Mendel and the Gene Idea - R. Wingerden · 2015-08-25 · •Mendel called the purple flower color a dominant trait and the white flower color a recessive trait ... •Crossing two

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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

BiologyEighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 14

Mendel and the Gene Idea

Overview: Drawing from the Deck of Genes

• What genetic principles account for the passingof traits from parents to offspring?

• The “blending” hypothesis is the idea thatgenetic material from the two parents blendstogether (like blue and yellow paint blend tomake green)

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

• The “particulate” hypothesis is the idea thatparents pass on discrete heritable units(genes)

• Mendel documented a particulate mechanismthrough his experiments with garden peas


Fig. 14-1

Concept 14.1: Mendel used the scientific approachto identify two laws of inheritance

• Mendel discovered the basic principles ofheredity by breeding garden peas in carefullyplanned experiments


Mendel’s Experimental, Quantitative Approach

• Advantages of pea plants for genetic study:

– There are many varieties with distinct heritablefeatures, or characters (such as flower color);character variants (such as purple or whiteflowers) are called traits

– Mating of plants can be controlled

– Each pea plant has sperm-producing organs(stamens) and egg-producing organs (carpels)

– Cross-pollination (fertilization between differentplants) can be achieved by dusting one plantwith pollen from another


2

Fig. 14-2

TECHNIQUE

RESULTS

Parentalgeneration(P) Stamens

Carpel

1

2

3

4

Firstfilialgener-ationoffspring(F1)

5

• Mendel chose to track only those charactersthat varied in an either-or manner

• He also used varieties that were true-breeding(plants that produce offspring of the samevariety when they self-pollinate)


• In a typical experiment, Mendel mated twocontrasting, true-breeding varieties, a processcalled hybridization

• The true-breeding parents are the Pgeneration

• The hybrid offspring of the P generation arecalled the F1 generation

• When F1 individuals self-pollinate, the F2generation is produced


The Law of Segregation

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

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

• Mendel discovered a ratio of about three toone, purple to white flowers, in the F2generation


Fig. 14-3-1

EXPERIMENT

P Generation(true-breeding parents) Purple

flowers Whiteflowers

×

Fig. 14-3-2

EXPERIMENT



×

F1 Generation (hybrids) All plants had

purple flowers

3

Fig. 14-3-3

EXPERIMENT



×

F1 Generation (hybrids) All plants had

purple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

• Mendel reasoned that only the purple flowerfactor was affecting flower color in the F1 hybrids

• Mendel called the purple flower color a dominanttrait and the white flower color a recessive trait

• Mendel observed the same pattern of inheritancein six other pea plant characters, eachrepresented by two traits

• What Mendel called a “heritable factor” is whatwe now call a gene


Table 14-1

Mendel’s Model

• Mendel developed a hypothesis to explain the3:1 inheritance pattern he observed in F2offspring

• Four related concepts make up this model

• These concepts can be related to what we nowknow about genes and chromosomes


• The first concept is that alternative versions ofgenes account for variations in inheritedcharacters

• For example, the gene for flower color in peaplants exists in two versions, one for purpleflowers and the other for white flowers

• These alternative versions of a gene are nowcalled alleles

• Each gene resides at a specific locus on aspecific chromosome


Fig. 14-4

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

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• The second concept is that for each characteran organism inherits two alleles, one from eachparent

• Mendel made this deduction without knowingabout the role of chromosomes

• The two alleles at a locus on a chromosomemay be identical, as in the true-breeding plantsof Mendel’s P generation

• Alternatively, the two alleles at a locus maydiffer, as in the F1 hybrids


• The third concept is that if the two alleles at alocus differ, then one (the dominant allele)determines the organism’s appearance, andthe other (the recessive allele) has nonoticeable effect on appearance

• In the flower-color example, the F1 plants hadpurple flowers because the allele for that trait isdominant


• The fourth concept, now known as the law ofsegregation, states that the two alleles for aheritable character separate (segregate) duringgamete formation and end up in differentgametes

• Thus, an egg or a sperm gets only one of thetwo alleles that are present in the somatic cellsof an organism

• This segregation of alleles corresponds to thedistribution of homologous chromosomes todifferent gametes in meiosis


