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CAMPBELL BIOLOGY IN FOCUSUrry Cain Wasserman Minorsky Jackson Reece

11Mendel and

the Gene Idea

(Ayo)

Ayo website:

http://myweb.nutn.edu.tw/~hycheng/

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Overview: Drawing from the Deck of Genes

What genetic principles account for the passing oftraits from parents to offspring?

blendinghypothesis

the way blue and yellow paint blend to make green

particulatehypothesis is the idea that parents

pass on discrete heritable units (genes).

Mendeldocumented a particulatemechanismthrough his experiments with garden peas.

2

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Chap.11 Mendel and the Gene Idea

11.1: Mendel used the scientific approachtoidentify two laws of inheritance

11.2: The laws of probability()govern

Mendelian inheritance 11.3: Inheritance patterns are often more complex

than predicted by simple Mendelian genetics

11.4: Many human traits follow Mendelian patternsof inheritance

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Gregor Mendel

4

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5

11.1: Mendel used the scientific approach toidentify two laws of inheritance

Mendel discovered the basic principles of heredity bybreeding garden peas in carefully planned

experiments

Mendel probably chose to work with peasbecause1. There are many varieties with distinct heritable

features, or characters (such as flower color);

character variants (such as purple or white flowers)

are called traits.

2. He could control mating between plants

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Figure 11.2

Parentalgeneration(P)

Stamens

First filialgenerationoffspring(F1)

Carpel

Technique

Results

1

2

3

4

5

Mendels Experimental,

Quantitative Approach

6

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In a typical experiment, Mendel mated twocontrasting varieties, a process called

hybridization.

The parents are the P generation.

The hybrid offspring of the P generation are called

the F1generation.

When F1individuals self-pollinate or cross- pollinate

with other F1hybrids, the F2generation is produced.

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The Law of Segregation

When Mendel crossed contrasting white- and purple-flowered pea plants, all of the F1hybrids were purple

When Mendel crossed the F1hybrids, many of the

F2

plants had purple flowers, but some had white

Mendel discovered a ratio of about three to one,

purple to white flowers, in the F2generation

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Figure 11.3-1

P Generation

Experiment

Purple flowers White flowers

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Figure 11.3-2

P Generation

Experiment

F1Generation

(hybrids)


All plants had purple flowers

Self- or cross-pollination

10

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Figure 11.3-3

P Generation

Experiment

F1Generation

F2Generation

(hybrids)


All plants had purple flowers

Self- or cross-pollination

705 purple-flowered

plants

224 white-flowered

plants 11

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

The factor for white flowers was not diluted or

destroyed because it reappeared in the F2

generation.

Mendel observed the same pattern of inheritance

in six other pea plant characters, each

represented by two traits. What Mendel called a heritable factor()

is what we now call a gene ( ).

T bl 11 1

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Table 11.1a

13

T bl 11 1b

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Table 11.1b

14

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MendelsModel

Four related concepts make up this model First,alternative versions of genesaccount for

variations in inherited characters.

These alternative versions of a gene are now calledalleles ( ).

Each generesides at a specific locus()on a

specific chromosome

Fig re 11 4

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Figure 11.4

Allele for purple flowers

Pair of

homologouschromosomes

Allele for white flowers

Locus for flower-color gene

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Second, for each character, an organism inheritstwo alleles, one from each parent.

Mendel made this deduction without knowing about

the existence of chromosomes

Two alleles at a particular locus may be identical, as

in the true-breeding plants of MendelsP generation

Alternatively, the two alleles at a locus may differ, as

in the F1hybrids

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Third, if the two alleles at a locus differ, then one(the dominant allele) determines the organisms

appearance, and the other (the recessive allele)

has no noticeable effect on appearance

In the flower-color example, the F1plants had purple

flowers because the allele for that trait is dominant

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Fourth(now known as the law of segregation),the two alleles for a heritable character separate

(segregate) during gamete formation and end up

in different gametes

Thus, an egg or a sperm gets only one of the two

alleles that are present in the organism

This segregation of alleles corresponds to the

distribution of homologous chromosomes to differentgametes in meiosis

Figure 11 5-1

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Figure 11.5-1

P Generation

Gametes:

