10.6 Gene Expression Can Appear to Alter Mendelian Ratios
A. Incomplete Dominance and Codominance Add Phenotype Classes
• Incomplete dominance– Heterozygote has 3rd
intermediate phenotype– Crossing of 2 pink
snapdragons yields red, white or pink offspring
Figure 10.14 Incomplete Dominance.
×
Parentalgeneration
r1r1
Red flowers
F1 generation
r2r2
White flowers
All pink flowers
r1r2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.14 Incomplete Dominance.
r1 r2
r 1
r 2
Female gametesF2 generation
r1r1 r1r2
r1r2 r2r2
Red (r1r1) : 25% chancePink (r1r2) : 50% chanceWhite (r2r2) : 25% chance
Mal
e g
amet
es
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
10.6 Gene Expression Can Appear to Alter Mendelian Ratios
A. Incomplete Dominance and Codominance Add Phenotype Classes
• Codominance– 2 different alleles expressed together– Human ABO blood type
• I gene – IA, IB, I – 3 alelles• A and B are codominant
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.15 Codominance.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
BB
B
B
B
B
AB
A
B
B
A
AA
AA
A
A
None Type Oii
Both A and B Type ABIAIB
IBIB
IBiOnly B Type B
IAIA
IAiOnly A Type A
ABO blood typeSurface molecules
PhenotypesGenotypes
Clicker Question
• A woman who has type O blood has a son with type O blood. Who below CANNOT be the father?
A.A man with type A bloodB.A man with type O bloodC.A man with type AB bloodD.A man with type B blood
10.6 Gene Expression Can Appear to Alter Mendelian Ratios
B. Some Inheritance Patterns Are Especially Difficult to Interpret
• Pleiotropy– One gene has multiple effect on the phenotype– One protein important in different pathways or
tissues– Marfan syndrome– Porphyria variegata
Figure 10.16 Pleiotropy.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Porphyrin accumulates in the...
Urine Digestivesystem
Nervoustissue
Muscles
Resulting in...
Delirium
Stupor
Madbehavior
ConvulsionsWeak limbsDark-
coloredurine
Abdominalpain
Constipation
Rapid pulse
10.6 Gene Expression Can Appear to Alter Mendelian Ratios
B. Some Inheritance Patterns Are Especially Difficult to Interpret
• Protein interactions– Can cause same phenotype from different
mutations– Epistasis- one gene’s product affects the
expression of another gene• ABO blood type inconsistencies• Paternity confusions
Figure 10.17 Epistasis.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
AA
A
A
A
A
Genotypes Phenotypes
ABO Gene H Gene
encodes
HH or Hh
encodes
Type A
Molecule Aattaches
to
Molecule H
Molecule Acannotattach
Molecule Habsent
mutanthh
Type O
IAIA
IAIA
encodes
10.6 Mastering Concepts
Compare and contrast dominant, recessive, incomplete dominant, and
codominant
10.7 Sex-Linked Genes Have Unique Inheritance Patterns
• Huntington Disease, cystic fibrosis and others from genes found on autosomes
– Affect both sexes equally• Red-green color blindness, hemophilia genes
are found on sex chromosomes– Affect one sex more than another– Called sex-linked
10.7 Sex-Linked Genes Have Unique Inheritance Patterns
A. X and Y Chromosomes Determine Sex in Humans
• Females XX• Males XY
Figure 10.18 The Sperm Determines the Sex of the Baby.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
X
Y
Mal
e g
amet
es
X
Y
X X
X X
SEM (false color) 2 µm Girl (XX) : 50% chanceBoy (XY) : 50% chance
SEM (false color) 2 µm
Female gametes
XXXX
XYXY
GirlGirl
BoyBoy
© Andrew Syred/Photo Researchers
10.7 Sex-Linked Genes Have Unique Inheritance Patterns
A. X and Y Chromosomes Determine Sex in Humans
• Y chromosome plays largest role in sex determination
– SRY gene– Few Y-linked disorders
• X-linked traits more common
10.7 Sex-Linked Genes Have Unique Inheritance Patterns
B. X-Linked Recessive Disorders Affect More Males Than Females
• Thomas Hunt Morgan and fruit flies• Male white-eyed flies
– Gene must be on X chromosome– Sex of parent mattered in crosses
Figure 10.19 Fly Eyes.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ma
le w
ith
wh
ite
ey
es
P
a. Cross of true breeding white-eyed male with red-eyed female
F1
X w
X W
Y
X W
X W X w X W X w
X W Y X W Y
W
w
Dominant allele; encodes red eyes
Recessive allele; encodes white eyes
X W
Y
X W X W X w
X W Y
X w
X W X W
YX w
P
b. Cross of true breeding red-eyed male with white-eyed female
F1
Y
X W X w X W X w
W
w
Dominant allele; encodes red eyes
Recessive allele; encodes white eyes
X W
Y
X W X w
X W Y Y
X w
X w
X w X w
X W
YX wYX w
X w X wX w
Female with red eyes Female with white eyes
Ma
le w
ith
wh
ite
ey
es
All females have red eyesAll males have white eyes
All off springHave red eyes
Female with red eyes
Ma
le w
ith
wh
ite
ey
es
All females have red eyes50% of males have red eyes
50% of males have white eyes
Female with red eyes
50% of females have red eyes50% of females have white eyes
50% of males have red eyes50% of males have white eyes
Ma
le w
ith
wh
ite
ey
es
10.7 Sex-Linked Genes Have Unique Inheritance Patterns
B. X-Linked Recessive Disorders Affect More Males Than Females
• Female shows recessive X-linked trait only if she inherits 2 recessive alleles
• Male expresses whatever is on his only X• Hemophilia A
– X-linked recessive inheritance
Figure 10.20 Hemophilia A.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
XH
XH Xh
YFa
the
r: h
eal
thy
Healthy daughter, noncarrier (XHXH): 25% chanceHealthy daughter, carrier (XHXh): 25% chanceHealthy son (XHY): 25% chanceAffected son (XhY): 25% chance
Mother: heterozygous
Female gametes
Healthydaughter(carrier)
Ma
le g
am
ete
s
Son withhemophilia
Healthyson
Healthydaughter
XHXHXHXh
XHY XhY
10.7 Sex-Linked Genes Have Unique Inheritance Patterns
C. X Inactivation Prevents “Double Dosing” of Proteins
• Males have a “double dose” of every gene on X
• Female cells shut off one X– Barr body– Random which X shut down– Calico and tortoiseshell cats are always female
Figure 10.21X Inactivation.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
X
X
a.
