Patterns of Patterns of InheritanceInheritance

Chapter 12 2

InheritanceInheritance

Inheritance is the process by which the Inheritance is the process by which the characteristics of individuals are characteristics of individuals are passed to their offspringpassed to their offspring

GenesGenes encode these characteristics encode these characteristicsA A genegene is a unit of heredity that encodes is a unit of heredity that encodes

information for the form of a particular information for the form of a particular characteristiccharacteristic

The location of a gene on a chromosome The location of a gene on a chromosome is called its is called its locuslocus

Chapter 12 3

AllelesAlleles

Homologous chromosomes carry the Homologous chromosomes carry the same kinds of genes for the same same kinds of genes for the same characteristicscharacteristics

Genes for the same characteristic are Genes for the same characteristic are found at the same loci on both found at the same loci on both homologous chromosomeshomologous chromosomes

Chapter 12 4

AllelesAlleles

Genes for a characteristic found on Genes for a characteristic found on homologous chromosomes may not homologous chromosomes may not be identicalbe identical

Alternate versions or forms of genes Alternate versions or forms of genes found at the same gene locus are found at the same gene locus are called called allelesalleles

Chapter 12 5

AllelesAllelesEach cell carries two alleles per Each cell carries two alleles per

characteristic, one on each of the two characteristic, one on each of the two homologous chromosomeshomologous chromosomes

If both homologous chromosomes carry the If both homologous chromosomes carry the samesame allele (gene form) at a given gene allele (gene form) at a given gene locus, the organism is locus, the organism is homozygoushomozygous at that at that locuslocus

If two homologous chromosomes carry If two homologous chromosomes carry differentdifferent alleles at a given locus, the alleles at a given locus, the organism is organism is heterozygousheterozygous at that locus (a at that locus (a hybridhybrid))

Chapter 12 6

11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 1616 1717 1818 1919 2020 2121 2222 2323 2424 2525 2626Loci:Loci:

Genes, Alleles,Genes, Alleles,Loci, and ChromosomesLoci, and Chromosomes

Chromosome from One ParentChromosome from One Parent

Homologous Chromosome from Other ParentHomologous Chromosome from Other Parent

11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515 1616 1717 1818 1919 2020 2121 2222 2323 2424 2525 2626Loci:Loci:

M locus has gene that

controls leaf color. Plant

homozygous for this gene

D locus has gene that

controls plant height. Plant homozygous for this gene

Bk locus has gene that

controls fruit shape. Plant

heterozygous for this gene

Chapter 12 7Definitions 1Definitions 1

Must know these!!!Must know these!!!TraitTrait—A variable characteristic of organism—A variable characteristic of organismGeneGene—A segment of chromosomal DNA —A segment of chromosomal DNA controlling a specific traitcontrolling a specific trait

LocusLocus—Chromosomal position where DNA —Chromosomal position where DNA for a specific gene livesfor a specific gene lives

GenomeGenome—Refers to all standard loci for a —Refers to all standard loci for a speciesspecies

Chapter 12 8

Definitions 2Definitions 2Must know these!!!Must know these!!!AllelesAlleles—Different forms of a —Different forms of a genegene

• ““Flower color” is a gene;Flower color” is a gene;• ““Purple” is one flower-color allelePurple” is one flower-color allele• ““White” is another flower-color alleleWhite” is another flower-color allele

GenotypeGenotype—List of alleles for an individual at —List of alleles for an individual at specific genesspecific genes

• Familiar organisms are diploidFamiliar organisms are diploid• One or two alleles per individualOne or two alleles per individual

Chapter 12 9

Definitions 3Definitions 3

HomozygousHomozygous—Maternal & paternal alleles —Maternal & paternal alleles samesame

• Father donates purple-flower alleleFather donates purple-flower allele• Mother donates purple-flower alleleMother donates purple-flower allele

HeterozygousHeterozygous—Maternal & paternal alleles —Maternal & paternal alleles differdiffer

• Father donates purple-flower alleleFather donates purple-flower allele• Mom donates white-flower alleleMom donates white-flower allele

