lecture 9: genetic inheritance - linn–benton community...

10/18/2015

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Biology 102Biology 102

Lecture 9: Genetic InheritanceLecture 9: Genetic Inheritance

•• Asexual reproduction = daughter cells genetically Asexual reproduction = daughter cells genetically

identical to parent (clones)identical to parent (clones)

•• Sexual reproduction = offspring are genetic Sexual reproduction = offspring are genetic

hybridshybrids

•• Tendency to inherit best traits of both Tendency to inherit best traits of both

parentsparents

•• Survival advantage against environmental Survival advantage against environmental

change, competition, disease, etc.change, competition, disease, etc.

Genetic VariabilityGenetic Variability

•• Siblings will often look similar, but not identicalSiblings will often look similar, but not identical

•• Each inherits 50% from each parent, but not the Each inherits 50% from each parent, but not the

same 50%same 50%

•• Crossing overCrossing over


•• Ultimate sources of variabilityUltimate sources of variability

•• MutationsMutations

•• Crossing over (recombination)Crossing over (recombination)

•• Independent assortmentIndependent assortment


•• Problem with inbreedingProblem with inbreeding

•• Limited number of genesLimited number of genes

•• Increased chances that deleterious mutations Increased chances that deleterious mutations

will show upwill show up


•• Remember how mutations affect genesRemember how mutations affect genes

•• Protein product altered in 1 of 4 ways…Protein product altered in 1 of 4 ways…

1) No effect1) No effect

•• Silent mutationSilent mutation

2) Protein is altered, but it doesn’t matter2) Protein is altered, but it doesn’t matter

•• Neutral change Neutral change –– HAT HAT vsvs CAPCAP

3) Protein loses some or all of its function3) Protein loses some or all of its function

•• Deleterious change Deleterious change -- HAT HAT vsvs CATCAT

4) Protein functions better4) Protein functions better

•• Example: HIV resistanceExample: HIV resistance

MutationsMutations

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•• All somatic cells contain 23 pairs of All somatic cells contain 23 pairs of

chromosomeschromosomes

•• 22 pairs of 22 pairs of autosomesautosomes

•• 1 pair of sex chromosomes1 pair of sex chromosomes

•• Genes contained in each pair of chromosomes are Genes contained in each pair of chromosomes are

identicalidentical

GeneticsGenetics

•• Gene:Gene: Portion of genetic material that codes for Portion of genetic material that codes for

a specific proteina specific protein

•• Allele:Allele: Any form of a given gene in the populationAny form of a given gene in the population

•• Humans are diploidHumans are diploid

•• For any given gene, we carry 2 allelesFor any given gene, we carry 2 alleles

•• Homozygous:Homozygous: Both alleles are the same for a Both alleles are the same for a

given genegiven gene

•• Heterozygous: Heterozygous: 2 different alleles for a given 2 different alleles for a given

genegene

GeneticsGenetics

•• 2 alleles for a given gene2 alleles for a given gene

•• Each codes for a slightly different proteinEach codes for a slightly different protein

•• Which will be made? Both?Which will be made? Both?

HeterozygosityHeterozygosity

•• One allele is usually chosen over the othersOne allele is usually chosen over the others

•• Consistently chosen across the speciesConsistently chosen across the species

•• Called the Called the dominant dominant alleleallele

•• Need only be present in one copy to be Need only be present in one copy to be

expressedexpressed

DominantDominant

•• Consistently ignored alleles are Consistently ignored alleles are recessiverecessive

•• Only expressed if present in 2 copiesOnly expressed if present in 2 copies

•• Can be passed on to offspring, even if not Can be passed on to offspring, even if not

expressedexpressed

•• Recessive does NOT mean rare, or even less Recessive does NOT mean rare, or even less

common! (Lab 9)common! (Lab 9)

RecessiveRecessive

•• Describes both alleles present for a given geneDescribes both alleles present for a given gene

•• Capital letter = dominantCapital letter = dominant

•• Lower case letter = recessiveLower case letter = recessive

•• Homozygous dominant = AAHomozygous dominant = AA

•• Heterozygous = Heterozygous = AaAa

•• Homozygous recessive = Homozygous recessive = aaaa

GenotypeGenotype

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•• Genotype is useful scientifically/medically, but Genotype is useful scientifically/medically, but

what does the organism look like?what does the organism look like?

