BIOL BIOL 102: 102: General Biology IIGeneral Biology II

Rob Rob SwatskiSwatski Assoc. Assoc. Prof. BiologyProf. Biology

HACCHACC--YorkYork

Chapter Chapter 1414 Mendel and Mendel and the Gene Ideathe Gene Idea

1

What genetic principles account for the What genetic principles account for the passing of traits from parents to passing of traits from parents to

offspring?offspring?

The “blending” hypothesis:

2

The “particulate” hypothesis:

parents pass on discrete heritable units (genes)

3

4

Augustinian Abbey Brno, Czech Republic 5

6

Mendel Sculpture Villanova University, Phila.

7

8

Mendel used the scientific method

to identify 2 laws of inheritance

- Experimentation

- Quantitative data

9

Advantages of Pea Plants for Genetic Study:Advantages of Pea Plants for Genetic Study:

– Many varieties with distinct heritable features (characters), such as flower color

- character variants = traits

– Mating of plants can be controlled

– Each flower produces sperm (stamens) & eggs (carpels)

– Cross-pollination

10

TECHNIQUETECHNIQUE

RESULTSRESULTS

Parental generation

(P) Stamens

Carpel

1

2

3

4

First filial

generation offspring

(F1)

5

11

Mendel tracked only those characters that varied in an “either-or” manner

- used pairs of traits

He also used varieties that were true-breeding

- plants that produce offspring of the same variety when they self-pollinate

12

In a typical experiment, Mendel mated 2 contrasting, true-breeding varieties (hybridization)

- P generation: the true-breeding parents

- F1 generation: the hybrid offspring of the P generation

- F2 generation: produced when F1 individuals self-

pollinate

13

EXPERIMENTEXPERIMENT

P Generation

(true-breeding parents) Purple

flowers White flowers

F1 Generation

(hybrids) All plants had purple flowers

F2 Generation

705 purple-flowered plants

224 white-flowered plants

14

Mendel’s ExperimentsMendel’s Experiments

• When Mendel crossed true-breeding white & purple flowered plants, all of the F1 hybrids were purple

• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white

• Mendel discovered a ratio of about 3:1 (purple to white flowers) in the F2 generation

15

• Mendel reasoned that only the purple flower “factor” was affecting flower color in the F1 hybrids

• Mendel called purple flower color a dominant trait & white

flower color a recessive trait • He observed the same pattern of inheritance in 6 other pea

plant characters, each represented by 2 traits - what Mendel called a “heritable factor” is what we now

call a gene

16

17

18

Mendel’s ModelMendel’s Model

• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

• 4 related concepts make up Mendel’s model

- we can now describe these concepts in light of what we know about genes & chromosomes

19

Concept 1Concept 1:: Alternative versions of genes account for variations in inherited

characters

- Ex: the gene for flower color in pea plants exists in 2 versions - purple flowers & whitewhite flowers

- Alleles

- each allele is found at a specific locus on a specific chromosome

20

Allele for purple flowers

Locus for flower-color gene

Allele for whitewhite flowers

Pair of homologous

chromosomes

21

Concept 2Concept 2:: For each character, an organism inherits 2 alleles, one from

each parent

- the 2 alleles at a locus may be identical, as in the true-breeding P generation

- the 2 alleles at a locus may also differ, as in the F1 hybrids

22

Concept 3Concept 3:: If the 2 alleles at a locus differ, then one dominant allele determines the organism’s appearance, while the

recessive allele has no noticeable effect

- Ex: the F1 plants had purple flowers because the allele for that trait is dominant

23

Concept 4Concept 4:: The 2 alleles for a heritable character separate (segregate) during

gamete formation & end up in different gametes

= The Law of SegregationThe Law of Segregation

- An egg or sperm receives only 1 of the 2 alleles present in the somatic cells of an organism

- Segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

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- Mendel’s segregation model accounts for the 3:1 ratio observed in his F2 generations

- The possible combinations of sperm & egg can be shown using a Punnett square

- helps predict the results of a genetic cross between individuals of known genetic makeup

Symbols:

- Capital letter represents a dominant allele

- lowercase letter represents a recessive allele

25

P Generation

Appearance: Genetic makeup:

Gametes:

Purple flowers White flowers

PP pp

P p

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P Generation

F1 Generation

Appearance: Genetic makeup:

Gametes:

Appearance: Genetic makeup:

Gametes:

Purple flowers White flowers

Purple flowers Pp

PP pp

P

P

p

p 1/2 1/2

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P GenerationP Generation

FF11 GenerationGeneration

FF22 GenerationGeneration

Appearance: Genetic makeup:

Gametes:

Appearance: Genetic makeup:

Gametes:

Purple flowers White flowers

Purple flowers

Sperm from F1 (Pp) plant

Pp

PP pp

P

P

P

P

p

p

p

p

Eggs from F1 (Pp) plant

PP

pp Pp

Pp

1/2 1/2

3 : 1

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Useful Genetic VocabularyUseful Genetic Vocabulary

Homozygous:

Heterozygous:

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An organism’s traits do not always reveal its genetic composition

- due to different effects of dominant & recessive alleles

Phenotype:

Genotype:

- PP

- Pp

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PhenotypePhenotype

Purple

Purple

Purple

White

3

1

1

1

2

Ratio 3:1 Ratio 1:2:1

GenotypeGenotype

PP (homozygous)

Pp (heterozygous)

Pp (heterozygous)

pp (homozygous)

31

The TestcrossThe Testcross

How can we determine the genotype of an individual with a dominant phenotype?

Testcross: breed the “mystery” individual with a known homozygous recessive individual

- if any offspring display the recessive phenotype, the mystery parent must be heterozygous

32

Dominant phenotype, unknown genotype:

PP or Pp?

Recessive phenotype, known genotype:

pp

Predictions If purple-flowered parent is PP

If purple-flowered parent is Pp

or

Sperm Sperm

Eggs Eggs

or

All offspring purple 1/2 offspring purple and 1/2 offspring white

Pp Pp

Pp Pp

Pp Pp

pp pp

p p p p

P

P

P

p

TECHNIQUETECHNIQUE

RESULTSRESULTS

33

Mendel derived the Law of Segregation by following one character

- the F1 offspring produced were monohybrids, individuals that are heterozygous for one

character

Monohybrid cross: between F1 monohybrids

34

Mendel derived his 2nd law of inheritance by following two characters at the same time

- crossing 2 true-breeding parents differing in 2 characters produces dihybrids in the F1 generation, heterozygous

for both characters

Dihybrid cross: between F1 dihybrids

- can determine whether 2 characters are transmitted to offspring as a package or independently

35

P Generation

F1 Generation

Predictions

Gametes

EXPERIMENTEXPERIMENT

RESULTSRESULTS

YYRR yyrr

yr YR

YyRr

Hypothesis of dependent assortment

Hypothesis of independent assortment

Predicted offspring of F2 generation

Sperm

Sperm or

Eggs

Eggs

Phenotypic ratio 3:1

Phenotypic ratio 9:3:3:1

Phenotypic ratio approximately 9:3:3:1 315 108 101 32

1/2 1/2

1/2

1/2

1/4 1/4

1/4 1/4

1/4

1/4

1/4

1/4

9/16 3/16

3/16 1/16

YR

YR

YR

YR

yr

yr

yr

yr

1/4 3/4

Yr

Yr

yR

yR

YYRR YyRr

YyRr yyrr

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

36

Law of Independent AssortmentLaw of Independent Assortment

- each pair of alleles segregates independently of other pairs of alleles during gamete formation

Applies only to genes on different non-homologous chromosomes

- genes located near each other on the same chromosome tend to be inherited together

37

Mendel’s laws reflect the rules of probability

- when tossing a coin, the outcome of 1 toss has no impact on the outcome of the next toss

- the alleles of one gene segregate into gametes independently of another gene’s alleles

38

39

The probability of 2 or more independent events occurring together is the product of their individual

probabilities

- determines probability in an F1 monohybrid cross

Each gamete has a:

½ chance of carrying the dominant allele &

½ chance of carrying the recessive allele

The Multiplication RuleThe Multiplication Rule

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Segregation of alleles into eggs

Segregation of alleles into sperm

Sperm

Eggs

1/2

1/2

1/2 1/2

1/4 1/4

1/4 1/4

Rr Rr

R

R

R

R

R

R

r

r

r

r r

r

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The probability of any one exclusive event occurring (out of 2 or more) is calculated by adding their

individual probabilities

- determines the probability that an F2 plant from a monohybrid cross will be heterozygous rather than

homozygous

The Addition RuleThe Addition Rule

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Segregation of alleles into eggs

Segregation of alleles into sperm

Sperm

Eggs

1/2

1/2

1/2 1/2

1/4 1/4

1/4 1/4

Rr Rr

R

R

R

R

R

R

r

r

r

r r

r

43

Inheritance patterns are often Inheritance patterns are often more complex more complex than predicted by simple than predicted by simple MendelianMendelian geneticsgenetics