Chapter 12 10

Definitions 4Definitions 4

PhenotypePhenotype::• List of traits exhibited by individualList of traits exhibited by individual• Doesn’t always represent genotypeDoesn’t always represent genotype

DominantDominant—Allele that is expressed 100% in —Allele that is expressed 100% in heterozygoteheterozygote

RecessiveRecessive—Allele is not expressed in —Allele is not expressed in heterozygoteheterozygote

Incomplete dominanceIncomplete dominance—heterozygote —heterozygote displays intermediate trait displays intermediate trait

Chapter 12 11

Genetic SymbolismGenetic Symbolism

Often use initial letter of dominant alleleOften use initial letter of dominant allele• CapitalCapital letter represents dominant letter represents dominant• Lower caseLower case of of same lettersame letter represents represents

recessiverecessive

If purple flower dominant to white…If purple flower dominant to white…• ““P” represents allele for purpleP” represents allele for purple• ““p” represents allele for whitep” represents allele for white

Chapter 12 12

Cross Fertilization of Parents Cross Fertilization of Parents

True-breedingTrue-breedingPurple-floweredPurple-flowered

ParentParent

True-breedingTrue-breedingWhite-floweredWhite-flowered

ParentParent

Cross-FertilizeCross-Fertilize

All Purple-floweredAll Purple-floweredOffspringOffspring

Pollen

Pollen

P P

F1

Chapter 12 13

Self-fertilization of FSelf-fertilization of F22

F1

Self-FertilizeSelf-Fertilize

F2 F2 F2 F2

75% Purple75% Purple25% White25% White

Chapter 12 14

Genotype vs PhenotypeGenotype vs Phenotype

Phenotype is how we look/behavePhenotype is how we look/behave• PurplePurple flowers flowers• WhiteWhite flowers flowers

Genotype is what our genes sayGenotype is what our genes say• WhiteWhiteFlowers / Flowers / WhiteWhiteFlowers Flowers • WhiteWhiteFlowers / Flowers / PurplePurpleFlowers Flowers • PurplePurpleFlowers / Flowers / PurplePurpleFlowers Flowers

Chapter 12 15

Genotype vs Phenotype 2Genotype vs Phenotype 2

GenotypesGenotypes• PP = homozygous for PP = homozygous for purplepurple flower flower• pp = homozygous for pp = homozygous for whitewhite flower flower• Pp = heterozygous for flower colorPp = heterozygous for flower color

Phenotype from genotype:Phenotype from genotype:• PP = PP = purplepurple flower flower• Pp = Pp = purplepurple flower flower• pP = pP = purplepurple flower flower• pp = pp = WhiteWhite flower flower

Chapter 12 16

How Meiosis Separates GenesHow Meiosis Separates Genes

The two alleles for a characteristic separate The two alleles for a characteristic separate during gamete formation (meiosis)during gamete formation (meiosis)

• Homologous chromosomes separate in Homologous chromosomes separate in meiosis anaphase Imeiosis anaphase I

• Each gamete receives one of each pair of Each gamete receives one of each pair of homologous chromosomes and thus one of homologous chromosomes and thus one of the two alleles per characteristicthe two alleles per characteristic

The separation of alleles in meiosis is The separation of alleles in meiosis is known as Mendel’s Law of Segregationknown as Mendel’s Law of Segregation

Chapter 12 17

Gametes of HomozygotesGametes of Homozygotes

A A A A

Homozygous ParentHomozygous Parent GametesGametes

All gametes identicalAll gametes identicalregarding this generegarding this gene

Chapter 12 18

Gametes of HeterozygotesGametes of Heterozygotes

A a A a

Heterozygous ParentHeterozygous Parent GametesGametes

Gametes 50/50Gametes 50/50regarding this generegarding this gene

Chapter 12 19

pphomozygous

recessive

Homozygous DominantHomozygous DominantX Homozygous RecessiveX Homozygous Recessive

