Biology Sylvia S. Mader Michael Windelspecht Chapter 11 Mendelian Patterns of Inheritance Lecture...

BiologySylvia S. Mader

Michael Windelspecht

Chapter 11Mendelian Patterns of Inheritance

Lecture Outline

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Outline

• 11.1 Gregor Mendel

• 11.2 Mendel’s Laws

• 11.3 Extending the Range of Mendelian Genetics

11.1 Gregor Mendel

• Concept of Blending Inheritance: Parents of contrasting appearance produce

offspring of intermediate appearance Popular concept during Mendel’s time

• Mendel’s findings were in contrast with this He formulated the Particulate Theory of

Inheritance• Inheritance involves reshuffling of genes from

generation to generation

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Gregor Mendel

• Austrian monk Studied science and mathematics at the University of

Vienna Conducted breeding experiments with the garden pea

Pisum sativum Carefully gathered and documented mathematical

data from his experiments

• Formulated fundamental laws of heredity in the early 1860s Had no knowledge of cells or chromosomes Did not have a microscope

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Gregor Mendel

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© Ned M. Seidler/Nationa1 Geographic Image Collection

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Gregor Mendel

• The garden pea: Organism used in Mendel’s experiments A good choice for several reasons:

• Easy to cultivate

• Short generation

• Normally self-pollinating, but can be cross-pollinated by hand

• True-breeding varieties were available

• Simple, objective traits

Garden Pea Anatomy

7

stamenanther

a.

Flower Structure

filament

stigma

style

ovules inovary

carpel


8

All peas are yellow whenone parent produces yellowseeds and the other parentproduces green seeds.

Brushingon pollenfrom anotherplant

Cutting awayanthers

Garden Pea AnatomyCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Garden Pea AnatomyTrait *Dominant

Characteristics*Recessive

Stem length

Pod shape

Seed shape

Seed color

Flower position

Flower color

Pod color

b.

Green

Purple

Axial

Yellow

Round

Inflated

Tall Short

Constricted

Wrinkled

Green

Terminal

White

Yellow


11.2 Mendel’s Laws

• Mendel performed cross-breeding experiments Used “true-breeding” (homozygous) plants Chose varieties that differed in only one trait

(monohybrid cross) Performed reciprocal crosses

• Parental generation = P

• First filial generation offspring = F1

• Second filial generation offspring = F2

Formulated the Law of Segregation10

Monohybrid Cross done by Mendel

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TT tt

Phenotypic Ratio

short1tall3

Allele Key

T tall plant=t short plant=


12

P generation

TT tt

Phenotypic Ratio

short1tall3

Allele Key




13


P generation

TT tt

tTP gametes

Phenotypic Ratio

short1tall3

Allele Key



14


tt

P generation

TT tt

tT

Tt

P gametes

F1 generation

Phenotypic Ratio

short1tall3

Allele Key



15


eggs

sp

erm

P generation

T t

T

t

TT tt

tT

Tt

P gametes

F1 gametes

F1 generation

Phenotypic Ratio

short1tall3

Allele Key


Monohybrid Cross done by MendelCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

eggs

sp

erm

P generation

T t

T

t

TT tt

tT

Tt

TT

P gametes

F1 gametes

F1 generation

Phenotypic Ratio

short1tall3

Allele Key


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17


eggs

sp

erm

T t

T

t

TT tt

tT

Tt

TT Tt

F1 gametes

Phenotypic Ratio

short1tall3

Allele key

F1 generation

P gametes

F1 generation



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eggs

sp

erm

P generation

T t

T

t

TT tt

tT

Tt

TT Tt

Tt

P gametes

F1 gametes

F1 generation

Phenotypic Ratio

short1tall3

Allele Key



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eggssp

erm

Offspring

P generation

T t

T

t

TT tt

tT

Tt

TT Tt

Tt tt

P gametes

F1 gametes

F2 generation

F1 generation

Phenotypic Ratio

short1tall3

Allele Key


Mendel’s Laws

• Law of Segregation:

Each individual has a pair of factors (alleles) for each trait

The factors (alleles) segregate (separate) during gamete (sperm & egg) formation

Each gamete contains only one factor (allele) from each pair of factors

Fertilization gives the offspring two factors for each trait

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Mendel’s Laws

• Classical Genetics and Mendel’s Cross:

Each trait in a pea plant is controlled by two alleles (alternate forms of a gene)

Dominant allele (capital letter) masks the expression of the recessive allele (lower-case)

Alleles occur on a homologous pair of chromosomes at a particular gene locus

• Homozygous = identical alleles

• Heterozygous = different alleles21

Classical View of Homologous Chromosomes

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Replication

alleles at agene locus

sister chromatids

b. Sister chromatids of duplicated chromosomes have same alleles for each gene.

a. Homologous chromosomes have alleles for same genes at specific loci.

G

R

S

t

G

R

S

t

G

R

S

t

g

r

s

T

g

r

s

T

g

r

s

T


Relationship Between Observed Phenotype and F2 Offspring

Trait

Stem length

Pod shape

Seed shape

Seed color

Flower position

Flower color

Pod color

DominantCharacteristics

Recessive

Tall

Inflated

Round

Yellow

Axial

Purple

Green

Short

Constricted

Wrinkled

Green

Terminal

White

Yellow

Dominant

F2Results

RatioRecessive

Totals:

2.84:1277787

882 299 2.95:1

2.96:1

3.01:1

3.14:1

3.15:1

2.82:1

2.98:1

2,001

1,8505,474

6,022

651

705

428

14,949

207

224

152

5,010


Mendel’s Laws

• Genotype

Refers to the two alleles an individual has for a specific trait

If identical, genotype is homozygous

If different, genotype is heterozygous

• Phenotype

Refers to the physical appearance of the individual

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Mendel’s Laws• A dihybrid cross uses true-breeding plants differing in two traits• Mendel tracked each trait through two generations.

Started with true-breeding plants differing in two traits The F1 plants showed both dominant characteristics F1 plants self-pollinated Observed phenotypes among F2 plants

• Mendel formulated the Law of Independent Assortment The pair of factors for one trait segregate independently of the factors for

other traits All possible combinations of factors can occur in the gametes

• P generation is the parental generation in a breeding experiment.• F1 generation is the first-generation offspring in a breeding

experiment.• F2 generation is the second-generation offspring in a breeding

experiment 25

Dihybrid Cross Done by MendelCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

×

====

TtGg

9331

ttggTTGG

P generation

P gametes

F1 generation

F1 gametes

F2 generation

sper

m

eggs

TtGg

TG tg

tgtGTgTG

TG

TtGg

Tg

tG

tg

TTGG TTGg TtGG

TtggTtGgTTggTTGg

ttGgttGGTtGgTtGG

ttggttGgTtggTtGg

Allele Key

Offspring

Phenotypic Ratio

Yellow podgreen podshort planttall plant tall plant, green pod

tall plant, yellow podshort plant, green podshort plant, yellow pod

Independent Assortment and Segregation during Meiosis

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A a

Bb

Parent cell has twopairs of homologouschromosomes.


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A

A A

a

a a

B

BB

b

b b

either



29

A

A A

a

a a

B

BB

b

b b

A A

b b BB

a a

either

or




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A

A A

a

a a

B

BB

b

b b

A A

b b BB

a a

either

or


All orientations of ho-mologous chromosomesare possible at meta-phase I in keeping withthe law of independentassortment.



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A

A A

A A

a

a

a

a

a

B

BB

BB

b

b b

A A

b b BB

a a

b b

either

or


All orientations of ho-mologous chromosomesare possible at metaphaseI in keeping with the law of independentassortment.