Appearance:Genetic makeup:

Purple flowersPP

White flowerspp

P p

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Figure 11 5-3

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Figure 11.5 3

P Generation

Gametes:


F1Generation

F2Generation

Purple flowersPP

White flowerspp

Gametes:


Eggs fromF1(Pp) plant

Sperm fromF1(Pp) plant

Purple flowersPp

P

P

p

p

P p

P

p

PP

pp

Pp

Pp

3 : 1 22

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Mendelssegregation model accounts for the 3:1ratio he observed in the F2generation of hisnumerous crosses

The possible combinations of sperm and egg can be

shown using a Punnett square, a diagram forpredicting the results of a genetic cross betweenindividuals of known genetic makeup

A capital letter represents a dominant allele, and alowercase letter represents a recessive allele

For example, Pis the purple-flower allele andpis thewhite-flower allele

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Useful Genetic Vocabulary

An organism with two identical alleles for acharacter is said to be homozygous for the gene

controlling that character.

An organism that has two different alleles for a

gene is said to be heterozygous for the gene

controlling that character.

Unlike homozygotes, heterozygotesare not true-

breeding.

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Because of the effects of dominant and recessivealleles, an organismstraits do not always reveal its

genetic composition

Therefore, we distinguish between an organisms

phenotype(

), or physical appearance, and its

genotype( ), or genetic makeup

In the example of flower color in pea plants, PP and

Ppplants have the same phenotype (purple) butdifferent genotypes

Figure 11.6

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g

Phenotype

1

Genotype

Purple

Purple

Purple

White

Ratio 3:1

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 1:2:1

2

3

1

1

26

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The Testcross

How can we tell the genotype of an individual withthe dominant phenotype?

Such an individual could be either homozygous

dominant or heterozygous

The answer is to carry out a testcross: breeding the

mystery individual with a homozygous recessive

individual

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

Figure 11.7T h i

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gTechnique

Predictions

Dominant phenotype,unknown genotype:

PPor Pp?

Eggs

Sperm

offspring purple and

offspring white

Recessive phenotype,known genotype:

pp

If purple-floweredparent isPP

If purple-floweredparent isPp

Eggs

Sperm

All offspring purple

Results

or

or

p p

P

p

Pp

pp

Pp

pp

p p

P

P

Pp

Pp

Pp

Pp

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The Law of Independent Assortment

Mendel derived the law of segregation by followinga single character

The F1offspring produced in this cross were

monohybrids, individuals that are heterozygous for

one character

A cross between such heterozygotes is called a

monohybrid cross

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Mendel identified his second law of inheritance byfollowing two characters at the same time

Crossing two true-breeding parents differing in

two characters produces dihybrids in the F1

generation, heterozygousfor both characters

A dihybrid cross, a cross between F1dihybrids,

can determine whether two characters are

transmitted to offspring as a package orindependently

Figure 11.8a

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Experiment

P Generation

F1Generation

Gametes

YYRR yyrr

YyRr

YR y r

31

Figure 11.8b

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YR yr

YR

y r

YYRR

yyr r

YyRr

YyRr

Predicted

offspring inF2generation

Results

Eggs

Eggs

Sperm

Sperm

Hypothesis of

dependent assortment

Phenotypic ratio 3:1

Hypothesis of

independent assortment

Phenotypic ratio approximately 9:3:3:1

Phenotypic ratio 9:3:3:1

YR yr

YR

yr

YYRR

yyr r

YYRr YyRr

Yr yR

Yr

yR

YyRR

YYRr YYrr YyrrYyRr

YyRR YyRr yyRryyRR

YyRr Yyrr yyRr

315 108 101 32

916 316316 116

32

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The results of Mendels dihybrid experiments arethe basis for the law of independent

assortment

It states that each pair of alleles segregates

independentlyof each other pair of allelesduring gamete formation

11 2 Th l f b bili M d li

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11.2: The laws of probability govern Mendelianinheritance