• Mendel’s segregation model accounts for the3:1 ratio he observed in the F2 generation ofhis numerous crosses

• The possible combinations of sperm and eggcan be shown using a Punnett square, adiagram for predicting the results of a geneticcross between individuals of known geneticmakeup

• A capital letter represents a dominant allele,and a lowercase letter represents a recessiveallele


Fig. 14-5-1

P Generation

Appearance:Genetic makeup:

Gametes:

Purple flowers White flowersPP

P

pp

p

Fig. 14-5-2

P Generation


Gametes:


P

pp

p

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Pp

P p1/2 1/2

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Fig. 14-5-3

P Generation


Gametes:


P

pp

p

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Pp

P p1/2 1/2

F2 GenerationSperm

Eggs

P

PPP Pp

p

pPp pp

3 1

Useful Genetic Vocabulary

• An organism with two identical alleles for acharacter is said to be homozygous for thegene controlling that character

• An organism that has two different alleles for agene is said to be heterozygous for the genecontrolling that character

• Unlike homozygotes, heterozygotes are nottrue-breeding


• Because of the different effects of dominantand recessive alleles, an organism’s traits donot always reveal its genetic composition

• Therefore, we distinguish between anorganism’s phenotype, or physicalappearance, and its genotype, or geneticmakeup

• In the example of flower color in pea plants, PPand Pp plants have the same phenotype(purple) but different genotypes


Fig. 14-6Phenotype

Purple

Purple3

Purple

Genotype

1 White

Ratio 3:1

(homozygous)

(homozygous)

(heterozygous)

(heterozygous)

PP

Pp

Pp

pp

Ratio 1:2:1

1

1

2

The Testcross

• How can we tell the genotype of an individualwith the dominant phenotype?

• Such an individual must have one dominantallele, but the individual could be eitherhomozygous dominant or heterozygous

• The answer is to carry out a testcross: breedingthe mystery individual with a homozygousrecessive individual

• If any offspring display the recessive phenotype,the mystery parent must be heterozygous


Fig. 14-7

TECHNIQUE

RESULTS

Dominant phenotype, unknown genotype:

PP or Pp?

Predictions

Recessive phenotype, known genotype: pp

×

If PP If PporSperm Spermp p p p

P

P

P

p

Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

orAll offspring purple 1/2 offspring purple and

1/2 offspring white

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The Law of Independent Assortment

• Mendel derived the law of segregation byfollowing a single character

• The F1 offspring produced in this cross weremonohybrids, individuals that areheterozygous for one character

• A cross between such heterozygotes is calleda monohybrid cross


• Mendel identified his second law of inheritanceby following two characters at the same time

• Crossing two true-breeding parents differing intwo characters produces dihybrids in the F1generation, heterozygous for both characters

• A dihybrid cross, a cross between F1 dihybrids,can determine whether two characters aretransmitted to offspring as a package orindependently


Fig. 14-8

EXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis ofdependentassortment

YYRR yyrr

YR yr

YyRr

×

Hypothesis ofindependentassortment

orPredictedoffspring ofF2 generation

Sperm

SpermYR

YR

yr

yr

Yr

YR

yR

Yr

yRyr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

Phenotypic ratio 9:3:3:1

1/2 1/2

1/2

1/2

1/4

yr

1/4 1/4 1/4 1/4

1/4

1/4

1/4

1/43/4

9/163/16 3/16

1/16

Phenotypic ratio approximately 9:3:3:1315 108 101 32

• Using a dihybrid cross, Mendel developed thelaw of independent assortment

• The law of independent assortment states thateach pair of alleles segregates independentlyof each other pair of alleles during gameteformation

• Strictly speaking, this law applies only to geneson different, nonhomologous chromosomes

• Genes located near each other on the samechromosome tend to be inherited together


Concept 14.2: The laws of probability governMendelian inheritance

• Mendel’s laws of segregation and independentassortment reflect the rules of probability

• When tossing a coin, the outcome of one tosshas no impact on the outcome of the next toss

• In the same way, the alleles of one genesegregate into gametes independently ofanother gene’s alleles