P

p

P

p

Purp

le P

aren

tPu

rple

Par

ent

PPhomozygous

dominant

Whi

te P

aren

tW

hite

Par

ent

spermspermnucleinuclei

eggeggnucleinuclei

spermspermnucleinuclei

eggeggnucleinuclei

Chapter 12 20

Pp

pP

P Sperm + p EggsP Sperm + p Eggssame as p Sperm + P Eggssame as p Sperm + P Eggs

Purp

le F

Purp

le F

11Pu

rple

FPu

rple

F11

P pspermsperm

nucleusnucleuseggegg

nucleusnucleus

++

p Peggegg

nucleusnucleusspermsperm

nucleusnucleus

++

Chapter 12 21

PurplePurplehomozygoushomozygous

dominant (PP)dominant (PP)PurplePurple

heterozygousheterozygous (Pp) (Pp)

PurplePurpleheterozygousheterozygous

(pP) (pP)WhiteWhite

homozygoushomozygousrecessive (pp)recessive (pp)

Pp X Pp CrossPp X Pp Cross

P

p

p

P

p

P

P

p

++++

++++

FF11 Sperm Sperm FF11 Eggs Eggs FF22 Offspring Offspring

Chapter 12 22Using Punnett SquaresUsing Punnett Squaresin Genetic Crossesin Genetic Crosses

Named after geneticist Reginald Named after geneticist Reginald PunnettPunnett

Figured using Figured using Punnett squaresPunnett squares• Considers only genes of interestConsiders only genes of interest• List sperm genotypes across topList sperm genotypes across top• List egg genotypes down sideList egg genotypes down side• Fill in boxes with zygote genotypesFill in boxes with zygote genotypes

Chapter 12 23

Consider Flower ColorConsider Flower Color

Pretend flower color affected by only one Pretend flower color affected by only one gene (gene (monohybrid crossmonohybrid cross))

Assume all alleles are purple or whiteAssume all alleles are purple or white

Purple (P) is dominant to white (p)Purple (P) is dominant to white (p)

HeterozygotesHeterozygotes will have flowers as purple will have flowers as purple as homozygous dominantsas homozygous dominants

Chapter 12 24

P p

1(25%)White

3 (75%)3 (75%)PurplePurple

FrequenciesFrequenciesPhenotypesPhenotypes

GenotypesGenotypes

FrequenciesFrequencies

Making a Punnett Square:Making a Punnett Square:Heterozygous X HeterozygousHeterozygous X Heterozygous

Eggs of Heterozygous PlantEggs of Heterozygous Plant

Pollen ofPollen ofHeterozygous Plant Heterozygous Plant

1111 22

P

p pP

PpPP

pp

PP pppP Pp

Chapter 12 25

Practical Application: The Test CrossPractical Application: The Test Cross

A A test crosstest cross is used to deduce the actual is used to deduce the actual genotype of an organism with a genotype of an organism with a dominant phenotype (i.e., is the dominant phenotype (i.e., is the organism organism PPPP or or PpPp?)?)

1.1. Cross the unknown dominant-phenotype Cross the unknown dominant-phenotype organism (organism (PP_) with a homozygous _) with a homozygous recessive organism (recessive organism (pppp)…)…

Chapter 12 26

Practical Application: The Test CrossPractical Application: The Test Cross

2. If the dominant-phenotype organism is 2. If the dominant-phenotype organism is homozygous dominant (homozygous dominant (PPPP), only ), only dominant-phenotype offspring will be dominant-phenotype offspring will be produced (produced (PpPp))

3.3. If the dominant-phenotype organism is If the dominant-phenotype organism is heterozygous (heterozygous (PpPp), approximately half of ), approximately half of the offspring will be of recessive the offspring will be of recessive phenotype (phenotype (pppp))

Chapter 12 27

p p

(50%)White

(50%)(50%)PurplePurple

FrequenciesFrequenciesPhenotypesPhenotypes

GenotypesGenotypes

FrequenciesFrequencies

Test Cross:Test Cross:Heterozygous X Homozygous RecessiveHeterozygous X Homozygous Recessive

Eggs of Homozygous RecessiveEggs of Homozygous Recessive

Pollen of unknownPollen of unknownplant with dominant plant with dominant

phenotypephenotype(Heterozygous)(Heterozygous)