b.
Blackinactivation
Orangeinactivation
a: © William E. Ferguson; b: © Horst Schaefer/Peter Arnold/Photolibrary
10.7 Mastering Concepts
Why do males and females express recessive X-linked alleles differently?
10.8 Pedigrees Show Modes of Inheritance
• Genes on autosomes exhibit– Autosomal dominant
• Appears in every generation
– Autosomal recessive• May seem to skip generations• Carriers have normal phenotype
• X-linked conditions have unique patterns• Pedigrees can be useful
Figure 10.22 Pedigrees Reveal Mode of Inheritance.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
II
21 1 2 3 4
1 2 3
1 2 3
4 5
I
2 3 4 5 6 7
1 2 3 4 5
1 8
21
2 3 4 5 6
1 2 3 4 5
1
a. Achondroplasia (autosomal dominant)
Female
Male
Normal Carrier Affected
II
III III
II
III
II
b. Albinism (autosomal recessive) c. Red-green color blindness (X-linked recessive)
a: © Rick Wilking/Reuters/Corbis; b: © Reuters/STRINGER Brazil; c: © BSIP/Photo Researchers
Clicker Question
• In determining the inheritance mode of a disorder, which factors below can cause problems?
A.Humans typically have only a few childrenB.Codominance or incomplete dominanceC.Protein interactions or epistasisD.All of the above
10.9 Most Traits Are Influenced by the Environments and Multiple Genes
A. The Environment Can Alter the Phenotype• A gene may be active in one circumstance but
inactive in another• Temperature in Siamese cats
• Fetal alcohol syndrome – exposure to alcohol alters phenotype
10.9 Most Traits Are Influenced by the Environments and Multiple Genes
B. Polygenic Traits Depend on More Than One Gene
• Phenotype reflects the activities of more than one gene– Eye color– Male pattern baldness
• Bell-shaped curve or continuum of gene expression
10.9 Most Traits Are Influenced by the Environments and Multiple Genes
B. Polygenic Traits Depend on More Than One Gene
• Bell-shaped curve or continuum of gene expression
Figure 10.24 Height Is Polygenic and Environmental.
b.a.
6’2”5’0” 6’0”5’6”5’4”5’2” 5’8” 5’10”4’10” 6’2”5’0” 6’0”5’6”5’4”5’2” 5’8” 5’10” 6’4”
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a–b: © Peter Morenus/University of Connecticut
Figure 10.25 Skin Color Is Polygenic.
×
1/ 6/ 15/64 20/6415/64 6/64 1/64
Possiblephenotypesof children
Possiblegenotypesof children
Light
Parent phenotypes: medium tone
= Unit of pigment
Parent genotypes: AaBbCc AaBbCc
aabbcc AabbccaaBbccaabbCc
AaBbccAabbCcAAbbccaaBBccaabbCCaaBbCc
AaBbCcaaBbCCAAbbCcAabbCCAABbccaaBBCcAaBBcc
aaBBCCAAbbCCAABBccAaBbCCAaBBCcAABbCc
AaBBCCAABbCCAABBCc
AABBCC
Dark
64 64
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Sarah Leen/National Geographic Image Collection
10.9 Mastering Concepts
How can the environment affect a phenotype?
10.10 Investigating Life: Heredity and the Hungry Hordes
• Pink bollworm (Pectinophora gossypiella)• Pink caterpillars damage cotton crop• Soil bacterium Bacillus thuringiensis or Bt
– Produces protein toxic to insects– Not toxic to humans– Used in organic farming
Figure 10.26 Hungry Caterpillar.
10.10 Investigating Life: Heredity and the Hungry Hordes
• Produce genetically modified crops that express Bt toxin
• Farmers must plant non-Bt buffer – Prevents selective pressure– Bt resistance genes are recessive
Figure 10.27 Bt Crops Require Buffers.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Re
sis
tan
t p
are
nt’
sg
am
ete
s
R R
r
r
Susceptible parent’sgametes
Susceptible
RR
Non-Btbuffer
Resistant
Bt crop
rr
All offspring areheterozygous
and susceptible
RrRrSusceptibleSusceptible
SusceptibleSusceptibleRrRr
Figure 10.28 Bt Resistance Is Recessive.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
×
× ×
0
7
00
SusceptibleResistant
Resistant ResistantSusceptible(heterozygous)
Susceptible(heterozygous)
Feed larvae 10 µg/ml purifiedBt toxin for 21 days
Feed larvae 1 µg/ml purifiedBt toxin for 11 days
14
Larva weight (mg) Larva weight (mg)
5010 20 30 40
14
10 20 30 40 50
Nu
mb
er o
f la
rvae
Nu
mb
er o
f la
rvae
7
0
10.10 Mastering Concepts
What do you predict will happen to the incidence of resistance alleles in pink bollworm populations if farmers
choose not to plant the required refuge?