Many heritable characters are not determined by only 1 gene with 2 alleles

But…the basic Laws of Segregation & Independent assortment do apply to more complex patterns

of inheritance

44

Extending Extending MendelianMendelian Genetics for Genetics for a Single Genea Single Gene

Inheritance of characters by one gene may deviate from simple Mendelian patterns:

- When alleles are not completely dominant or recessive

- When a gene has more than 2 alleles

- When a gene produces multiple phenotypes

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Degrees of Dominance Degrees of Dominance

• Complete dominance: phenotypes of the heterozygote & dominant homozygote are identical

• Incomplete dominance: the phenotype of F1 hybrids is somewhere between the 2 parental phenotypes

• Codominance: the 2 dominant alleles affect the phenotype in separate & distinguishable ways

46

P Generation

F1 Generation

F2 Generation

1/2 1/2

1/2 1/2

1/2

1/2

Red White

Gametes

Pink

Gametes

Sperm

Eggs

CWCW CRCR

CR CW

CRCW

CR CW

CW CR

CR

CW

CRCR CRCW

CRCW CWCW Incomplete Dominance 47

A dominant allele does not subdue a recessive allele

- alleles don’t interact: they are simply variations in a gene’s nucleotide sequence

For any character, dominance/recessive relationships of alleles depends on the level at which one examines the

phenotype

48

TayTay--SachsSachs DiseaseDisease - result of dysfunctional enzyme

- lipids accumulate in the brain

At the organismal level:

- the allele is recessive

At the biochemical level:

- the phenotype (enzyme activity) is incompletely dominant

At the molecular level:

- the alleles are codominant

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Frequency of Dominant AllelesFrequency of Dominant Alleles Dominant alleles are not necessarily more common in

populations than recessive alleles

- Ex: 1 in 400 U.S. babies is born with extra fingers or toes (polydactyly)

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Multiple AllelesMultiple Alleles

Most genes exist in more than 2 allelic forms

Ex: ABO blood group phenotypes (A, B, AB, O)

- determined by 3 alleles for an enzyme (I) that attaches A or B carbohydrates to red blood cells:

IA, IB, and i

- IA allele: enzyme adds the A carbohydrate

- IB allele: enzyme adds the B carbohydrate

- i allele: enzyme adds neither carbohydrate

51

Carbohydrate

Allele

(a) The three alleles for the ABO blood groups and their carbohydrates

(b) Blood group genotypes and phenotypes

Genotype

Red blood cell appearance

Phenotype (blood group)

A

A

B

B AB

none

O

IA IB i

ii IAIB IAIA or IAi IBIB or IBi

52

B = brown eye color B = blue eye color

G = green or hazel eyes g = lighter colored eyes 53

PleiotropyPleiotropy

Most genes have multiple phenotypic effects

- Ex: pleiotropic alleles are responsible for multiple symptoms of hereditary

diseases like cystic fibrosis & sickle-cell

disease

54

Complete dominance of one allele

Relationship among alleles of a single gene

Description Example

Incomplete dominance of either allele

Codominance

Multiple alleles

Pleiotropy

Heterozygous phenotype same as that of homo- zygous dominant

Heterozygous phenotype intermediate between the two homozygous phenotypes

Both phenotypes expressed in heterozygotes

In the whole population, some genes have more than two alleles

One gene is able to affect multiple phenotypic characters

ABO blood group alleles

Sickle-cell disease

PP Pp

CRCR CRCW CWCW

IAIB

IA, IB, i

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EpistasisEpistasis

Some traits may be determined by 2 or more genes

Epistasis: a gene at 1 locus alters the phenotypic expression of a gene at a 2nd locus

Ex: Coat color in dogs depends on 2 genes:

- one gene determines pigment color (B = black, b = brown)

- 2nd gene determines if pigment will be deposited in hair (E = color, e = no color)

56

Sperm

Eggs

9 : 3 : 4

1/4 1/4

1/4 1/4

1/4

1/4

1/4

1/4

BbEe BbEe

BE

BE

bE

bE

Be

Be

be

be

BBEE BbEE BBEe BbEe

BbEE bbEE BbEe bbEe

BBEe BbEe BBee Bbee

BbEe bbEe Bbee bbee

57

Polygenic InheritancePolygenic Inheritance

- an additive effect of 2 or more genes on a single phenotype

- Quantitative characters: vary along a continuum

- Ex: skin color in humans

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Eggs

Sperm

Phenotypes:

Number of dark-skin alleles: 0 1 2 3 4 5 6

1/64 6/64

15/64 20/64

15/64 6/64

1/64

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8 1/8

1/8 1/8

1/8 1/8

1/8 1/8

AaBbCc AaBbCc

59

Epistasis

Polygenic inheritance

Relationship among two or more genes

Description Example

The phenotypic expression of one gene affects that of another

A single phenotypic character is affected by two or more genes

9 : 3 : 4

BbEe BbEe

BE

BE

bE

bE

Be

Be

be

be

AaBbCc AaBbCc

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The Environmental Impact on PhenotypeThe Environmental Impact on Phenotype

Another variation from Mendelian genetics occurs when a character’s phenotype depends on the environment

in addition to the genotype

= Norm of reaction

- Ex: hydrangea flowers of same genotype can range in color from blue-violet to pink, depending on soil acidity

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Integrating a Integrating a MendelianMendelian View of Heredity View of Heredity & Variation& Variation

An organism’s phenotype includes its:

- physical appearance

- internal anatomy

- physiology

- behavior

Phenotype reflects an organism’s overall genotype and unique environmental history

62

Humans are not good subjects for genetic research:

- our generation time is too long

- parents produce relatively few offspring

- ethics of breeding experiments

However, basic Mendelian genetics is the foundation of human genetics

63

Pedigree AnalysisPedigree Analysis

Pedigree: a family tree describing the interrelationships of parents & children across generations

- trace & describe inheritance patterns of certain traits

- can make predictions about future offspring

- use the multiplication & addition rules to predict the probability of specific phenotypes

64

KeyKey

Male

Female

Affected male

Affected female

Mating

Offspring, in birth order (first-born on left)

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1st generation (grandparents)

2nd generation (parents, aunts, and uncles)

3rd generation (two sisters)

Widow’s peak No widow’s peak

(a) Is a widow’s peak a dominant or recessive trait?

Ww ww

Ww Ww ww ww

ww

ww Ww

Ww

ww WW

Ww

or

66

Attached earlobe

1st generation (grandparents)

2nd generation (parents, aunts, and uncles)

3rd generation (two sisters)

Free earlobe

(b) Is an attached earlobe a dominant or recessive trait?

Ff Ff

Ff Ff Ff

ff Ff

ff ff ff

ff

FF or

or FF

Ff

67

Recessively Inherited DisordersRecessively Inherited Disorders

- appear only in individuals homozygous for the allele

- Carriers: heterozygous individuals who have the recessive allele, but are phenotypically normal

- Ex: Albinism is a recessive condition characterized by a lack of pigmentation in skin & hair

68

ParentsParents

Normal Normal

Sperm

Eggs

Normal Normal (carrier)

Normal (carrier) Albino

Aa Aa

A

A AA

Aa

a

Aa aa

a

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If a recessive allele causing a disease is rare, then the chance of 2 carriers meeting & mating is low

Consanguineous matings (between close relatives) increase the chance of mating between 2 carriers of

the same rare allele

- most societies & cultures have laws or taboos against marriages between close relatives

70

Dominantly Inherited DisordersDominantly Inherited Disorders

Result from dominant alleles that cause a lethal disease

- rare & arise by mutation

Achondroplasia: a form of dwarfism due to a rare dominant allele

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Parents

Dwarf Dd

Sperm

Eggs

Dd Dwarf

dd Normal

Dd Dwarf

dd Normal

D

d

d

d

Normal dd

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Degenerative disease of the nervous system showing no obvious phenotypic effects until person is

approx. 35-40 years of age

Huntington’s DiseaseHuntington’s Disease

73

MultifactorialMultifactorial DisordersDisorders

Diseases having both genetic & environmental components

- Ex: most diseases (heart disease, cancer)

- little is understood about the genetic contribution to most multifactorial diseases

74

Genetic Testing & CounselingGenetic Testing & Counseling

Genetic counselors: inform prospective parents concerned about family history for specific disease

- use family histories & carrier ID tests to help couples determine odds of their children having genetic

disorders

75

Fetal TestingFetal Testing

- Amniocentesis: removes & tests amniotic fluid that bathes fetus

- Chorionic villus sampling (CVS): removes & tests a sample of placenta

- Ultrasound & fetoscopy: visually assesses fetal health in utero

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(a) Amniocentesis (b) Chorionic villus sampling (CVS)

Ultrasound monitor

Amniotic fluid withdrawn

Fetus

Placenta

Uterus Cervix

Centrifugation

Fluid

Fetal cells

Several hours

Several weeks

Several weeks

Biochemical and genetic

tests

Karyotyping

Ultrasound monitor

Fetus

Placenta

Chorionic villi

Uterus

Cervix

Suction tube inserted through cervix

Several hours

Fetal cells

Several hours

1

1

2

2

3

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