Mendel and GenesChapter 14

Fig. 14-1

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

•The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)

• How can we tell the genotype of an individual with the dominant phenotype?– The answer is to carry out a testcross:

Fig. 14-7TECHNIQUE

RESULTS

Dominant phenotype, unknown genotype:

PP or Pp?

Predictions

Recessive phenotype, known genotype: pp

If PP If Ppor

Sperm Spermp p p p

P

P

P

p

Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

or

All offspring purple 1/2 offspring purple and1/2 offspring white

Concept 14.2: The laws of probability govern Mendelian inheritance

• Mendel’s laws of segregation and independent assortment 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 independently of another gene’s alleles

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• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

– Probability in an F1 monohybrid cross can be determined using the multiplication rule

• 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 the recessive allele

The Multiplication and Addition Rules Applied to Monohybrid Crosses

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1

2

1

2

Fig. 14-9 Rr RrSegregation of

alleles 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 probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

• The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

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Fig. 14-9 Rr RrSegregation of

alleles 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

Summary of Mendel’s Principles

• The inheritance of biological characteristics is determined by individual units known as genes. In organisms that reproduce sexually, genes are passed from parents to their offspring.

• In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.

• In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed.

• The alleles for different genes usually segregate independently of one another.

Concept 14.3: Inheritance patterns are often more complex than predicted by simple

Mendelian genetics

• The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied

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Beyond dominant/recessive

• incomplete dominance - situation in which one allele is not completely dominant over another

• There is no white no red: new phenotype pink

Fig. 14-10-3

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

Beyond dominant and recessive alleles

• Codominance - situation in which both alleles of a gene contribute to the phenotype of the organism

Beyond dominant and recessive alleles

• multiple alleles - three or more alleles of the same gene

Fig. 14-11

IA

IB

i

A

B

none(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

Polygenic Inheritance

• Quantitative characters are those that vary in the population along a continuum

• Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Height and Skin color in humans is an example of polygenic 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

• 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

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Frequency of Dominant Alleles

Pleiotropy

• Most genes have multiple phenotypic effects, a property called pleiotropy

• For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as– cystic fibrosis

– PKU

– Sickle-cell disease

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Epistasis

• In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus– For example, in mice and many other mammals, coat

color depends on two genes

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

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

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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

Nature and Nurture: The Environmental Impact on Phenotype

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

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

• multifactorialCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Pedigree Analysis

• A pedigree is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced and described using pedigrees

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

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

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

(Start Next)The Behavior of Recessive Alleles

• Recessively inherited disorders show up only in individuals homozygous for the allele

• Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)

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

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Fig. 14-16

Parents

Normal Normal

Sperm

Eggs

NormalNormal(carrier)

Normal(carrier) Albino

Aa Aa

A

AAA

Aa

a

Aaaa

a

Autosomal Disorders

• Genes for these disorders are located on autosomes– Recessive disorder– Dominant disorders– Codominant disorders

Cystic Fibrosis

Fig. 14-17

Eggs

Parents

Dwarf Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd dd

dD

Dd dd

ddDd

d

d

Multifactorial Disorders

• Many diseases, such as heart disease and cancer, have both genetic and environmental components– Little is

understood about the genetic contribution to most multifactorial diseases

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Genetic Testing and Counseling

• Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

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

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

Karyotype• Making a

Karyotype– Photograph

chromosomes during mitosis

– Cut chromosomes out of photograph

– Group them in order, in pairs

– Male 46XY– Female

46XX

Sex-Linked Genes

• Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.

• Some disorders caused by recessive alleles on the X chromosome in humans:– Color blindness– Duchenne muscular dystrophy– Hemophilia

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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