• The multiplication rule states that theprobability that two or more independentevents will occur together is the product of theirindividual probabilities

• Probability in an F1 monohybrid cross can bedetermined using the multiplication rule

• Segregation in a heterozygous plant is likeflipping a coin: Each gamete has a chance ofcarrying the dominant allele and a chance ofcarrying the recessive allele

The Multiplication and Addition Rules Applied toMonohybrid Crosses


12

12

7

Fig. 14-9Rr Rr×

Segregation ofalleles into eggs

Sperm

R

R

R RR

R rrr

r

r

r1/2

1/2

1/2

1/2

Segregation ofalleles into sperm

Eggs1/4

1/4

1/41/4

• The rule of addition states that the probabilitythat any one of two or more exclusive eventswill occur is calculated by adding together theirindividual probabilities

• The rule of addition can be used to figure outthe probability that an F2 plant from amonohybrid cross will be heterozygous ratherthan homozygous


Solving Complex Genetics Problems with the Rulesof Probability

• We can apply the multiplication and additionrules to predict the outcome of crossesinvolving multiple characters

• A dihybrid or other multicharacter cross isequivalent to two or more independentmonohybrid crosses occurring simultaneously

• In calculating the chances for variousgenotypes, each character is consideredseparately, and then the individual probabilitiesare multiplied together


Fig. 14-UN1

Concept 14.3: Inheritance patterns are often morecomplex than predicted by simple Mendeliangenetics

• The relationship between genotype andphenotype is rarely as simple as in the peaplant characters Mendel studied

• Many heritable characters are not determinedby only one gene with two alleles

• However, the basic principles of segregationand independent assortment apply even tomore complex patterns of inheritance


Extending Mendelian Genetics for a Single Gene

• Inheritance of characters by a single gene maydeviate from simple Mendelian patterns in thefollowing situations:

– When alleles are not completely dominant orrecessive

– When a gene has more than two alleles

– When a gene produces multiple phenotypes


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Degrees of Dominance

• Complete dominance occurs whenphenotypes of the heterozygote and dominanthomozygote are identical

• In incomplete dominance, the phenotype ofF1 hybrids is somewhere between thephenotypes of the two parental varieties

• In codominance, two dominant alleles affectthe phenotype in separate, distinguishableways


Fig. 14-10-1

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

Fig. 14-10-2

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/2 1/2

Fig. 14-10-3

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/2 1/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/2 1/2

1/2

1/2

• A dominant allele does not subdue a recessiveallele; alleles don’t interact

• Alleles are simply variations in a gene’snucleotide sequence

• For any character, dominance/recessivenessrelationships of alleles depend on the level atwhich we examine the phenotype


The Relation Between Dominance and Phenotype

• Tay-Sachs disease is fatal; a dysfunctionalenzyme causes an accumulation of lipids in thebrain

– At the organismal level, the allele is recessive

– At the biochemical level, the phenotype (i.e.,the enzyme activity level) is incompletelydominant

– At the molecular level, the alleles arecodominant


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Frequency of Dominant Alleles

• Dominant alleles are not necessarily morecommon in populations than recessive alleles

• For example, one baby out of 400 in the UnitedStates is born with extra fingers or toes


• The allele for this unusual trait is dominant tothe allele for the more common trait of fivedigits per appendage

• In this example, the recessive allele is far moreprevalent than the population’s dominant allele


Multiple Alleles

• Most genes exist in populations in more thantwo allelic forms

• For example, the four phenotypes of the ABOblood group in humans are determined bythree alleles for the enzyme (I) that attaches Aor B carbohydrates to red blood cells: IA, IB,and i.

