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BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 9
Patterns of Inheritance
Modules 9.11 – 9.23
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
VARIATIONS ON MENDEL’S PRINCIPLES
The relationship of genotype to phenotype is rarely
simple
• Mendel’s principles are valid for all sexually
reproducing species
– However, often the genotype does not dictate the
phenotype in the simple way his principles
describe
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1. Incomplete dominance results in intermediate
phenotypes
• When an offspring’s
phenotype—such
as flower color— is
in between the
phenotypes of its
parents, it exhibits
incomplete
dominance
P GENERATION
White
rr
Red
RR
Gametes
R
r
Pink
Rr
F1 GENERATION
1/
1/
Eggs
1/
F2 GENERATION
2
2
2
R
1/
2
r
1/
R
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R
Red
RR
r
Pink
Rr
Sperm
1/
Pink
rR
White
rr
Figure 9.12A
2
2
r
• Incomplete dominance in human
hypercholesterolemia
GENOTYPES:
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
PHENOTYPES:
LDL
LDL
receptor
Cell
Normal
Mild disease
Figure 9.12B
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Severe disease
2. Many genes have more than two alleles in the
population
• In a population, multiple alleles often exist for a
characteristic
– The three alleles for ABO blood type in humans
is an example
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– 3. Codominance--The alleles for A and B blood
types are codominant, and both are expressed in
the phenotype
Blood
Group
(Phenotype)
Genotypes
Antibodies
Present in
Blood
Reaction When Blood from Groups Below Is Mixed with
Antibodies from Groups at Left
O
O
ii
Anti-A
Anti-B
A
IA IA
or
IA i
Anti-B
B
IB IB
or
IB i
Anti-A
AB
IA IB
Figure 9.13
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A
B
AB
Calico coloring is
a mix of
phaeomelanin
based colors (red)
and eumelanin
based color
(black, chocolate
and cinnamon).
Cats of this
coloration are
believed to bring
good luck in the
folklore of many
cultures.[1]
The spotting gene causes white patches to cover
the colored fur
The Calico Cat-codominance
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4. A single gene may affect many phenotypic
characteristics
• A single gene may affect phenotype in many
ways
– This is called pleiotropy
– The allele for sickle-cell disease is an example
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Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of red
blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Pain and
fever
Paralysis
Brain
damage
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
Figure 9.14
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5. A single characteristic may be influenced by
many genes
• This situation creates a continuum of
phenotypes
– Example: skin color
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P GENERATION
aabbcc
AABBCC
(very light) (very dark)
F1 GENERATION
Eggs
Sperm
Fraction of population
AaBbCc AaBbCc
Skin pigmentation
F2 GENERATION
Figure 9.16
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9.15 Connection: Genetic testing can detect
disease-causing alleles
• Genetic testing can be of
value to those at risk of
developing a genetic disorder
or of passing it on to offspring
Figure 9.15B
• Dr. David Satcher, former U.S.
surgeon general, pioneered
screening for sickle-cell disease
• Thallasemia, Cystic Fibrosis,
Tay Sachs,
Figure 9.15A
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Canavan Disease -- This condition is most common in people of Ashkenazi Jewish ancestry,
with a carrier incidence of 1 in 40. Canavan disease is a central nervous system disease that is
usually fatal in childhood, with a few people surviving to adulthood. This disease is the result of
a substance that destroys the central nervous system over time. There is presently no effective
treatment for Canavan disease.
Fragile X Syndrome -- The Fragile X syndrome is not specific to a certain ethnic background.
It is an inherited condition that can cause a range of intellectual and behavioral problems, from
learning disabilities to mental retardation to autism. While Fragile X syndrome tends to be
more severe in boys, it occurs in both males or females. It can be passed on to family members
by individuals who have no signs of the syndrome. Review of your family history with a genetic
counselor may help determine if Fragile X carrier testing is indicated.
Sickle Cell Disease -- This condition is most common in persons of African-American,
African, Mediterranean, Hispanic and South American ancestry, with the carrier risk ranging
from 1/10 to 1/40, depending on your ethnic background. Sickle cell disease is caused by a
variant hemoglobin that changes the shape of the red blood cells. This causes anemia, severe
pain, a tendency toward infection, and other serious health problems. Frequent blood
transfusions and infection preventing antibiotics are available treatment.
Tay Sachs Disease -- People of both Ashkenazi Jewish and French Canadian ancestry have
the greatest chance of being carriers of Tay Sachs disease, about 1/30 versus 1/250 in the
general population. The disease results from a build up of certain substances in the brain, and is
fatal in early childhood. There is presently no effective treatment for Tay Sachs disease.