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A

A A

A A

a

a

a

a

a

A A

aa

B

BB

BB

BB

b

b b

A A

b b BB

a a

b b

bb

either

or


All orientations of ho-mologous chromosomesare possible at metaphaseI in keeping withthe law of independentassortment.



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A

A A

A A

a

a

a

a

a

A A

aa

B

BB

BB

BB

b

b b

A A

b b BB

a a

b b

bb

either

or


All orientations of ho-mologous chromosomesare possible at meta-phase I in keeping withthe law of independentassortment.

At metaphase II, eachdaughter cell has onlyone member of eachhomologous pair inkeeping with the law ofsegregation



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All possible combi -tions of chromosomesand alleles occur inthe gametes assuggested by Mendel'stwo laws.

All orientations of ho-mologous chromosomesare possible at metaphase I in keepingwith the law of Independent assortment.

At metaphase II, eachdaughter cell has onlyone member of eachhomologous pair inkeeping with the law ofsegregation

A

A A

A A

A

A

AB

ab

Ab

aB

a

a

a

a

aa

a

AA

bA

aa aB

B

BB

B

B

B

B

B

Ba

B

b

b b

A A

b b BB

a a

b

b

b

b

b

A

b

b

either

or


Mendel’s Laws

• Punnett Square

Table listing all possible genotypes resulting from a cross

• All possible sperm genotypes are lined up on one side

• All possible egg genotypes are lined up on the other side

• Every possible zygote genotypes are placed within the squares

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Mendel’s Laws

• Punnett Square Allows us to easily calculate probability, of genotypes

and phenotypes among the offspring Punnett square in next slide shows a 50% (or ½)

chance • The chance of E = ½• The chance of e = ½

An offspring will inherit:• The chance of EE =½ ½=¼ • The chance of Ee =½ ½=¼ • The chance of eE =½ ½=¼ • The chance of ee =½ ½=¼

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Punnett Square

37

eggs

spem

Pu

nn

ett

squ

are

Offspring

Parents

E e

E

e

Ee

EeEE

Ee Ee

Allele key Phenotypic Ratio

unattached earlobes31

E =

e =

unattached earlobes

attached earlobes attached earlobes


ee

Mendel’s Laws

• Testcrosses

Individuals with recessive phenotype always have the homozygous recessive genotype

However, individuals with dominant phenotype have indeterminate genotype

• May be homozygous dominant, or

• Heterozygous

A testcross determines the genotype of an individual having the dominant phenotype

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Mendel’s Laws

• Two-trait testcross:

An individual with both dominant phenotypes is crossed with an individual with both recessive phenotypes.

If the individual with the dominant phenotypes is heterozygous for both traits, the expected phenotypic ration is 1:1:1:1.

Mendel’s Laws

• Genetic disorders are medical conditions caused by alleles inherited from parents

• Autosome - Any chromosome other than a sex chromosome (X or Y)

• Genetic disorders caused by genes on autosomes are called autosomal disorders Some genetic disorders are autosomal dominant

• An individual with AA has the disorder• An individual with Aa has the disorder• An individual with aa does NOT have the disorder

Other genetic disorders are autosomal recessive• An individual with AA does NOT have the disorder• An individual with Aa does NOT have the disorder, but is a carrier• An individual with aa DOES have the disorder

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Autosomal Recessive Pedigree

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I

II

III

IVKey

Gen

erat

ion

s

Autosomal recessive disorders• Most affected children have unaffected parents.

• Heterozygotes (Aa) have an unaffected phenotype.• Two affected parents will always have affected children.• Close relatives who reproduce are more likely to have affected children.

• Both males and females are affected with equal frequency.

A?

aa A?

A? Aa Aa A?

Aa*

Aa A?

A?aaaaaa = affectedAa = carrier (unaffected)AA = unaffectedA? = unaffected (one allele unknown)

Autosomal Dominant Pedigree

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• Affected children will usually have anaffected parent.

• Heterozygotes (Aa) are affected.• Two affected parents can produce an unaffected child.• Two unaffected parents will not have affected children.• Both males and females are affected with equal frequency.

