Chapter 14 Mendel & the Gene Idea (2005)

8/3/2019 Chapter 14 Mendel & the Gene Idea (2005)

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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for

Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 14

Mendel and the Gene Idea


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Overview: Drawing from the Deck of Genes

What genetic principles account for the

transmission of traits from parents to offspring?


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One possible explanation of heredity is a

blending hypothesis The idea that genetic material contributed by

two parents mixes in a manner analogous to

the way blue and yellow paints blend to makegreen


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An alternative to the blending model is the

particulate hypothesis of inheritance: thegene idea

Parents pass on discrete heritable units, genes


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Gregor Mendel

Documented a particulate mechanism ofinheritance through his experiments with

garden peas

Figure 14.1


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Concept 14.1: Mendel used the scientific

approach to identify two laws of inheritance Mendel discovered the basic principles of

heredity

By breeding garden peas in carefully planned

experiments


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Mendels Experimental, Quantitative Approach

Mendel chose to work with peas

Because they are available in many varieties

Because he could strictly control which plants

mated with which



Crossing pea plants

Figure 14.2

1

5

4

3

2

Removed stamens

from purple flower

Transferred sperm-

bearing pollen from

stamens of white

flower to egg-

bearing carpel of

purple flower

Parental

generation(P)

Pollinated carpel

matured into pod

Carpel

(female)

Stamens

(male)

Planted seeds

from pod

Examined

offspring:

all purple

flowers

First

generation

offspring

(F1)

APPLICATION By crossing (mating) two true-breedingvarieties of an organism, scientists can study patterns of

inheritance. In this example, Mendel crossed pea plants

that varied in flower color.

TECHNIQUETECHNIQUE

When pollen from a white flower fertilizes

eggs of a purple flower, the first-generation hybrids all have purple

flowers. The result is the same for the reciprocal cross, the transfer

of pollen from purple flowers to white flowers.

TECHNIQUERESULTS



Some genetic vocabulary

Character: a heritable feature, such as flowercolor

Trait: a variant of a character, such as purple

or white flowers



Mendel chose to track

Only those characters that varied in an either-or manner

Mendel also made sure that

He started his experiments with varieties that

were true-breeding



In a typical breeding experiment

Mendel mated two contrasting, true-breedingvarieties, a process called hybridization

The true-breeding parents

Are called the P generation



The hybrid offspring of the P generation

Are called the F1 generation

When F1 individuals self-pollinate

The F2 generation is produced



The Law of Segregation

When Mendel crossed contrasting, true-

breeding white and purple flowered pea plants All of the offspring were purple

When Mendel crossed the F1 plants

Many of the plants had purple flowers, but

some had white flowers



Mendel discovered

A ratio of about three to one, purple to white flowers,in the F2 generation

Figure 14.3

P Generation

(true-breeding

parents) Purpleflowers

Whiteflowers

F1 Generation

(hybrids)

All plants had

purple flowers

F2 Generation

EXPERIMENT True-breeding purple-flowered pea plants and

white-flowered pea plants were crossed (symbolized by ). The

resulting F1 hybrids were allowed to self-pollinate or were cross-

pollinated with other F1 hybrids. Flower color was then observed

in the F2

generation.

RESULTS Both purple-flowered plants and white-

flowered plants appeared in the F2 generation. In Mendels

experiment, 705 plants had purple flowers, and 224 had white

flowers, a ratio of about 3 purple : 1 white.


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Mendel reasoned that

In the F1 plants, only the purple flower factor

was affecting flower color in these hybrids

Purple flower color was dominant, and white

flower color was recessive


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Mendel observed the same pattern

In many other pea plant characters

Table 14.1


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Mendels Model

Mendel developed a hypothesis

To explain the 3:1 inheritance pattern that heobserved among the F2 offspring

Four related concepts make up this model


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First, alternative versions of genes

Account for variations in inherited characters,which are now called alleles

Figure 14.4

Allele for purple flowers

Locus for flower-color gene

Homologous

pair of

chromosomes

Allele for white flowers


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Second, for each character

An organism inherits two alleles, one fromeach parent

A genetic locus is actually represented twice


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Third, if the two alleles at a locus differ

Then one, the dominant allele, determines theorganisms appearance

The other allele, the recessive allele, has no

noticeable effect on the organismsappearance


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Fourth, the law of segregation

The two alleles for a heritable characterseparate (segregate) during gamete formation

and end up in different gametes


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Does Mendels segregation model account for

the 3:1 ratio he observed in the F2 generationof his numerous crosses?