Thalassemia -- Individuals of Mediterranean, Southeast Asian and African ancestry have the
greatest chance - 1 in 3 and 1 in 30, respectively -- of being carriers for thalassemia. In general,
this group of blood disorders affects a person's ability to produce hemoglobins, the protein in
our blood that carries oxygen and nutrients to all parts of the body. In severe cases, children
with thalassemia may not survive. Others have anemia, bone growth problems and liver and
spleen involvement. Blood transfusions may be needed for treatment.
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Ethnic disorders continued
• Cystic fibrosis (CF) is a progressive disorder that causes the body to
produce an abnormally thick, sticky mucus which is present in the
lungs and digestive system. There are a variety of symptoms
including frequent respiratory infections, poor weight gain, and
progressive lung damage. Treatment of CF depends upon the stage
of the disease and the organs involved. The condition is life
shortening. The average age of death is in the early 30's. Although
CF is no more common among Ashkenazi Jews than it is among
other caucasians, it is one of the most common genetic disorders
among Jews and non-Jews alike.
• Disease frequency: One in every 3,200 live Caucasian births.
Carrier frequency:Approximately 1 in 25 in Caucasians and similar
frequency in those of Jewish ancestry. Diagnosis:By measuring
amount of salt in sweat ("sweat test") or by testing the CF gene.
Inheritance:Autosomal Recessive Carrier testing:Available by
testing the CF gene. Prenatal diagnosis:Available by testing the CF
gene
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Prenatal diagnosis for couples testing
positive after genetic testing
• Chorionic villus sampling at 8 weeks of
gestation.
• Amniocentesis at 12-18 weeks of gestation.
• Pre-implantation genetic testing and In vitro
fertilization
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THE CHROMOSOMAL BASIS OF
INHERITANCE
9.17 Chromosome behavior accounts for Mendel’s
principles
• Genes are located on chromosomes
– Their behavior during meiosis accounts for
inheritance patterns
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• The chromosomal basis of Mendel’s principles
Figure 9.17
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Question?
• What is the phenotypic ratio between a
dihybrid cross involving two heterozygotes?
•
9:3:3:1
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9.18 Genes on the same chromosome tend to be
inherited together
• Certain genes are linked
– They tend to be inherited together because they
reside close together on the same chromosome
When would
they not be
inherited
together?
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If you cross PpLl x PpLl, what
Phenotypic ratio do you
expect?
When the organism was
selved or self pollinated, most
of the progeny looked like the
parent but the 9:3:3:1 ratio
was not realized!! Linked
genes were then suspected.
Figure 9.18
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9.19 Crossing over produces new combinations of
alleles
• This produces gametes with recombinant
chromosomes
• The fruit fly Drosophila melanogaster was used
in the first experiments to demonstrate the
effects of crossing over
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A
B
a
b
a
B
A B
a
b
Tetrad
A
b
Crossing over
Gametes
When does
crossing over
take place?
Figure 9.19A, B
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Question?
• What is the phenotypic ratio between a
dihybrid cross involving two heterozygotes?
•
9:3:3:1
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When you cross a di-hybrid
heterozygote with a homozygous
recessive, you expect a ¼
distribution of each potential
phenotype.
GgLl X ggll
When this particular cross was
performed, this was not the case.
Most of the organisms resembled
the parents. A few had the
independently assorted phenotypes.
What had occurred is that genes for
body color and wing type were on
the same chromosome (linked) and
in some gametes, crossing over did
occur, but in most gamestes, G and
L were linked as were g and l.
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Figure 9.19C
Crossing Over and Linkage
Is crossing over
more likely to
occur between
genes that are
close together
or farther
apart?
Does crossing
over comply
with Mendel’s
Law of
Independent
Assortment?
Law of
Segregation?
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9.20 Geneticists use crossover data to map genes
• Crossing over is more likely to occur between genes that are
farther apart
– Recombination frequencies can be used to map the
relative positions of genes on chromosomes.
– Determine the location of g and c and l on the
chromosome. Recombination frequency between g and c
is 9%, between c and l is 9.5%, and between g and l is
17%.