Mendels laws of segregation and independentassortment reflect the rules of probability

When tossing a coin, the outcome of one toss has

no impact on the outcome of the next toss

In the same way, the alleles of one gene

segregate into gametes independentlyof another

genes alleles

Th M l i li i d Addi i R l A li d

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The multiplication rulestates that the probabilitythat two or more independent events will occur

together is the product of their individual

probabilities

This can be applied to an F1monohybrid cross

Segregation in a heterozygous plant is like flipping a

coin: Each gamete has a chance of carrying the

dominant allele and a chance of carrying therecessive allele

The Multiplication and Addition Rules Appliedto Monohybrid Crosses

12

12

Figure 11.9

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R r

R

r

R

r

Segregation ofalleles into eggs

Eggs

Sperm

Segregation ofalleles into sperm

Rr Rr

R

R

r

r

R

r

36

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The addition rulestates that the probability that anyone of two or more mutually exclusive events will

occur is calculated by adding together their

individual probabilities

It can be used to figure out the probability that an F2plant from a monohybrid cross will be heterozygous

rather than homozygous

S l i C l G ti P bl ith th

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Solving Complex Genetics Problems with theRules of Probability

We can apply the rules of probability to predict theoutcome of crosses involving multiple characters

A dihybrid or other multicharacter cross is equivalent

to two or more independent monohybrid crosses

occurring simultaneously

In calculating the chances for various genotypes,

each character is considered separately, and then

the individual probabilities are multiplied

Figure 11.UN01

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39

For example, if we cross F1heterozygotes of

genotype YyRr, we can calculate the

probability of different genotypes among the

F2generation

Figure 11.UN02

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40

For example, for the cross PpYyRr Ppyyrr,

we can calculate the probability of offspringshowing at least two recessive traits

11 3: Inheritance patterns are often more complex

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11.3: Inheritance patterns are often more complexthan predicted by simple Mendelian genetics

Not all heritable characters are determined assimply as the traits Mendel studied

However, the basic principles of segregation and

independent assortment apply even to more

complex patternsof inheritance

E t di M d li G ti f Si l G

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Extending Mendelian Genetics for a Single Gene

Inheritance of characters by a single gene maydeviatefrom simple Mendelian patterns in the

following situations

1. When alleles are not completely dominant or

recessive

2. When a gene has more than two alleles

3. When a single gene influences multiple

phenotypes

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1. Degrees of Dominance

Complete dominanceoccurs when phenotypes ofthe heterozygote and dominant homozygote are

identical

In incomplete dominance, the phenotype of F1

hybrids is somewhere between the phenotypes ofthe two parental varieties

In codominance, two dominant alleles affect the

phenotype in separate, distinguishable ways

Figure 11.10-1

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P Generation

Gametes

WhiteCWCW

RedCRCR

CWCR

44

Figure 11.10-2

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P Generation

F1Generation

Gametes

Gametes

WhiteCWCW

PinkCRCW

RedCRCR

CWCR

CWCR

45

Figure 11.10-3

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Eggs

P Generation

F1Generation

Gametes

F2Generation

Gametes

Sperm

WhiteCWCW

PinkCRCW

RedCRCR

CWCW

CRCWCRCR

CWCR

CWCR

CW

CR

CW

CR

CRCW

46

Th R l ti hi B t D i d

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The Relationship Between Dominance and

Phenotype

Alleles are simply variations in a genesnucleotide

sequence

When a dominant allele coexists with a recessive

allele in a heterozygote, they do not actually interact

at all

For any character, dominant/recessive relationships

of alleles depend on the level at which we examine

the phenotype

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Tay-Sachsdisease (

) is fatal; adysfunctional enzyme causes an accumulation of

lipids in the brain

At the organismallevel, the allele is recessive

At the biochemicallevel, the phenotype (i.e., the

enzyme activity level) is incompletely dominant

At the molecularlevel, the alleles are codominant

()

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Frequency of Dominant Alleles

