozygotes are less susceptible to the
malaria parasite, so there is an advantage to
being heterozygous
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Common Human Disorders
• NONDISJUNCTION  Failure
of homologous chromosomes to
separate during meiosis (abnormal
#s of chromosomes can result)
EXAMPLES
-Down Syndrome (three copies of
chromosome 21) aka “Trisomy 21”
-Turner’s Syndrome (only one X
chromosome)  Sterile
-Klinefelter’s Syndrome (extra X
in males, XXY)  Sterile
Dominantly Inherited Disorders
• Some human disorders are caused by
dominant alleles
• Dominant alleles that cause a lethal disease
are rare and arise by mutation
• Achondroplasia is a form of dwarfism caused
by a rare dominant allele
© 2011 Pearson Education, Inc.
Figure 14.17
Parents
Dwarf
Dd
Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Normal
Eggs
Huntington’s Disease: A Late-Onset Lethal
Disease
• The timing of onset of a disease significantly
affects its inheritance
• Huntington’s disease is a degenerative disease
of the nervous system
• The disease has no obvious phenotypic effects
until the individual is about 35 to 40 years of age
• Once the deterioration of the nervous system
begins the condition is irreversible and fatal
© 2011 Pearson Education, Inc.
Multifactorial Disorders
• Many diseases, such as heart disease,
diabetes, alcoholism, mental illnesses, and
cancer have both genetic and environmental
components
• Little is understood about the genetic
contribution to most multifactorial diseases
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Genetic Testing and Counseling
• Genetic counselors can provide information to
prospective parents concerned about a family
history for a specific disease
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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
• Probabilities are predicted on the most
accurate information at the time; predicted
probabilities may change as new information
is available
© 2011 Pearson Education, Inc.
Tests for Identifying Carriers
• For a growing number of diseases, tests are
available that identify carriers and help define the
odds more accurately
© 2011 Pearson Education, Inc.
Figure 14.18
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
• Other techniques, such as ultrasound and
fetoscopy, allow fetal health to be assessed
visually in utero
Video: Ultrasound of Human Fetus I
© 2011 Pearson Education, Inc.
Figure 14.19
(a) Amniocentesis
1
(b) Chorionic villus sampling (CVS)
Ultrasound monitor
Amniotic
fluid
withdrawn
Ultrasound
monitor
Fetus
1
Placenta
Chorionic villi
Fetus
Placenta
Uterus
Cervix
Cervix
Uterus
Suction
tube
inserted
through
cervix
Centrifugation
Fluid
Fetal
cells
Several hours
2
Several
weeks
Biochemical
and genetic
tests
Several
hours
Fetal cells
2
Several hours
Several weeks
3
Karyotyping
Newborn Screening
• Some genetic disorders can be detected at birth
by simple tests that are now routinely performed
in most hospitals in the United States
© 2011 Pearson Education, Inc.
Figure 14.UN06