•• PhenotypePhenotype describes observable characteristics describes observable characteristics

based on expression of the genotypebased on expression of the genotype

•• Homozygous dominant = AA = brown eyesHomozygous dominant = AA = brown eyes

•• Heterozygous = Heterozygous = AaAa = brown eyes= brown eyes

•• Homozygous recessive = Homozygous recessive = aaaa = blue eyes= blue eyes

PhenotypePhenotype

•• Much of what we know about patterns of Much of what we know about patterns of

inheritance started with experiments done by inheritance started with experiments done by

this man…this man…

GregorGregor MendelMendel

Mendel’s Pea PlantsMendel’s Pea Plants Mendel’s Pea PlantsMendel’s Pea Plants

•• Mendel observed 7 characteristics Mendel observed 7 characteristics –– let’s just let’s just

look at seed colorlook at seed color

•• Examined patterns of inheritance of phenotypeExamined patterns of inheritance of phenotype

•• Experiment: cross plant with yellow seeds by Experiment: cross plant with yellow seeds by

plant with green seedsplant with green seeds

•• Result: all offspring had yellow seedsResult: all offspring had yellow seeds

YY GG

YY YY YY YY

ParentParent

F1F1

Mendel’s Pea PlantsMendel’s Pea Plants

•• Experiment: selfExperiment: self--pollinated one of the new pollinated one of the new

yellowyellow--seeded plantsseeded plants

•• Result: 25% of new plants had green seeds!Result: 25% of new plants had green seeds!

YY

YY YY YY GG

F1F1

F2F2


•• Experiment: selfExperiment: self--pollinated all of the F2 pollinated all of the F2

generationgeneration

YY YY YY GG F2F2

F3F3

YY YY YY YY

4:04:0

YY YY YY GG

3:13:1YY YY YY GG

3:13:1

G G G G G G GG

0:40:4

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Mendel’s ConclusionsMendel’s Conclusions

1.1. Factors for traits come in pairs Factors for traits come in pairs –– only one will only one will

be passed from parent to offspringbe passed from parent to offspring

•• Carry 2 alleles for each geneCarry 2 alleles for each gene

•• Separated during meiosisSeparated during meiosis

•• Inherit one allele from each parentInherit one allele from each parent


2.2. If factors are identical, only that factor can be If factors are identical, only that factor can be

passed to offspringpassed to offspring

•• HomozygousHomozygous


3.3. If factors are different, there is a 50/50 If factors are different, there is a 50/50

chance of each trait being passed onchance of each trait being passed on

•• HeterozygousHeterozygous

Another of Mendel’s ConclusionsAnother of Mendel’s Conclusions

4.4. Some factors are inherited as a group, others Some factors are inherited as a group, others

are inherited randomlyare inherited randomly

•• When genes are on the same chromosome, When genes are on the same chromosome,

they are often inherited togetherthey are often inherited together

•• Chromosomes are sorted randomly, so genes Chromosomes are sorted randomly, so genes

on different chromosomes are not inherited on different chromosomes are not inherited

togethertogether

•• (More on this later)(More on this later)

PunnettPunnett SquareSquare

•• Once Once diploiditydiploidity was discovered, Mendel’s was discovered, Mendel’s

observations were easily explainedobservations were easily explained

•• PunnettPunnett Square: a box diagram used to Square: a box diagram used to

determine the probability of a given genotypedetermine the probability of a given genotype

•• Yellow seed color = dominant alleleYellow seed color = dominant allele

•• Green seed color = recessive alleleGreen seed color = recessive allele

PunnettPunnett SquareSquare

Maternal allelesMaternal alleles

Y Y YY

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•• Possible offspring genotypes? Phenotypes?Possible offspring genotypes? Phenotypes?

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Explaining MendelExplaining Mendel

YYYY gggg

YYgg YYgg YYgg YYgg

ParentParent

F1F1


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Y Y gg

YYYY YYgg YYgg gggg

F1F1

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Explaining MendelExplaining MendelPa

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YYYY YYgg YYgg gggg F2F2

F3F3

YYYY YYYY YYYY YYYY

YYYY YYYY YYYY gggg

YYYY YYYY YYYY gggg

gggg gggg gggg gggg

Huntington’s DiseaseHuntington’s Disease

•• Enough with the peas!Enough with the peas!

•• Let’s look at a human disease: Huntington’s Let’s look at a human disease: Huntington’s DiseaseDisease

•• AutosomalAutosomal dominant, 100% dominant, 100% penetrancepenetrance

•• Neurodegenerative disorderNeurodegenerative disorder

•• Decrease in physical coordinationDecrease in physical coordination

•• Mental declineMental decline

•• Behavioral symptomsBehavioral symptoms

•• Symptoms usually do not appear until after age Symptoms usually do not appear until after age 35, after the gene may have been passed on to 35, after the gene may have been passed on to offspringoffspring

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•• Scenario: A male is diagnosed with Huntington’s Scenario: A male is diagnosed with Huntington’s

Disease. His wife is tested for the disease gene Disease. His wife is tested for the disease gene

and has two healthy alleles. They have three and has two healthy alleles. They have three

children.children.