Dominant alleles are not necessarily more common

in populations than recessive alleles

For example, one baby out of 400 in the United

States is born with extra fingers or toes, a dominant

trait called polydactyly

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Mul tiple Alleles

Most genes exist in populations in more than two

allelic forms

For example, the four phenotypes of the ABO blood

group in humans are determined by three alleles of

the gene: IA

, IB

, and i. The enzyme (I) adds specific carbohydrates to the

surface of blood cells

The enzyme encoded by I

A

adds the A carbohydrate,and the enzyme encoded by IBadds the B

carbohydrate; the enzyme encoded by the iallele

adds neither

Figure 11.11

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Carbohydrate

(b) Blood group genotypes and phenotypes

Allele

Red blood cellappearance

Genotype

noneBA

IB

Phenotype(blood group)

iIA

IA IB i iIA IA orIA i IBIB orIBi

BA OAB

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

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Pleiotropy

Most genes have multiple phenotypic effects, aproperty called pleiotropy.

For example, pleiotropic alleles are responsible for

the multiple symptoms of certain hereditary diseases,

such as cystic fibrosis (

) and sickle-cell disease.

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Epistasis (

)

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

For example, in Labrador retrievers and many other

mammals, coat color depends on two genes.

One gene determines the pigment color (with alleles B

for black and bfor brown).

The other gene (with alleles Cfor color and cfor no

color) determines whether the pigment will bedeposited in the hair.

Figure 11.12

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BE Be

BE

be

BBEE

bbee

BbEE BbEe

bE be

bE

Be

BBEe

BbEE bbEE bbEeBbEe

BBEe BbEe BbeeBBee

BbEe bbEe Bbee

9 : 4: 3

Eggs

Sperm

BbEe BbEe

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Polygenic Inheritance

Quantitative charactersare those that vary in thepopulation along a continuum

Quantitative variation usually indicates polygenic

inheritance, an additive effect of two or more genes

on a single phenotype

Skin color in humans is an example of polygenic

inheritance

Figure 11.13

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Eggs

Sperm

AaBbCc AaBbCc

Phenotypes:

0

Number of

dark-skin alleles: 1 2 3 4 5 6

164

164

664

664

164

1564

1564

2064

18

1 8

18

18

18

18

18

18

1

8

1

8

1

8

1

8

1

8

1

8

1

8

1

8

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Nature and Nurture: The Environmental Impact

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Nature and Nurture: The Environmental Impacton Phenotype

Another departure from Mendelian genetics ariseswhen the phenotype for a character depends on

environmentas well as genotype

The norm of reaction is the phenotypic range of a

genotype influenced by the environment.

The phenotypic range is generally broadest for

polygenic characters.

Such characters are called multifactorialbecause

genetic and environmental factors collectively

influence phenotype.

11 4: Many human traits follow Mendelian

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11.4: Many human traits follow Mendelianpatterns of inheritance

Humans are notgood subjects for genetic research1. Generation time is too long

2. Parents produce relatively few offspring

3. Breeding experiments are unacceptable

However, basic Mendelian genetics endures as the

foundation of human genetics

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Pedigree Analysis

A pedigreeis a family tree that describes theinterrelationships of parents and children across

generations.

Inheritance patterns of particular traits can be traced

and described using pedigrees.

Pedigreescan also be used to make predictions

about future offspring.

Figure 11.14

K

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WW

or

Ww

ww

wwww

ww ww

WwWw

Ww Ww

No widows peakWidows peak

wwWw

1st generation(grandparents)

3rd generation(two sisters)

2nd generation(parents,

aunts, anduncles)

Affectedmale

Affectedfemale

Male Female

Key

Mating

Attachedearlobe

Freeearlobe

Offspring, inbirth order(first-born on left)

FF

or

Ff

ff

ff ff

Ff ff

FfFF or

Ff

Ff Ff

ffFf

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

or recessive trait?

60

Figure 11.14a

Key

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WW

or

Ww

ww

ww ww

ww ww

WwWw

Ww Ww

Widows peak

wwWw



2nd generation(parents, aunts,and uncles)

Affectedmale

Affectedfemale

Male

Female

Mating


(a) Is a widows peak a dominant or recessive trait?