22

P

p pp

PpPP

pp

Pp pppP pp22

Chapter 12 28

p p

(100%)(100%)PurplePurple

FrequenciesFrequenciesPhenotypesPhenotypes

GenotypesGenotypes

FrequenciesFrequencies

Test Cross:Test Cross:Homozygous X Homozygous RecessiveHomozygous X Homozygous Recessive

Eggs of Homozygous RecessiveEggs of Homozygous Recessive

Pollen of unknownPollen of unknownplant with dominant plant with dominant

phenotypephenotype(Homozygous)(Homozygous)

P

Pp

PpPp

Pp

Pp PpPp Pp

P

44

Chapter 12 29Traits of PeasTraits of PeasStudied by MendelStudied by Mendel

Plant size

Flower location

Flower color

Pod colorPod shape

Seed shapeSeed color

Chapter 12 30

Traits Are Inherited IndependentlyTraits Are Inherited Independently

Seed color (yellow vs. green peas) and seed Seed color (yellow vs. green peas) and seed shape (smooth vs. wrinkled peas) were shape (smooth vs. wrinkled peas) were the characteristics studiedthe characteristics studied

The allele symbols were assigned:The allele symbols were assigned:• YY = yellow (dominant), = yellow (dominant), yy = green (recessive) = green (recessive)• SS = smooth (dominant), = smooth (dominant), ss = wrinkled (recessive) = wrinkled (recessive)

Two trait cross was between two true Two trait cross was between two true breeding varieties for each characteristicbreeding varieties for each characteristic

• P: P: SSYYSSYY x x ssyyssyy

Chapter 12 31

RecombinationRecombination

Genes on the same chromosome do not Genes on the same chromosome do not alwaysalways sort together sort together

Crossing overCrossing over in Prophase I of meiosis in Prophase I of meiosis creates new gene combinationscreates new gene combinations

Crossing over involves the exchange of Crossing over involves the exchange of DNA between chromatids of paired DNA between chromatids of paired homologous chromosomes in homologous chromosomes in synapsissynapsis

Chapter 12 32

red

red

Purple

Purple

round

round

Long

Long

PP

PP

pp

pp

LL

LL

ll

ll

PP

pp

pp

LL

LL

ll

ll

PP

LL

pp LL

ll

ll

PP

pp

PP

LL

pp LL

ll

ll

PP

pp

LL

LL

ll

ll

PP

PP

pp

pp

PP

PP

pp

pp

LL

LL

ll

ll

PP

pp

pp

LL

LL

ll

ll

Crossing OverCrossing Over

SisterSisterChromatidsChromatids

DuplicatedDuplicatedChromosomeChromosome

DuplicatedDuplicatedChromosomeChromosome

LL

LL

ll

ll

PP

PP

pp

pp

SisterSisterChromatidsChromatids Ho

mol

ogou

sHo

mol

ogou

sCh

rom

osom

esCh

rom

osom

es

PP

PP

pp

pp

LL

LL

ll

ll

PP

pp

pp

LL

LL

ll

ll

pp LL

PP ll

LLPP

llpp

old combinationold combination

new combinationnew combinationnew combinationnew combination

old combinationold combination

FlowerColor

PollenShape

Chapter 12 33

Chapter 12 34

Chapter 12 35

Sex Chromosomes and AutosomesSex Chromosomes and Autosomes

Mammals and many insect species have a Mammals and many insect species have a set of set of sex chromosomessex chromosomes that dictate that dictate gendergender

• Females have two Females have two X chromosomesX chromosomes• Males have an Males have an X chromosomeX chromosome and a and a YY

chromosomechromosome• Sex chromosomesSex chromosomes segregate during segregate during

meiosismeiosis• [The rest of the (non-sex) chromosomes [The rest of the (non-sex) chromosomes

are called are called autosomes]autosomes]

Chapter 12 36

Chapter 12 37

XX11 XX22

Sex DeterminationSex Determinationin Mammalsin Mammals

EGGSEGGS

Male ParentMale ParentYYXXmm

SSPPEERRMM

Female OffspringFemale Offspring

Male OffspringMale OffspringYY

XXmmXXmmXX11 XX22XXmm

YY YYXX11 XX22

XX11 XX22Female ParentFemale Parent

Chapter 12 38

Sex-Linked Genes Are on the X or the YSex-Linked Genes Are on the X or the YGenes carried on one sex chromosome are Genes carried on one sex chromosome are sex-sex-

linkedlinked• X chromosome is much larger than the Y and X chromosome is much larger than the Y and

carries over 1000 genescarries over 1000 genes• Y chromosome is smaller and carries only 78 Y chromosome is smaller and carries only 78

genesgenesThe X and the Y have very few genes in commonThe X and the Y have very few genes in common