AA = affectedAa = affectedA? = affected (one allele unknown)aa = unaffected

I

II

III Aa

aa

Aa

Aa

*

Aa

A?

aa

aa aa aa

aaaaaa

Aa

Key

Gen

erat

ion

s

Autosomal dominant disorders

11.3 Extending the Range of Mendelian Genetics

• Some traits are controlled by multiple alleles (multiple allelic traits)

• The gene exists in several allelic forms (but each individual only has two alleles)

• ABO blood types The alleles:

• IA = A antigen on red blood cells, anti-B antibody in plasma

• IB = B antigen on red blood cells, anti-A antibody in plasma

• i = Neither A nor B antigens on red blood cells, both anti-A and anti-B antibodies in plasma

• The ABO blood type is also an example of codominance More than one allele is fully expressed Both IA and IB are expressed in the presence of the other

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ABO Blood Type

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GenotypeIAIA, IAiIBIB, IBiIAIB

ii

PhenotypeABABO

Extending the Range of Mendelian Genetics

• Incomplete Dominance:

Heterozygote has a phenotype intermediate between that of either homozygote

• Homozygous red has red phenotype

• Homozygous white has white phenotype

• Heterozygote has pink (intermediate) phenotype

Phenotype reveals genotype without a test cross

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Incomplete Dominance

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R1 R2

R1

R2

R1R2

R1R2R1R1

R2R2

R1R2 R1R2

eggs

sperm

Offspring

Key

1 R1R12 R1R21 R2R2

redpinkwhite

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• Human examples of incomplete dominance:

Incomplete penetrance• The dominant allele may not always lead to the

dominant phenotype in a heterozygote

• Many dominant alleles exhibit varying degrees of penetrance

• Example: polydactyly– Extra digits on hands, feet, or both

– Not all individuals who inherit the dominant polydactyly allele will exhibit the trait


• Pleiotropy occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits.

• Marfan syndrome is pleiotropic and results in the following phenotypes: disproportionately long arms, legs, hands, and

feet a weakened aorta poor eyesight

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• Polygenic Inheritance: Occurs when a trait is governed by two or more genes

having different alleles Each dominant allele has a quantitative effect on the

phenotype These effects are additive Results in continuous variation of phenotypes within a

population The traits may also be affected by the environment Examples

• Human skin color

• Height

• Eye color

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Polygenic Inheritance

50


aabbcc

Aabbcc

AaBbcc

AaBbCc

AABbCc

AABBCc

AABBCC

20—64

15—64

6—64

164

Pro

po

rtio

n o

f P

op

ula

tio

n

Genotype Examples

F2 generation

—

F1 generation

P generation


• X-Linked Inheritance In mammals

• The X and Y chromosomes determine gender

• Females are XX • Males are XY

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• X-Linked Inheritance The term X-linked is used for genes that have

nothing to do with gender• X-linked genes are carried on the X chromosome. • The Y chromosome does not carry these genes • Discovered in the early 1900s by a group at

Columbia University, headed by Thomas Hunt Morgan.

– Performed experiments with fruit flies» They can be easily and inexpensively raised in

simple laboratory glassware» Fruit flies have the same sex chromosome pattern as

humans

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X-Linked Recessive Pedigree

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XBXB XbY grandfather

daughterXBXbXBY XBY XbXb

XbY

XBXb grandsonXBY XBXB XbY

KeyXBXB = Unaffected femaleXBXb = Carrier femaleXbXb = Color-blind femaleXbY = Unaffected maleXbY = Color-blind maleX-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.

Muscular Dystrophy

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(Abnormal): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center;(Boy): Courtesy Muscular Dystrophy Association; (Normal): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory,

University of Rochester Medical Center.

abnormalmuscle

normaltissue

fibroustissue

Biology Sylvia S. Mader Michael Windelspecht Chapter 11 Mendelian Patterns of Inheritance Lecture...

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