We can answer this question using a Punnett

square


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Mendels law of segregation, probability and

the Punnett square

Figure 14.5

P Generation

F1 Generation

F2 Generation

P p

P p

P p

P

p

PpPP

ppPp

Appearance:

Genetic makeup:Purple flowers

PP

White flowers

pp

Purple flowers

Pp

Appearance:

Genetic makeup:

Gametes:

Gametes:

F1 sperm

F1 eggs

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Each true-breeding plant of the

parental generation has identical

alleles, PPorpp.

Gametes (circles) each contain only

one allele for the flower-color gene.

In this case, every gamete produced

by one parent has the same allele.

Union of the parental gametesproduces F1 hybrids having a Pp

combination. Because the purple-

flower allele is dominant, all

these hybrids have purple flowers.

When the hybrid plants produce

gametes, the two alleles segregate,

half the gametes receiving the P

allele and the other half thep allele.

3 : 1

Random combination of the gametes

results in the 3:1 ratio that Mendel

observed in the F2 generation.

This box, a Punnett square, shows

all possible combinations of allelesin offspring that result from an

F1 F1 (Pp Pp) cross. Each square

represents an equally probable productof fertilization. For example, the bottom

left box shows the genetic combination

resulting from a p egg fertilized by

a P sperm.


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Useful Genetic Vocabulary

An organism that is homozygous for a

particular gene Has a pair of identical alleles for that gene

Exhibits true-breeding

An organism that is heterozygous for a

particular gene

Has a pair of alleles that are different for thatgene


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An organisms phenotype

Is its physical appearance

An organisms genotype

Is its genetic makeup


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Phenotype versus genotype

Figure 14.6

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP

(homozygous)

Pp

(heterozygous)

Pp

(heterozygous)

pp

(homozygous)

Ratio 3:1 Ratio 1:2:1

Th T


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The Testcross

In pea plants with purple flowers

The genotype is not immediately obvious


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A testcross

Allows us to determine the genotype of anorganism with the dominant phenotype, but

unknown genotype

Crosses an individual with the dominantphenotype with an individual that is

homozygous recessive for a trait


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The testcross

Figure 14.7

Dominant phenotype,

unknown genotype:

PPorPp?

Recessive phenotype,

known genotype:

pp

IfPP,

then all offspring

purple:

IfPp,

then 12 offspring purple

and 12 offspring white:

p p

P

P

Pp Pp

PpPp

pp pp

PpPpP

p

p p

APPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous for

the dominant allele or heterozygous. To determine the organisms

genotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with the

unknown genotype is crossed with a homozygous individual

expressing the recessive trait (white f lowers in this example).

By observing the phenotypes of the offspring resulting from thiscross, we can deduce the genotype of the purple-flowered

parent.

RESULTS

Th L f I d d t A t t


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The Law of Independent Assortment

Mendel derived the law of segregation

By following a single trait

The F1 offspring produced in this cross

Were monohybrids, heterozygous for onecharacter


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Mendel identified his second law of inheritance

By following two characters at the same time

Crossing two, true-breeding parents differing in

two characters

Produces dihybrids in the F1 generation,

heterozygous for both characters


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How are two characters transmitted from

parents to offspring?

As a package?

Independently?


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YYRRP Generation

Gametes YR yr

yyrr

YyRrHypothesis of

dependent

assortment

Hypothesis of

independent

assortment

F2 Generation

(predicted

offspring)

12 YR

YR

yr

12

12

12 yr

YYRR YyRr

yyrrYyRr

34 14

Sperm

Eggs

Phenotypic ratio 3:1

YR14

Yr14

yR14

yr14

916316

316116

YYRR YYRr YyRR YyRr

YyrrYyRrYYrrYYrr

YyRR YyRr yyRR yyRr

yyrryyRrYyrrYyRr

Phenotypic ratio 9:3:3:1

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

F1 Generation

Eggs

YR Yr yR yr 1414

1414

Sperm

RESULTS

CONCLUSION The results support the hypothesis of

independent assortment. The alleles for seed color and seed

shape sort into gametes independently of each other.

