Introduction: Mendelian Genetics

Introduction: Mendelian Genetics

Copyright © 2009 Pearson Education, Inc.

Chapter 9

The science of genetics has ancient roots

Pangenesis was an early explanation for inheritance

– It was proposed by Hippocrates

– Particles called pangenes came from all parts of theorganism to be incorporated into eggs or sperm

– Characteristics acquired during the parents’ lifetime couldbe transferred to the offspring

– Aristotle rejected pangenesis and argued that instead ofparticles, the potential to produce the traits was inherited

Blending was another idea, based on plant breeding

– Hereditary material from parents mixes together to forman intermediate trait, like mixing paint


Experimental genetics began in an abbeygarden

Gregor Mendel discovered principles of genetics inexperiments with the garden pea

– Mendel showed that parents pass heritable factors tooffspring (heritable factors are now called genes)

– Advantages of using pea plants

– Controlled matings

– Self-fertilization or cross-fertilization

– Observable characteristics with two distinct forms

– True-breeding strains


Flower color White

Axial

Purple

Flower position Terminal

YellowSeed color Green

RoundSeed shape Wrinkled

InflatedPod shape Constricted

GreenPod color Yellow

TallStem length Dwarf

MENDEL’S LAWS

Mendel’s first law describes the inheritance of asingle character

Example of a monohybrid cross

– Parental generation: purple flowers × white flowers

– F1 generation: all plants with purple flowers

– F2 generation: of plants with purple flowers of plants with white flowers

Mendel needed to explain

– Why one trait seemed to disappear in the F1generation

– Why that trait reappeared in one quarter of the F2offspring


3/41/4

P generation(true-breedingparents)

Purple flowers White flowers



F1 generation All plants havepurple flowers



F1 generation All plants havepurple flowers

F2 generation

Fertilizationamong F1 plants(F1 X F1)

of plantshave purple flowers

3–4 of plantshave white flowers

1–4

Mendel’s law of segregation describes theinheritance of a single character

Four Hypotheses

1. Genes are found in alternative versions called alleles;a genotype is the listing of alleles an individualcarries for a specific gene

2. For each characteristic, an organism inherits twoalleles, one from each parent; the alleles can be thesame or different

– A homozygous genotype has identical alleles

– A heterozygous genotype has two different alleles


Mendel’s law of segregation describes theinheritance of a single character

Four Hypotheses

3. If the alleles differ, the dominant allele determinesthe organism’s appearance, and the recessive allelehas no noticeable effect

– The phenotype is the appearance or expression of a trait

– The same phenotype may be determined by more thanone genotype

4. Law of segregation: Allele pairs separate (segregate)from each other during the production of gametes sothat a sperm or egg carries only one allele for eachgene


P plants

1–21–2

Genotypic ratio1 PP : 2 Pp : 1 pp

Phenotypic ratio3 purple : 1 white

F1 plants(hybrids)

Gametes

Genetic makeup (alleles)⇣

All

All Pp

Sperm

Eggs

PP

p

ppPp

Pp

P

pP

pP

P

p

PP pp

All

Gametes

F2 plants

Homologous chromosomes bear the alleles foreach character

For a pair of homologous chromosomes, alleles ofa gene reside at the same locus

– Homozygous individuals have the same allele on bothhomologues

– Heterozygous individuals have a different allele oneach homologue

Gene loci

Homozygousfor thedominant allele

Dominantallele

Homozygousfor therecessive allele

Heterozygous

Recessive allele

Genotype:

P Ba

P

PP

a

aa

b

Bb

Mendel’s second law is revealed by tracking twocharacters at once

Example of a dihybrid cross

– Parental generation: round yellow seeds × wrinkled greenseeds

– F1 generation: all plants with round yellow seeds

– F2 generation: of plants with round yellow seeds of plants with round green seeds

of plants with wrinkled yellow seeds of plants with wrinkled green seeds


9/163/163/161/16

Mendel needed to explain

–Why nonparental combinations were observed

–Why a 9:3:3:1 ratio was observed among the F2offspring

The law of independent assortment is revealedby tracking two characters at once

Law of independent assortment

– Each pair of alleles segregates independently of theother pairs of alleles during gamete formation