• Females (XX) can be homozygous or Females (XX) can be homozygous or heterozygous for a characteristicheterozygous for a characteristic

• Males (XY) have only Males (XY) have only one copyone copy of the genes on of the genes on the X or the Ythe X or the Y

Chapter 12 39How Sex-Linkage Affects How Sex-Linkage Affects InheritanceInheritance

Patterns of sex-linked inheritance were first Patterns of sex-linked inheritance were first discovered in fruit flies (discovered in fruit flies (DrosophilaDrosophila) in ) in early 1900searly 1900s

Eye color genes were found to be carried by Eye color genes were found to be carried by the X chromosomethe X chromosome

• RR = red eyes (dominant) = red eyes (dominant)• rr = white eyes (recessive) = white eyes (recessive)

Chapter 12 40How Sex-Linkage Affects How Sex-Linkage Affects InheritanceInheritance

Sex-linked (specifically Sex-linked (specifically X-linkedX-linked) recessive ) recessive alleles displayed their phenotype more alleles displayed their phenotype more often in malesoften in males

• Males showed recessive white-eyed Males showed recessive white-eyed phenotype more often than females in anphenotype more often than females in an

XXRRXXrr xx X XrrY crossY crossMales do not have a second X-linked gene Males do not have a second X-linked gene

(as do females) which can mask a (as do females) which can mask a recessive gene if dominantrecessive gene if dominant

Chapter 12 41

25%25%Normal fNormal f Carrier fCarrier f Normal mNormal m

25%25% 25%25% 25%White-e m

FrequenciesFrequenciesPhenotypesPhenotypes

GenotypesGenotypes

FrequenciesFrequencies

Sex Linkage:Sex Linkage:Eye Color in Fruit FliesEye Color in Fruit Flies

Eggs of Eggs of XR Xr FemaleFemale

Sperm ofSperm ofXXRRY Male Y Male

1111

YXR

XRXrXRXR

YXr

XRXR XrYXRXr XRY

R r

R

Female Female

Male Male

11 11

Chapter 12 42

RR RR

(100%)(100%)Pink (intermediate)Pink (intermediate)

FrequenciesFrequenciesPhenotypesPhenotypes

GenotypesGenotypes

FrequenciesFrequencies

Incomplete Dominance:Incomplete Dominance:Homozygous-X Homo RecessiveHomozygous-X Homo Recessive

Eggs of HomozygousEggs of Homozygous RR Red Parent Red Parent

Pollen ofPollen ofHomozygous Homozygous R'R'

White ParentWhite Parent

R'

R'R'R

R'RR'R

R'R

R'R R'RR'R R'R

Pink Pink

Pink Pink

11

Chapter 12 43

CodominanceCodominance

Some alleles are always expressed even Some alleles are always expressed even in combination with other allelesin combination with other alleles

Heterozygotes display phenotypes of Heterozygotes display phenotypes of both the homozygote phenotypes in both the homozygote phenotypes in codominancecodominance

Chapter 12 44

CodominanceCodominance

Example: Human blood group allelesExample: Human blood group alleles• Alleles A and B are codominant Alleles A and B are codominant • Type AB blood is seen where individual Type AB blood is seen where individual

has the genotype ABhas the genotype AB

Chapter 12 45

10%10%

40%40%

46%46%

4%4%

B or ABB or AB

A or ABA or AB

O,AB,O,AB,A,BA,B

(universal)(universal)

ABAB(universal)(universal)

B or OB or O

A or OA or O

OO

AB, A,AB, A,B, OB, O

(universal)(universal)