EXPERIMENT Two true-breeding pea plants

one with yellow-round seeds and the other with

green-wrinkled seedswere crossed, producingdihybrid F1 plants. Self-pollination of the F1 dihybrids,

which are heterozygous for both characters,

produced the F2 generation. The two hypotheses

predict different phenotypic ratios. Note that yellow

color (Y) and round shape (R) are dominant.

A dihybrid cross

Illustrates the inheritance of two characters

Produces four phenotypes in the F2 generation

Figure 14.8


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Using the information from a dihybrid cross,

Mendel developed the law of independent

assortment

Each pair of alleles segregates independently

during gamete formation


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Concept 14.2: The laws of probability govern

Mendelian inheritance

Mendels laws of segregation and independent

assortment

Reflect the rules of probability

The Multiplication and Addition Rules Applied to


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The Multiplication and Addition Rules Applied toMonohybrid Crosses

The multiplication rule States that the probability that two or more

independent events will occur together is the

product of their individual probabilities


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Probability in a monohybrid cross

Can be determined using this ruleRr

Segregation of

alleles into eggs

Rr

Segregation of

alleles into sperm

R r

rR

RR

R12

1212

1414

1414

12 r

rR r

r

Sperm

Eggs

Figure 14.9


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The rule of addition

States that the probability that any one of twoor more exclusive events will occur is

calculated by adding together their individual

probabilities

Solving Complex Genetics Problems with the Rules


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Solving Complex Genetics Problems with the Rulesof Probability

We can apply the rules of probability To predict the outcome of crosses involving

multiple characters


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A dihybrid or other multicharacter cross

Is equivalent to two or more independentmonohybrid crosses occurring simultaneously

In calculating the chances for various

genotypes from such crosses

Each character first is considered separately

and then the individual probabilities are

multiplied together


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Concept 14.3: Inheritance patterns are often

more complex than predicted by simple

Mendelian genetics

The relationship between genotype and

phenotype is rarely simple

Extending Mendelian Genetics for a Single Gene


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Extending Mendelian Genetics for a Single Gene

The inheritance of characters by a single gene

May deviate from simple Mendelian patterns

The Spectrum of Dominance


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The Spectrum of Dominance

Complete dominance

Occurs when the phenotypes of theheterozygote and dominant homozygote are

identical


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In codominance

Two dominant alleles affect the phenotype inseparate, distinguishable ways

The human blood group MN

Is an example of codominance


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In incomplete dominance

The phenotype of F1 hybrids is somewhere betweenthe phenotypes of the two parental varieties

Figure 14.10

P Generation

F1 Generation

F2 Generation

Red

CRCR

Gametes CR CW

White

CWCW

Pink

CRCW

Sperm

CR

CR

CR

Cw

CR

CRGametes

12 12

12

12

12

Eggs12

CRCR CRCW

CWCWCRCW


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The Relation Between Dominance and

Phenotype

Dominant and recessive alleles

Do not really interact

Lead to synthesis of different proteins that

produce a phenotype


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Frequency of Dominant Alleles

Dominant alleles

Are not necessarily more common in

populations than recessive alleles

Multiple Alleles


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Multiple Alleles

Most genes exist in populations

In more than two allelic forms


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The ABO blood group in humans

Is determined by multiple alleles

Table 14.2

Pleiotropy


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Pleiotropy

In pleiotropy

A gene has multiple phenotypic effects

Extending Mendelian Genetics for Two or More Genes


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Extending Mendelian Genetics for Two or More Genes

Some traits

May be determined by two or more genes

Epistasis


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Epistasis

In epistasis

A gene at one locus alters the phenotypicexpression of a gene at a second locus


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An example of epistasis

Figure 14.11

BC bC Bc bc1414

1414

BC

bC

Bc

bc

14

14

14

14

BBCc BbCc BBcc Bbcc

Bbcc bbccbbCcBbCc

BbCC bbCC BbCc bbCc

BBCC BbCC BBCc BbCc

916316

416

BbCc BbCc

Sperm

Eggs

Polygenic Inheritance


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Polygenic Inheritance

Many human characters

Vary in the population along a continuum andare called quantitative characters


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AaBbCc AaBbCc

aabbccAabbcc AaBbcc AaBbCc AABbCcAABBCcAABBCC

2064

1564

664

164

Frac

tion

of

pro

geny

Quantitative variation usually indicates

polygenic inheritance

An additive effect of two or more genes on a

single phenotype

Figure 14.12

Nature and Nurture: The Environmental Impact


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N u e d Nu u e: e v o e p con Phenotype