• The enzyme encoded by the IA allele adds theA carbohydrate, whereas the enzyme encodedby the IB allele adds the B carbohydrate; theenzyme encoded by the i allele adds neither


Fig. 14-11

IA

IB

i

ABnone

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

Allele Carbohydrate

GenotypeRed blood cell

appearancePhenotype

(blood group)

IAIA or IA i A

BIBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

Pleiotropy

• Most genes have multiple phenotypic effects, aproperty called pleiotropy

• For example, pleiotropic alleles are responsiblefor the multiple symptoms of certain hereditarydiseases, such as cystic fibrosis and sickle-celldisease


Extending Mendelian Genetics for Two or MoreGenes

• Some traits may be determined by two or moregenes


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Epistasis

• In epistasis, a gene at one locus alters thephenotypic expression of a gene at a secondlocus

• For example, in mice and many othermammals, coat color depends on two genes

• One gene determines the pigment color (withalleles B for black and b for brown)

• The other gene (with alleles C for color and cfor no color) determines whether the pigmentwill be deposited in the hair


Fig. 14-12

BbCc BbCc

Sperm

EggsBC bC Bc bc

BC

bC

Bc

bc

BBCC

1/41/4

1/41/4

1/4

1/4

1/4

1/4

BbCC BBCc BbCc

BbCC bbCC BbCc bbCc

BBCc BbCc

BbCc bbCc

BBcc Bbcc

Bbcc bbcc

9 : 3 : 4

×

Polygenic Inheritance

• Quantitative characters are those that vary inthe population along a continuum

• Quantitative variation usually indicatespolygenic inheritance, an additive effect oftwo or more genes on a single phenotype

• Skin color in humans is an example ofpolygenic inheritance


Fig. 14-13

Eggs

Sperm

Phenotypes:Number ofdark-skin alleles: 0 1 2 3 4 5 6

1/646/64

15/6420/64

15/646/64

1/64

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/81/8

1/81/8

1/81/8

1/81/8

AaBbCc AaBbCc

× Nature and Nurture: The Environmental Impacton Phenotype

• Another departure from Mendelian geneticsarises when the phenotype for a characterdepends on environment as well as genotype

• The norm of reaction is the phenotypic rangeof a genotype influenced by the environment

• For example, hydrangea flowers of the samegenotype range from blue-violet to pink,depending on soil acidity


Fig. 14-14

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• Norms of reaction are generally broadest forpolygenic characters

• Such characters are called multifactorialbecause genetic and environmental factorscollectively influence phenotype


Integrating a Mendelian View of Heredity andVariation

• An organism’s phenotype includes its physicalappearance, internal anatomy, physiology, andbehavior

• An organism’s phenotype reflects its overallgenotype and unique environmental history


Concept 14.4: Many human traits followMendelian patterns of inheritance

• Humans are not good subjects for geneticresearch

– Generation time is too long

– Parents produce relatively few offspring

– Breeding experiments are unacceptable

• However, basic Mendelian genetics endures asthe foundation of human genetics


Pedigree Analysis

• A pedigree is a family tree that describes theinterrelationships of parents and childrenacross generations

• Inheritance patterns of particular traits can betraced and described using pedigrees


Fig. 14-15a

KeyMale

Female

AffectedmaleAffectedfemale

Mating

Offspring, inbirth order(first-born on left)

Fig. 14-15b

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 Wwww ww

ww

wwWw

Ww

wwWW

Wwor

12

Fig. 14-15c

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

orFF

Ff

• Pedigrees can also be used to makepredictions about future offspring

• We can use the multiplication and additionrules to predict the probability of specificphenotypes


Recessively Inherited Disorders

• Many genetic disorders are inherited in arecessive manner


The Behavior of Recessive Alleles

• Recessively inherited disorders show up onlyin individuals homozygous for the allele

• Carriers are heterozygous individuals whocarry the recessive allele but arephenotypically normal (i.e., pigmented)

• Albinism is a recessive condition characterizedby a lack of pigmentation in skin and hair


Fig. 14-16

ParentsNormal Normal

Sperm

Eggs

Normal Normal(carrier)

Normal(carrier) Albino

Aa Aa

A

A AA Aa

a

Aa aaa

×• If a recessive allele that causes a disease is

rare, then the chance of two carriers meetingand mating is low

• Consanguineous matings (i.e., matingsbetween close relatives) increase the chanceof mating between two carriers of the samerare allele

• Most societies and cultures have laws ortaboos against marriages between closerelatives


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Cystic Fibrosis

• Cystic fibrosis is the most common lethalgenetic disease in the United States,strikingone out of every 2,500 people of Europeandescent

• The cystic fibrosis allele results in defective orabsent chloride transport channels in plasmamembranes