•• Disease is Disease is autosomalautosomal dominantdominant

•• How many disease alleles must be present to How many disease alleles must be present to

cause Huntington’s Disease? cause Huntington’s Disease?

•• Let’s assume he is heterozygous: Let’s assume he is heterozygous: HhHh

•• His wife is homozygous: His wife is homozygous: hhhh

•• What is the probability that any one of their What is the probability that any one of their

children will develop Huntington’s Disease?children will develop Huntington’s Disease?


h h hh

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•• Based on this information, the affected Based on this information, the affected

individual’s children decided to be tested, and to individual’s children decided to be tested, and to

have their children testedhave their children tested

•• This information was compiled into a This information was compiled into a pedigreepedigree

Huntington’s DiseaseHuntington’s Disease PedigreesPedigrees

•• A phenotypic family treeA phenotypic family tree

•• Used to determine genotype and track allelesUsed to determine genotype and track alleles

•• Females are circlesFemales are circles

•• Males are squaresMales are squares

•• Darkened individuals have the condition or trait Darkened individuals have the condition or trait

being trackedbeing tracked

PedigreesPedigrees

•• Note that there is at least one affected Note that there is at least one affected

individual in every generationindividual in every generation

•• Hallmark of a dominant traitHallmark of a dominant trait

9 10 11 12 13 14 15 16 17 18

1 2

3 4 5 6 7 8


•• Assign a genotype to all individuals in the familyAssign a genotype to all individuals in the family

•• Step 1: Assign a genotype to anyone we know is Step 1: Assign a genotype to anyone we know is

homozygous (remember: dominant disease)homozygous (remember: dominant disease)

•• Step 2: Assign all offspring of healthy Step 2: Assign all offspring of healthy

individuals one healthy alleleindividuals one healthy allele

•• Step 3: Assign all affected individuals one Step 3: Assign all affected individuals one

disease alleledisease allele

•• Step 4: Work from siblings or offspring to fill Step 4: Work from siblings or offspring to fill

in any missing information (if possible in any missing information (if possible –– some some

alleles may remain unknown)alleles may remain unknown)

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Hh

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hh

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PunnettPunnett SquaresSquares

•• Let’s look at a another human disease : Let’s look at a another human disease : TayTay--

Sach’sSach’s Disease (TSD)Disease (TSD)

•• AutosomalAutosomal recessiverecessive

•• Affects the enzyme Affects the enzyme hexosaminidasehexosaminidase A A

•• LysosomalLysosomal enzymeenzyme

•• Fatty substance builds up in brainFatty substance builds up in brain

•• Mental, physical deterioration; death by age 4Mental, physical deterioration; death by age 4

PunnettPunnett SquaresSquares

•• Scenario: 2 healthy individuals have a child with Scenario: 2 healthy individuals have a child with

TayTay--Sach’sSach’s

•• AutosomalAutosomal recessive disease so child must be recessive disease so child must be

homozygoushomozygous

•• One allele inherited from each parent, yet each One allele inherited from each parent, yet each

parent is healthyparent is healthy

•• Both parents must be Both parents must be heterozygousheterozygous

•• We call these individuals We call these individuals carrierscarriers

•• Have the disease gene, but do not have the Have the disease gene, but do not have the

diseasedisease


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TayTay--Sach’sSach’s DiseaseDisease

DeafnessDeafness

•• Let’s do a pedigree for an Let’s do a pedigree for an autosomalautosomal recessive recessive condition: hereditary deafness (condition: hereditary deafness (dddd))

•• Trait may skip a generationTrait may skip a generation

•• Assign a genotype to each individualAssign a genotype to each individual

DeafnessDeafness

•• Step 1: Assign a genotype to anyone we know is Step 1: Assign a genotype to anyone we know is

homozygoushomozygous

•• Step 2: Give all unaffected individuals one DStep 2: Give all unaffected individuals one D

•• Step 3: Give all offspring of affected Step 3: Give all offspring of affected

individuals one dindividuals one d

•• Step 4: Work backwards Step 4: Work backwards –– look at affected look at affected

individuals; d must be present in both parentsindividuals; d must be present in both parents

•• Step 5: Double check, but some will remain a Step 5: Double check, but some will remain a

mysterymystery

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DeafnessDeafness

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InheritanceInheritance

•• In reality, inheritance is much more complicatedIn reality, inheritance is much more complicated