Another departure from simple Mendeliangenetics arises

When the phenotype for a character depends

on environment as well as on genotype


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The norm of reaction

Is the phenotypic range of a particulargenotype that is influenced by the environment

Figure 14.13


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Multifactorial characters

Are those that are influenced by both geneticand environmental factors

Integrating a Mendelian View of Heredity and Variation


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g g y

An organisms phenotype

Includes its physical appearance, internalanatomy, physiology, and behavior

Reflects its overall genotype and unique

environmental history


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Even in more complex inheritance patterns

Mendels fundamental laws of segregation andindependent assortment still apply


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Concept 14.4: Many human traits follow

Mendelian patterns of inheritance

Humans are not convenient subjects for

genetic research

However, the study of human geneticscontinues to advance

Pedigree Analysis


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g y

A pedigree

Is a family tree that describes theinterrelationships of parents and children

across generations


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Inheritance patterns of particular traits

Can be traced and described using pedigrees

Figure 14.14 A, B

Ww ww ww Ww

wwWwWwwwwwWw

WW

or

Ww

ww

First generation

(grandparents)

Second generation

(parents plus aunts

and uncles)

Third

generation

(two sisters)

Ff Ff ff Ff

ffFfFfffFfFForFf

ff FF

or

Ff

Widows peak No Widows peak Attached earlobe Free earlobe

(a) Dominant trait (widows peak) (b) Recessive trait (attached earlobe)


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Pedigrees

Can also be used to make predictions aboutfuture offspring

Recessively Inherited Disorders


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Many genetic disorders

Are inherited in a recessive manner


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Recessively inherited disorders

Show up only in individuals homozygous forthe allele

Carriers

Are heterozygous individuals who carry the

recessive allele but are phenotypically normal

Cystic Fibrosis


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Symptoms of cystic fibrosis include

Mucus buildup in the some internal organs

Abnormal absorption of nutrients in the small

intestine

Sickle-Cell Disease


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Sickle-cell disease

Affects one out of 400 African-Americans

Is caused by the substitution of a single amino

acid in the hemoglobin protein in red blood

cells

Symptoms include

Physical weakness, pain, organ damage, andeven paralysis

Mating of Close Relatives


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Matings between relatives

Can increase the probability of the appearanceof a genetic disease

Are called consanguineous matings

Dominantly Inherited Disorders


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Some human disorders

Are due to dominant alleles


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One example is achondroplasia

A form of dwarfism that is lethal whenhomozygous for the dominant allele

Figure 14.15


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Huntingtons disease

Is a degenerative disease of the nervoussystem

Has no obvious phenotypic effects until about

35 to 40 years of age

Figure 14.16

Multifactorial Disorders


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Many human diseases

Have both genetic and environmentcomponents

Examples include

Heart disease and cancer

Genetic Testing and Counseling


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Genetic counselors

Can provide information to prospective parentsconcerned about a family history for a specific

disease

Counseling Based on Mendelian Genetics andP b bili R l


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Probability Rules

Using family histories

Genetic counselors help couples determine the

odds that their children will have genetic

disorders

Tests for Identifying Carriers


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For a growing number of diseases

Tests are available that identify carriers andhelp define the odds more accurately


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Fetal testing

Figure 14.17 A, B

(a) Amniocentesis

Amniotic

fluidwithdrawn

Fetus

Placenta Uterus Cervix

Centrifugation

A sample of

amniotic fluid canbe taken starting at

the 14th to 16thweek of pregnancy.

(b) Chorionic villus sampling (CVS)

Fluid

Fetal

cells

Biochemical tests can bePerformed immediately on

the amniotic fluid or later

on the cultured cells.

Fetal cells must be cultured

for several weeks to obtainsufficient numbers for

karyotyping.

Several

weeks

Biochemical

tests

Severalhours

Fetalcells

Placenta Chorionic viIIi

A sample of chorionic villus

tissue can be taken as earlyas the 8th to 10th week ofpregnancy.

Suction tube

Inserted through

cervix

Fetus

Karyotyping and biochemicaltests can be performed on

the fetal cells immediately,providing results within a day

or so.

Karyotyping

Newborn Screening


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Some genetic disorders can be detected at

birth

By simple tests that are now routinely

performed in most hospitals in the United

States

Documents

Chapter 14 Mendel & the Gene Idea (2005)