• Symptoms include mucus buildup in someinternal organs and abnormal absorption ofnutrients in the small intestine


Sickle-Cell Disease

• Sickle-cell disease affects one out of 400African-Americans

• The disease is caused by the substitution of asingle amino acid in the hemoglobin protein inred blood cells

• Symptoms include physical weakness, pain,organ damage, and even paralysis


Dominantly Inherited Disorders

• Some human disorders are caused bydominant alleles

• Dominant alleles that cause a lethal diseaseare rare and arise by mutation

• Achondroplasia is a form of dwarfism causedby a rare dominant allele


Fig. 14-17

Eggs

ParentsDwarf Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd × dd

dD

Dd dd

ddDd

d

d

• Huntington’s disease is a degenerativedisease of the nervous system

• The disease has no obvious phenotypic effectsuntil the individual is about 35 to 40 years ofage

Huntington’s Disease


Multifactorial Disorders

• Many diseases, such as heart disease andcancer, have both genetic and environmentalcomponents

• Little is understood about the geneticcontribution to most multifactorial diseases


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Genetic Testing and Counseling

• Genetic counselors can provide information toprospective parents concerned about a familyhistory for a specific disease


Counseling Based on Mendelian Genetics andProbability Rules

• Using family histories, genetic counselors helpcouples determine the odds that their childrenwill have genetic disorders


Tests for Identifying Carriers

• For a growing number of diseases, tests areavailable that identify carriers and help definethe odds more accurately


Fetal Testing

• In amniocentesis, the liquid that bathes thefetus is removed and tested

• In chorionic villus sampling (CVS), a sampleof the placenta is removed and tested

• Other techniques, such as ultrasound andfetoscopy, allow fetal health to be assessedvisually in utero


Video: Ultrasound of Human Fetus IVideo: Ultrasound of Human Fetus I

Fig. 14-18

Amniotic fluidwithdrawn

Fetus

Placenta

Uterus Cervix

Centrifugation

Fluid

Fetalcells

Severalhours

Severalweeks

Severalweeks

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

Severalhours

Severalhours

Fetalcells

Bio-chemical

tests

Karyotyping

Placenta Chorionicvilli

Fetus

Suction tubeinsertedthroughcervix

Newborn Screening

• Some genetic disorders can be detected atbirth by simple tests that are now routinelyperformed in most hospitals in the UnitedStates


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Fig. 14-UN2

Degree of dominance

Complete dominanceof one allele

Incomplete dominanceof either allele

Codominance

Description

Heterozygous phenotypesame as that of homo-zygous dominant

Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes

Heterozygotes: Bothphenotypes expressed

Multiple alleles

Pleiotropy

In the whole population,some genes have morethan two alleles

One gene is able toaffect multiplephenotypic characters

CRCR CRCW CWCW

IAIB

IA , IB , i

ABO blood group alleles

Sickle-cell disease

PP Pp

Example

Fig. 14-UN3

DescriptionRelationship amonggenes

Epistasis One gene affectsthe expression ofanother

Example

Polygenicinheritance

A single phenotypiccharacter isaffected bytwo or more genes

BbCc BbCc

BCBC

bC

bC

Bc

Bc

bc

bc

9 : 3 : 4

AaBbCc AaBbCc

You should now be able to:

1. Define the following terms: true breeding,hybridization, monohybrid cross, Pgeneration, F1 generation, F2 generation

2. Distinguish between the following pairs ofterms: dominant and recessive; heterozygousand homozygous; genotype and phenotype

3. Use a Punnett square to predict the results ofa cross and to state the phenotypic andgenotypic ratios of the F2 generation


4. Explain how phenotypic expression in theheterozygote differs with completedominance, incomplete dominance, andcodominance

5. Define and give examples of pleiotropy andepistasis

6. Explain why lethal dominant genes are muchrarer than lethal recessive genes

7. Explain how carrier recognition, fetal testing,and newborn screening can be used ingenetic screening and counseling


Documents

Mendel and the Gene Idea - R. Wingerden · 2015-08-25 · •Mendel called the purple flower color a dominant trait and the white flower color a recessive trait ... •Crossing two