No widows peak

61

Figure 11.14b

Aff t dM l

Key

M ti

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Affectedmale

Affectedfemale

Male

Female

Mating




2nd generation(parents, aunts,and uncles)

Attached earlobe Free earlobe

FF

or

Ff

ff

ff ff

Ff ff

FfFF or

Ff

Ff Ff

ffFf

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

62

Recessively Inherited Disorders

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Recessively Inherited Disorders

Many genetic disorders are inherited in a recessive

manner

These range from relatively mild to life-threatening

The Behavior of Recessive Alleles

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The Behavior of Recessive Alleles

Recessively inherited disorders show up only in

individuals homozygous for the allele

Carriersare heterozygous individuals who carry the

recessive allele but are phenotypically normal

Most people who have recessive disorders are born

to parents who are carriers of the disorder

Figure 11.15

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Parents

Sperm

NormalAa

NormalAa

EggsAA

Normal

AaNormal(carrier)

AaNormal

(carrier)

aaAlbino

A

a

A a

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If a recessive allele that causes a disease is rare,

then the chance of two carriers meeting and mating

is low

Consanguineous (between close relatives) matings

increase the chance of mating between two carriersof the same rare allele

Most societies and cultures have laws or taboos

against marriages between close relatives

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Cystic F ibrosis

Cystic fibrosisis the most common lethal genetic

disease in the United States,striking one out of every

2,500 people of European descent

The cystic fibrosis allele results in defective or

absent chloride transport channels in plasmamembranes leading to a buildup of chloride ions

outside the cell

Symptoms include mucus buildup in some internalorgans and abnormal absorption of nutrients in the

small intestine

Sickle-Cell Disease: A Genetic Disorder with

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Evolutionary Implications

Sickle-cell diseaseaffects one out of 400 African-Americans

The disease is caused by the substitution of a single

amino acid in the hemoglobin protein in red blood

cells

In homozygous individuals, all hemoglobin is

abnormal (sickle-cell)

Symptoms include physical weakness, pain, organ

damage, and even paralysis

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Heterozygotes (said to have sickle-cell trait) are

usually healthy but may suffer some symptoms

About one out of ten African-Americans has sickle-cell

trait, an unusually high frequency of an allele with

detrimental effects in homozygotes

Heterozygotes are less susceptible to the malaria

parasite, so there is an advantage to being

heterozygous

D i tl I h it d Di d

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Dominantly Inherited Disorders

Some human disordersare caused by dominant

alleles

Dominant alleles that cause a lethal disease are

rare and arise by mutation

Achondroplasiais a form of dwarfismcaused by a

rare dominant allele

Figure 11.16

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Parents

Sperm

DwarfDd

Normaldd

Eggs

DdDwarf

ddNormal

DdDwarf

ddNormal

d

d

D d

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The timing of onset of a disease significantly affects

its inheritance.

Huntingtonsdiseaseis a degenerative disease of

the nervous system.

The disease has no obvious phenotypic effects until

the individual is about 35 to 45 years of age.

Once the deterioration of the nervous system begins

the condition is irreversible and fatal.

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Multifactorial Disorders

Many diseases, such as heart disease, diabetes,

alcoholism, mental illnesses, and cancer, have both

geneticand environmentalcomponents.

Lifestyle has a tremendous effect on phenotype for

cardiovascular health and other multifactorialcharacters.

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Genetic Counseling Based on Mendelian Genetics

Genetic counselors can provide information to

prospective parents concerned about a family history

for a specific disease.

Avoiding simple Mendelian disorders is possible

when the risk of a particular genetic disorder can beassessed before a child is conceived or during the

early stages of the pregnancy.

Many hospitals have genetic counselors who canprovide information to prospective parents concerned

about a family history for a specific disease.

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Ayo NUTN website: http://myweb.nutn.edu.tw/~hycheng/
http://myweb.nutn.edu.tw/~hycheng/http://myweb.nutn.edu.tw/~hycheng/

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