10.6 gene expression can appear to alter mendelian ratios a. incomplete dominance and codominance...

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



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


Figure 10.15 Codominance.


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


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.


Porphyrin accumulates in the...

Urine Digestivesystem

Nervoustissue

Muscles

Resulting in...

Delirium

Stupor

Madbehavior

ConvulsionsWeak limbsDark-

coloredurine

Abdominalpain

Constipation

Rapid pulse


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.


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


A. X and Y Chromosomes Determine Sex in Humans

• Females XX• Males XY

Figure 10.18 The Sperm Determines the Sex of the Baby.


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


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


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.


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


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.


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


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.


X

X

a.

b.

Blackinactivation

Orangeinactivation

a: © William E. Ferguson; b: © Horst Schaefer/Peter Arnold/Photolibrary


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.


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


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


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”


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


© Sarah Leen/National Geographic Image Collection


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.


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.


×

× ×

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


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?

10.6 gene expression can appear to alter mendelian ratios a. incomplete dominance and codominance...

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