– For genotype RrYy, four gamete types are possible:RY, Ry, rY, and ry

P generation

1–2

Hypothesis: Dependent assortment Hypothesis: Independent assortment

1–2

1–2

1–2

1–4

1–4

1–4

1–4

1–41–4

1–4

1–4

9––16

3––16

3––16

1––16

RRYY

Gametes

Eggs

F1generation

SpermSperm

F2generation

Eggs

Gametes

rryy

RrYy

ryRY

ryRY

ry

RY

Hypothesized(not actually seen)

Actual results(support hypothesis)

RRYY rryy

RrYy

ryRY

RRYY

rryy

RrYy

ry

RY

RrYy

RrYy

RrYy

rrYYRrYY

RRYyRrYY

RRYy

rrYy

rrYy

Rryy

Rryy

RRyy

rY

Ry

ry

YellowroundGreenround

Greenwrinkled

Yellowwrinkled

RY rY Ry

PhenotypesGenotypes

Mating of heterozygotes(black, normal vision)

Phenotypic ratioof offspring

Black coat, normal visionB_N_

9 black coat,normal vision

Black coat, blind (PRA)B_nn

3 black coat,blind (PRA)

Chocolate coat, normal visionbbN_

3 chocolate coat,normal vision

Chocolate coat, blind (PRA)bbnn

1 chocolate coat,blind (PRA)

Blind Blind

BbNn BbNn

Mendel’s laws reflect the rules of probability

The probability of a specific event is the numberof ways that event can occur out of the totalpossible outcomes.

Rule of multiplication

– Multiply the probabilities of events that must occurtogether

Rule of addition

– Add probabilities of events that can happen inalternate ways


F1 genotypes

1–2

1–2

1–2

1–2

1–41–4

1–41–4

Formation of eggs

Bb female

F2 genotypes

Formation of sperm

Bb male

B

BB B B

B

b

b

bbbb

Parents NormalDd

Offspring

Sperm

Eggs

ddDeafd

DdNormal(carrier)

DDNormalD

D d

DdNormal(carrier)

NormalDd×

VARIATIONS ON MENDEL’SLAWS


Incomplete dominance results in intermediatephenotypes

Incomplete dominance

– Neither allele is dominant over the other

– Expression of both alleles is observed as anintermediate phenotype in the heterozygousindividual


P generation

1–21–2

1–21–2

1–2

1–2

F1 generation

F2 generation

RedRR

Gametes

Gametes

Eggs

Sperm

RR rR

Rr rr

R

r

R r

R r

PinkRr

R r

Whiterr

HHHomozygous

for ability to makeLDL receptors

hhHomozygous

for inability to makeLDL receptors

HhHeterozygous

LDL receptor

LDL

Normal Cell Mild disease Severe disease

Genotypes:

Phenotypes:

Codominance–Neither allele is dominant over the other

–Expression of both alleles is observed as a distinctphenotype in the heterozygous individual

Many genes have more than two alleles in thepopulation

Multiple alleles

– More than two alleles are found in the population

– A diploid individual can carry any two of these alleles

– The ABO blood group has three alleles, leading to fourphenotypes: type A, type B, type AB, and type Oblood