AA

BB

BothBoth

NeitherNeither

BB or BOBB or BO

AA or AOAA or AO

OOOO

ABAB

OO

ABAB

BB

AA

FreqFreqDonatesDonatesRe-Re-ceivesceives

Anti-Anti-bodiesbodiesRBCsRBCsGenotypeGenotypeTypeType

Human ABO Blood GroupHuman ABO Blood Group

Chapter 12 46

Polygenic InheritancePolygenic Inheritance

Phenotypes produced by Phenotypes produced by polygenic polygenic inheritanceinheritance are governed by the are governed by the interaction of more than two genes at interaction of more than two genes at multiple locimultiple loci

Human skin color is controlled by at least 3 Human skin color is controlled by at least 3 genes, each with pairs of incompletely genes, each with pairs of incompletely dominant allelesdominant alleles

Chapter 12 47

Chapter 12 48

Pedigree AnalysisPedigree Analysis

Records of gene expression over several Records of gene expression over several generations of a family can be generations of a family can be diagrammeddiagrammed

Careful analysis of this diagram (a Careful analysis of this diagram (a pedigreepedigree) can reveal inheritance ) can reveal inheritance pattern of a traitpattern of a trait

Pedigree analysis is often combined with Pedigree analysis is often combined with molecular genetics technology to molecular genetics technology to elucidate gene action and expressionelucidate gene action and expression

Chapter 12 49

How to Read PedigreesHow to Read Pedigrees= male= male = female= female

= parents= parents

oror = individual who shows the trait = individual who shows the trait

oror = heterozygous carrier of= heterozygous carrier ofautosomal trait autosomal trait

= offspring = offspring 11 22 33

I, II, III, IV, or VI, II, III, IV, or V = generation = generation

Chapter 12 50

A Recessive PedigreeA Recessive Pedigree

Chapter 12 51Pedigrees:Pedigrees:Legacy of Queen VictoriaLegacy of Queen Victoria

Chapter 12 52

Sickle-Cell AnemiaSickle-Cell AnemiaHemoglobin is an oxygen-transporting protein Hemoglobin is an oxygen-transporting protein

found in red blood cellsfound in red blood cellsA mutant hemoglobin gene causes A mutant hemoglobin gene causes

hemoglobin molecules in blood cells to hemoglobin molecules in blood cells to clump togetherclump together

• Red blood cells take on a sickle (crescent) Red blood cells take on a sickle (crescent) shape and easily breakshape and easily break

• Blood clots can form, leading to oxygen Blood clots can form, leading to oxygen starvation of tissues and paralysisstarvation of tissues and paralysis

• Condition is known as Condition is known as sickle-cell anemiasickle-cell anemia

Chapter 12 53

Normal Red Blood CellsNormal Red Blood Cells

Chapter 12 54

Sickled CellsSickled Cells

Chapter 12 55

Sex-Linked Genetic DisordersSex-Linked Genetic Disorders

Several defective alleles for Several defective alleles for characteristics encoded on the X characteristics encoded on the X chromosome are knownchromosome are known

Sex-linked disorders appear more Sex-linked disorders appear more frequently in males and often skip frequently in males and often skip generationsgenerations

Examples of sex-linked (X-linked) Examples of sex-linked (X-linked) disordersdisorders

• Red-green color blindnessRed-green color blindness

Chapter 12 56

Chapter 12 57

Chapter 12 58

Non-DisjunctionNon-Disjunction

Incorrect separation of chromosomes or Incorrect separation of chromosomes or chromatids in meiosis known as chromatids in meiosis known as non-non-disjunctiondisjunction

Most embryos arising from gametes with Most embryos arising from gametes with abnormal chromosome numbers abort abnormal chromosome numbers abort spontaneously (are miscarried)spontaneously (are miscarried)

Some combinations of abnormal Some combinations of abnormal chromosome number survive to birth chromosome number survive to birth or beyondor beyond

Chapter 12 59

Chapter 12 60

Chapter 12 61

Incidence of Down SyndromeIncidence of Down Syndrome

1010 2020 3030 4040 505000

100100

200200

300300

400400

Age of Mother (years)Age of Mother (years)

Num

ber p

er 1

000

Birth

sNu

mbe

r per

100

0 Bi

rths

Chapter 12

The endThe end