Chromosome
g
c
l
17%
9%
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9.5%
Figure 9.20B
answer
• Answer: 3.6% recombination
• 3.6 map units
• 3.6 centiMorgans
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• In corn C (colored) is dominant to c (colorless) and for
the endosperm (part of seed where food is stored for
embryo) Full (F) is dominant to shrunken (f). When
an CcFf was test crossed, the results were as follows:
• Colored, full 4032
• colored, shrunken 149
• Colorless, full 152
• Colorless, shrunken 4035
• What do you expect if the genes are not linked? CcFf x
ccff? 1:1:1:1
• We got 27:1:1:27
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• The Chinese primrose-slate color (s) is recessive to
blue (S), red stigma(r) is recessive to green stigma (R),
and long style (l) is recessive to short style (L). All
three genes are on the same chromosome. The F1 of a
cross of true breeding strains, was test crossed and
gave the following:
•
Slate, green, short
27
•
Slate, red, short
85
•
Blue, red short
402
•
Slate, red, long
977
•
Slate, green, long
427
•
Blue, green, long
95
•
Blue, green, short
960
•
Blue, red ,long
27
• Total
3000
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Answer the following questions:
• What were the genotypes of the parents in the
cross of the two true-breeding strains?
• Make a map of the genes, showing gene order
and distance between them.
• Answer: hum??????????????????
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• slate (s)
red (r)
long (l)
• Blue (S)
Green (R)
short (L)
• SsRrLl x ssrrll Expect 1:1:1:1 If not expect crossing over.
• To determine placement of genes on chromosomes:
– Inspect for highest frequencies for parental phenotypes. (NO
crossing over)
– Inspect for other phenotypes to show single and double crossing
over. (Lowest # is double crossing over)
– Determine position of genes on chromosome. The gene that has
changed position relative to the other two is the central (observe
double cross overs only).
– Designate regions I and II and calculate crossing over in those
regions. Add all combinations in each region (both single cross
over and double) to determine cross over frequency in that region.
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• Blue, Green, Short and slate, red, long are parental
phenotypes
• Blue, red, long and slate, Green, Sort are the double
crossovers.
• Flower color (Blue vs slate)has changed position
relative to stigma and style. “S: is in the middle. Rsl
and RSL
• Region I ---Rsl and rSL + RsL and rSl = 427 + 402 +
27 + 27 = 883/3000 = 29.43%
• Region II ---RSl and rsL + RsL and rSl = 95 + 85 + 27
+ 27 = 234/3000 = 7.8%
• Thus, R----------------S-----L
•
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answer
• Between r and s the distance is 29.4 map units
• Between s and l is 7.8 map units.
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• Alfred H. Sturtevant, seen here at a party with
T. H. Morgan and his students, used
recombination data from Morgan’s fruit fly
crosses to map genes
Figure 9.20A
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• A partial genetic map of a fruit fly chromosome
Mutant phenotypes
Short
aristae
Black
body
(g)
Long aristae
(appendages
on head)
Gray
body
(G)
Cinnabar
eyes
(c)
Red
eyes
(C)
Vestigial
wings
(l)
Brown
eyes
Normal
wings
(L)
Red
eyes
Wild-type phenotypes
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Figure 9.20C
SEX CHROMOSOMES AND SEX-LINKED
GENES
9.21 Chromosomes determine sex in many species
• A human male has one X chromosome and one
Y chromosome
• A human female has two X chromosomes
• Whether a sperm cell has an X or Y
chromosome determines the sex of the
offspring
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(male)
(female)
Parents’
diploid
cells
X
Y
Male
Sperm
Egg
Offspring
(diploid)
Figure 9.21A
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• Other systems of sex determination exist in
other animals and plants
– The X-O system
– The Z-W system
– Chromosome number
Figure 9.21B-D
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9.22 Sex-linked genes exhibit a unique pattern of
inheritance
• All genes on the sex chromosomes are said to be
sex-linked
– In many organisms, the X chromosome carries
many genes unrelated to sex
– Fruit fly eye
color is a
sex-linked
characteristic
Figure 9.22A
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– Their inheritance pattern reflects the fact that
males have one X chromosome and females
have two
– These figures illustrate inheritance patterns for
white eye color (r) in the fruit fly, an X-linked
recessive trait
Female
XRXR
Male
Xr Y
XR
Female
XRXr
Xr
XRXr
Male
XRY
XRY
Xr
XRXR
XrXR
XRY
XrY
R = red-eye allele
r = white-eye allele
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Male
XRXr
XR
XR
Y
Female
XrY
Xr
XR
Y
Xr
XRXr
Xr Xr
Y
XRY
XrY
Figure 9.22B-D
9.23 Connection: Sex-linked disorders affect
mostly males
• Most sex-linked human
disorders are due to
recessive alleles
– Examples: hemophilia,
red-green color blindness
– These are mostly seen in males
Figure 9.23A
– A male receives a single X-linked allele from his
mother, and will have the disorder, while a
female has to receive the allele from both
parents to be affected
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• A high incidence of hemophilia has plagued the
royal families of Europe
Albert
Queen
Victoria
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
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