•• Many factors at play that can alter expected Many factors at play that can alter expected

inheritance patternsinheritance patterns

•• More than two alleles for one geneMore than two alleles for one gene

•• More than one gene affects a traitMore than one gene affects a trait

•• One gene modifies expression of another One gene modifies expression of another

gene (gene (epistasisepistasis))

•• We will look at 2 factors here:We will look at 2 factors here:

•• Incomplete dominanceIncomplete dominance

•• CodominanceCodominance

Incomplete DominanceIncomplete Dominance

•• Sometimes there is not one clear dominant alleleSometimes there is not one clear dominant allele

•• In a heterozygous individual, both alleles are In a heterozygous individual, both alleles are

expressedexpressed

•• Phenotype is a blend of both traitsPhenotype is a blend of both traits


•• Example: snapdragon colorExample: snapdragon color

•• Both red (RR) and white (Both red (RR) and white (rrrr) are dominant) are dominant

•• Heterozygous (Heterozygous (RrRr) = pink) = pink

•• Use a Use a PunnettPunnett square to predict the ratio of square to predict the ratio of

red:pink:whitered:pink:white offspring if 2 pink snapdragons offspring if 2 pink snapdragons

are crossedare crossed


•• Genotype? Phenotype?Genotype? Phenotype?


•• Example in humans: hairExample in humans: hair

•• Both curly (CC) and straight (SS) are dominantBoth curly (CC) and straight (SS) are dominant

•• Heterozygous (CS) = wavyHeterozygous (CS) = wavy

•• Use a Use a PunnettPunnett square to predict the probability square to predict the probability

of a child with wavy hair from a father with wavy of a child with wavy hair from a father with wavy

hair and a mother with straight hair hair and a mother with straight hair

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•• Genotype? Phenotype?Genotype? Phenotype?


S S SS

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CodominanceCodominance

•• Commonly seen when more than 2 alleles exist Commonly seen when more than 2 alleles exist

for the same genefor the same gene

•• Both dominant alleles are expressed at onceBoth dominant alleles are expressed at once

•• Not a blend of the 2 traits Not a blend of the 2 traits –– both distinct both distinct

traits can be seen at the same timetraits can be seen at the same time

Incomplete vs. Incomplete vs. CodominanceCodominance

Dominant Dominant•• Incomplete dominanceIncomplete dominance

and and codominancecodominance areare

NOT the same thing!!NOT the same thing!!

•• Incomplete dominance:Incomplete dominance:

phenotype is a blendphenotype is a blend

of the two traitsof the two traits

•• CodominanceCodominance: both: both

traits are seen attraits are seen at

the same timethe same time

CodominanceCodominance

•• Human example: A, B, O blood typesHuman example: A, B, O blood types

•• Both type A and type B are dominant (IBoth type A and type B are dominant (IAA and Iand IBB))

•• Make different Make different glycoproteinsglycoproteins on the on the

membrane of red blood cellsmembrane of red blood cells

•• Type O is recessiveType O is recessive

•• Makes no such glycoproteinMakes no such glycoprotein

•• If IIf IAA and Iand IBB are both present, both will be are both present, both will be

expressedexpressed

Blood TypeBlood Type CodominanceCodominance

•• Consider the following genotypes, and determine Consider the following genotypes, and determine

the phenotype (blood type) that would be the phenotype (blood type) that would be

present in each individualpresent in each individual

Genotype PhenotypeIAIA

IAi

ii

IBIB

IBi

IAIB

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Chaplin Paternity CaseChaplin Paternity Case

•• Before the days of DNA testing, blood type was Before the days of DNA testing, blood type was

used to settle paternity suitsused to settle paternity suits

•• Doesn’t always work thoughDoesn’t always work though

•• Charlie Chaplin was involved in such a case in Charlie Chaplin was involved in such a case in

1942 with actress Joan Barry1942 with actress Joan Barry


•• Charlie Chaplin’s blood type: ABCharlie Chaplin’s blood type: AB

•• Joan Barry’s blood type: OJoan Barry’s blood type: O

•• Child’s blood type: OChild’s blood type: O

•• Use a Use a PunnettPunnett square to determine whether square to determine whether

Charlie Chaplin could have been the child’s Charlie Chaplin could have been the child’s

fatherfather


•• Charlie Chaplin’s blood type: ABCharlie Chaplin’s blood type: AB

•• Only possible genotype: Only possible genotype:

•• Joan Berry’s blood type: OJoan Berry’s blood type: O


•• Child’s blood type: OChild’s blood type: O




ii iiPa

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IIAAIIBB


•• Could Charlie Chaplin have been the child’s Could Charlie Chaplin have been the child’s

father?father?

lecture 9: genetic inheritance - linn–benton community...

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