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Chapter 12

Lecture Outline

Patterns of InheritanceChapter 12

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Mystery of heredity

• Before the 20th century, 2 concepts were the basis for ideas about heredity– Heredity occurs within species– Traits are transmitted directly from parent to

offspring• Thought traits were borne through fluid and

blended in offspring• Paradox – if blending occurs why don’t all

individuals look alike?

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Early work

• Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents– Additional variation observed in 2nd generation

offspring contradicts direct transmission• T.A. Knight – 1823 – crossed 2 varieties of

garden pea, Pisum sativa– Crossed 2 true-breeding strains– 1st generation resembled only 1 parent strain– 2nd generation resembled both

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Gregor Mendel

Chose to study pea plants because:1. Other research showed that pea hybrids could be produced2. Many pea varieties were available3. Peas are small plants and easy to grow4. Peas can self-fertilize or be cross-fertilized

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Mendel’s experimental method

• Usually 3 stages1. Produce true-breeding strains for each trait

he was studying2. Cross-fertilize true-breeding strains having

alternate forms of a trait – Also perform reciprocal crosses

3. Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait

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StigmaStyle

Anthers (male) 1. The anthers are cut away on the purple flower.

PetalsCarpel (female)

4. All progeny result in purple lowers.

3. Pollen is transferred to the purple flower.

2. Pollen is obtained from the white flower.



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Monohybrid crosses

• Cross to study only 2 variations of a single trait

• Mendel produced true-breeding pea strains for 7 different traits– Each trait had 2 variants

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F1 generation

• First filial generation• Offspring produced by crossing 2 true-

breeding strains• For every trait Mendel studied, all F1 plants

resembled only 1 parent– Referred to this trait as dominant– Alternative trait was recessive

• No plants with characteristics intermediate between the 2 parents were produced

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F2 generation

• Second filial generation• Offspring resulting from the self-

fertilization of F1 plants• Although hidden in the F1 generation, the

recessive trait had reappeared among some F2 individuals

• Counted proportions of traits– Always found about 3:1 ratio

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Purple White

Yellow Green

Round Wrinkled

Green Yellow

1. Flower Color

2. Seed Color

4. Pod Color

Dominant Recessive

3.15:1X

X

X

X

3.01:1

2.96:1

2.82:1

F2 Generation

705 Purple:224 White

6022 Yellow:2001 Green

5474 Round:1850 Wrinkled

428 Green:152 Yellow

3. Seed Texture

Inflated Constricted

X2.95:1

Axial Terminal

Tall Short

6. Flower Position

7. Plant Height

X

X

3.14:1

2.84:1

882 Inflated:299 Constricted

651 Axial:207 Terminal

787 T all:277 Short

5. Pod Shape

Dominant Recessive F2 Generation


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3:1 is actually 1:2:1

• F2 plants– ¾ plants with the dominant form– ¼ plants with the recessive form– The dominant to recessive ratio was 3:1

• Mendel discovered the ratio is actually:– 1 true-breeding dominant plant– 2 not-true-breeding dominant plants– 1 true-breeding recessive plant

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Parent generation

Self-cross Self-cross Self-cross Self-cross

Cross-fertilize

Self-cross

True-breedingPurpleParent

True-breeding

WhiteParent

PurpleOffspring

F1 generation

F2 generation(3:1 phenotypicratio)

F3 generation(1:2:1 genotypicratio)

PurpleDominant

PurpleDominant

PurpleDominant

WhiteRecessive

True-breeding

Non-true-breeding

Non-true-breeding

True-breeding

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Conclusions

• His plants did not show intermediate traits– Each trait is intact, discrete

• For each pair, one trait was dominant, the other recessive

• Pairs of alternative traits examined were segregated among the progeny of a particular cross

• Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive

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Five-element model

1. Parents transmit discrete factors (genes)2. Each individual receives one copy of a

gene from each parent3. Not all copies of a gene are identical

– Allele – alternative form of a gene– Homozygous – 2 of the same allele– Heterozygous – different alleles

4. Alleles remain discrete – no blending5. Presence of allele does not guarantee

expression– Dominant allele – expressed– Recessive allele – hidden by dominant allele