BloodGroup(Phenotype) Genotypes

O

A

ii

IAIAorIAi

Red Blood Cells

Carbohydrate A

BIBIBorIBi

Carbohydrate B

AB IAIBCodominanceobserved fortype AB blood

A single gene may affect many phenotypiccharacters

Pleiotropy

– One gene influencing many characteristics

– The gene for sickle cell disease

– Affects the type of hemoglobin produced

– Affects the shape of red blood cells

– Causes anemia

– Causes organ damage

– Is related to susceptibility to malaria


Clumping of cellsand clogging of

small blood vessels

Pneumoniaand otherinfections

Accumulation ofsickled cells in spleen

Pain andfever

Rheumatism

Heartfailure

Damage toother organs

Braindamage

Spleendamage

Kidneyfailure

Anemia

ParalysisImpairedmental

function

Physicalweakness

Breakdown ofred blood cells

Individual homozygousfor sickle-cell allele

Sickle cells

Sickle-cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped

A single character may be influenced by manygenes

Polygenic inheritance - Many genes influenceone trait

– Skin color is affected by at least three genes

– Height

– Intelligence


P generation

1–8

F1 generation

F2 generation

Frac

tion

of p

opul

atio

n

Skin color

Eggs

Sperm1–8

1–81–8

1–81–8

1–81–8

1–8

1–81–8

1–8

1–81–8

1–8

1–8

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

1––6415––64

6––641––64

15––646––64

20––64

1––64

15––64

6––64

20––64

The environment affects many characters

Phenotypic variations are influenced by theenvironment

– Skin color is affected by exposure to sunlight

– Susceptibility to diseases, such as cancer, hashereditary and environmental components


Incompletedominance

RedRR

Singlegene

Single characters(such as skin color)

Multiple charactersPleiotropy

PolygenicinheritanceMultiple

genes

Whiterr

PinkRr

THE CHROMOSOMAL BASIS OFINHERITANCE


Sex-linked genes are located on either of thesex chromosomes

– Reciprocal crosses show different results

– White-eyed female × red-eyed male red-eyed femalesand white-eyed males

– Red-eyed female × white-eyed male red-eyed femalesand red-eyed males

– X-linked genes are passed from mother to son andmother to daughter

– X-linked genes are passed from father to daughter

– Y-linked genes are passed from father to son

Sex-linked genes exhibit a unique pattern ofinheritance


Female Male

XR XR Xr Y

XR YXR Xr

YXr

XR

Sperm

Eggs

R = red-eye alleler = white-eye allele

All RED means Red is ?

Female Male

XR Xr XR Y

XR YXR XR

YXR

XR

Sperm

Eggs

Xr XR Xr YXr

3 RED : 1 whitemeans?

Sex-linked disorders affect mostly males

Males express X-linked disorders such as thefollowing when recessive alleles are present inone copy

– Hemophilia

– Red/Green Colorblindness

– Duchenne muscular dystrophy


QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

SEX CHROMOSOMES ANDSEX-LINKED GENES


Chromosomes determine sex in many species

X-Y system in mammals, fruit flies

– XX = female; XY = male


(male)

Sperm

(female)44+

XYParents’diploidcells

44+

XX

22+X

22+Y

22+X

44+

XY

44+

XX

Egg

Offspring(diploid)


Other Systems

1. X-O system in grasshoppers and roaches

1. XX = female; XO = male


22+X

22+

XX

76+

ZZ76+

ZW

2. Z-W in system in birds, butterflies, and some fishes

– ZW = female, ZZ = male


Other Systems

3. Chromosome number in ants and bees

– Diploid = female; haploid = male


1632

Nonchromosomal Sex Determination

Reptiles - temperature at a stage of egg development

Shifts dependent of ratio of the sexes in a population

Both sexes in same organism

– Earthworms, some plants

Genes

locatedon

(b)

(a)

at specificlocations called

alternativeversions called

if both same,genotype called

expressedallele called

inheritance when phenotypeIn between called

unexpressedallele called

if different,genotype called

chromosomes

heterozygous

(d)

(c)

(f)

(e)

loci homozygous

alleles

dominant recessive

incomplete dominance

Mutant phenotypesShortaristae

Blackbody(g)

Cinnabareyes(c)

Vestigialwings(l)

Browneyes

Long aristae(appendageson head)

Graybody(G)

Redeyes(C)

Normalwings(L)

Redeyes

Wild-type phenotypes

A few Non-Sex Linked Alleles

1. Explain and apply Mendel’s laws of segregationand independent assortment

2. Distinguish between terms in the followinggroups: allele—gene; dominant—recessive;genotype—phenotype; F1—F2;heterozygous—homozygous; incompletedominance—codominance

3. Explain the meaning of the terms locus, multiplealleles, pedigree, pleiotropy, polygenicinheritance

You should now be able to


4. Describe the difference in inheritance patterns forlinked genes and explain how recombination canbe used to estimate gene distances

5. Describe how sex is inherited in humans andidentify the pattern of inheritance observed forsex-linked genes

6. Solve genetics problems involving monohybridand dihybrid crosses for autosomal and sex-linked traits, with variations on Mendel’s laws

You should now be able to


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Introduction: Mendelian Genetics