• Genotype – total set of alleles an individual contains

• Phenotype – physical appearance

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Principle of Segregation

• Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization

• Physical basis for allele segregation is the behavior of chromosomes during meiosis

• Mendel had no knowledge of chromosomes or meiosis – had not yet been described

Punnett square

• Cross purple-flowered plant with white-flowered plant

• P is dominant allele – purple flowers• p is recessive allele – white flowers• True-breeding white-flowered plant is pp

– Homozygous recessive• True-breeding purple-flowered plant is PP

– Homozygous dominant• Pp is heterozygote purple-flowered plant

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P

P

p

p pp

P

P

p

p pp

Pp

P

P

p

p pppP

P

P

p

p pp

Pp

pP

PpPP

a.

1. p + p = pp. 2. P + p = Pp.

3. p + P = pP. 4. P + P = PP.

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p

P

p

PPp

Pp

Pp

Pp

White parent pp

b.

P

P

p

ppp

Pp

PurpleparentPP

PurpleheterozygotePp

Purpleheterozygote Pp

F1 generation

PP

pP

F2 generation 3 Purple:1 White(1PP: 2Pp :1pp )

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Human traits• Some human traits are controlled by a single

gene– Some of these exhibit dominant and recessive

inheritance• Pedigree analysis is used to track

inheritance patterns in families• Dominant pedigree – juvenile glaucoma

– Disease causes degeneration of optic nerve leading to blindness

– Dominant trait appears in every generation

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21

2 3 4 51

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Dominant Pedigree

Generation I

Generation II

Generation III

Key

affected female

affected male

unaffected female

unaffected male

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• Recessive pedigree – albinism– Condition in which the pigment melanin is not

produced– Pedigree for form of albinism due to a

nonfunctional allele of the enzyme tyrosinase– Males and females affected equally– Most affected individuals have unaffected

parents

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1 2

1 2

1 2

3

3

1 2 3

4

4

5

5 6 7

Recessive Pedigree

Generation I

Generation II

Generation III

Generation IV

Heterozygous

Homozygous recessive

Keymale carrier

female carrieraffected female

affected male

unaffected female

unaffected male


One of these personsis heterozygous

Mating betweenfirst cousins

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Dihybrid crosses

• Examination of 2 separate traits in a single cross

• Produced true-breeding lines for 2 traits• RRYY x rryy• The F1 generation of a dihybrid cross

(RrYy) shows only the dominant phenotypes for each trait

• Allow F1 to self-fertilize to produce F2

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F1 self-fertilizes•RrYy x RrYy•The F2 generation shows all four possible phenotypes in a set ratio

– 9:3:3:1– R_Y_:R_yy:rrY_:rryy– Round yellow:round green:wrinkled

yellow:wrinkled green

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Cross-fertilization

RY Ry rY ry

Meiosis Meiosis

rr yy

Parent generation

RR YY

Rr Yy

F1 generation

Meiosis(chromosomes assort independently

into four types of gametes)

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RY Ry rY ry

RR yy Rr yy

Rr yy rr yy

9/16

3/16

3/16

1/16

round, yellow

round, green

wrinkled, yellow

wrinkled, green

RY

Ry

rY

ry

F1 X F1 (RrYy X RrYy)

F2 generation

RR YY RR Yy Rr YY Rr Yy

RR Yy Rr Yy

rr Yy

rr Yy

rr YYRr YyRr YY

Rr Yy

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Principle of independent assortment

• In a dihybrid cross, the alleles of each gene assort independently

• The segregation of different allele pairs is independent

• Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs

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Probability

• Rule of addition– Probability of 2 mutually exclusive events

occurring simultaneously is the sum of their individual probabilities

• When crossing Pp x Pp, the probability of producing Pp offspring is – probability of obtaining Pp (1/4), PLUS

probability of obtaining pP (1/4)– ¼ + ¼ = ½

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• Rule of multiplication– Probability of 2 independent events occurring

simultaneously is the product of their individual probabilities

• When crossing Pp x Pp, the probability of obtaining pp offspring is– Probability of obtaining p from father = ½ – Probability of obtaining p from mother = ½ – Probability of pp = ½ x ½ = ¼

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Testcross

• Cross used to determine the genotype of an individual with dominant phenotype

• Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)

• Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

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P

p

P P

p

p

Heterozygousdominant

Homozygousrecessive

Alternative 2:Half of the offspring are white and the unknown

flower is heterozygous (Pp)

PP or Pp

thenIf Pp

DominantPhenotype(unknowngenotype)

If PPthen

Alternative 1:All offspring are purple and the unknown

flower is homozygous dominant (PP)

Homozygousrecessive

Homozygousdominant

PpPp Pp pp

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Extensions to Mendel

• Mendel’s model of inheritance assumes that– Each trait is controlled by a single gene– Each gene has only 2 alleles– There is a clear dominant-recessive

relationship between the alleles

• Most genes do not meet these criteria

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Polygenic inheritance

• Occurs when multiple genes are involved in controlling the phenotype of a trait

• The phenotype is an accumulation of contributions by multiple genes

• These traits show continuous variation and are referred to as quantitative traits– For example – human height– Histogram shows normal distribution

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30

20

10

00 5′6″ '6′0″5′0″


Num

ber o

f Ind

ivid

uals

(top): From Albert F. Blakeslee, “CORN AND MEN: The Interacting Infl uence of Heredity and Environment—Movements for Betterment of Men, or Corn, or Any Other Living Thing, One-sided Unless Th ey Take Both Factors into Account,” Journal of

Heredity, 1914, 5:511-8, by permission of Oxford University Press

Height

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Pleiotropy

• Refers to an allele which has more than one effect on the phenotype

• Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions

• This can be seen in human diseases such as cystic fibrosis or sickle cell anemia– Multiple symptoms can be traced back to one

defective allele

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Multiple alleles

• May be more than 2 alleles for a gene in a population

• ABO blood types in humans– 3 alleles

• Each individual can only have 2 alleles• Number of alleles possible for any gene is

constrained, but usually more than two alleles exist for any gene in an outbreeding population

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• Incomplete dominance– Heterozygote is intermediate in phenotype

between the 2 homozygotes– Red flowers x white flowers = pink flowers

• Codominance– Heterozygote shows some aspect of the

phenotypes of both homozygotes– Type AB blood

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Parent generation

1 : 2 : 1

CR CW

Cross-fertilization

CWCWCRCR

F1 generation

CRCW

CRCWCRCR

CR

CW

CRCW CWCW

CRCR: CRCW: CWCW

F2 generation


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Human ABO blood group

• The system demonstrates both– Multiple alleles

• 3 alleles of the I gene (IA, IB, and i)– Codominance

• IA and IB are dominant to i but codominant to each other

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Alleles

AB

NoneO

GalactosamineA

GalactoseB

BloodType

SugarsExhibited

Donates andReceives

Receives A and ODonates to A and ABReceives B and ODonates to B and ABUniversal receiverDonates to ABReceives OUniversal donor

Both galactose andgalactosamine

IAIA, IAi(IA dominant to i)

IBIB, IBi(IB dominant to i)

IAIB

(codominant)ii

(i is recessive)

Environmental influence• Coat color in

Himalayan rabbits and Siamese cats– Allele

produces an enzyme that allows pigment production only at temperatures below 30oC

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© DK Limited/Corbis

Temperaturebelow33º C, tyrosinaseactive, dark pigment

Temperature above33º C, tyrosinaseinactive, no pigment

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Epistasis

• Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment

• R.A. Emerson crossed 2 white varieties of corn– F1 was all purple

– F2 was 9 purple:7 white – not expected

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AB Ab aB ab

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBB AaBb aaBB aaBb

AaBb Aabb aaBb aabb

9/16 Purple: 7/16 White

AB

Ab

aB

ab

Cross-fertilization

a.

b.

Parentalgeneration

F1 generation

F2 generation

Pigment(purple)

EnzymeB

EnzymeAPrecursor

(colorless)Intermediate(colorless)

White(aaBB)

White(AAbb)

All Purple(AaBb)

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