1 mendelelian genetics copyright cmassengale a tale of two families copyright cmassengale2 modes of...

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Mendelelian Mendelelian GeneticsGenetics

copyright cmassengale

A Tale of Two Families

copyright cmassengale 2

Modes of inheritance are the rules explaining the common patterns of inheritance.

Huntington’s Disease and Cystic Fibrosis demonstrate two important modes of inheritance.

HD is autosomal dominant and CF is autosomal recessive.

Fig. 4.1

Autosomal Dominant – affects both sexes and appears every generation

Fig. 4.2Autosomal Recessive – affects both sexes and skip generations through carriers

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Gregor Gregor MendelMendel

(1822-1884)(1822-1884)

Responsible Responsible for the Laws for the Laws governing governing

Inheritance Inheritance of Traitsof Traits


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Gregor Johann MendelGregor Johann MendelAustrian monkAustrian monkStudied the Studied the inheritanceinheritance of of traits in traits in pea plantspea plantsDeveloped the Developed the laws of inheritancelaws of inheritanceMendel's work Mendel's work was not recognized was not recognized until the turn of until the turn of thethe 20th century 20th century


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Gregor Johann MendelGregor Johann MendelBetween Between 1856 1856 and 1863,and 1863, Mendel Mendel cultivated and cultivated and tested some tested some 28,000 pea plants28,000 pea plantsHe found that He found that the plants' the plants' offspring retained offspring retained traits of the traits of the parentsparentsCalled theCalled the ““Father of Father of Genetics"Genetics"


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Site of Site of Gregor Gregor MendelMendel’’s s experimentexperimental garden al garden in the in the Czech Czech RepublicRepublic


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Mendel stated Mendel stated that physical traits that physical traits are inherited as are inherited as ““particlesparticles””Mendel did not Mendel did not know that the know that the ““particlesparticles”” were were actually actually Chromosomes & Chromosomes & DNADNA

Particulate InheritanceParticulate Inheritance


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Genetic TerminologyGenetic Terminology

TraitTrait - any characteristic - any characteristic that can be passed from that can be passed from parent to offspring parent to offspring HeredityHeredity - passing of - passing of traits from parent to traits from parent to offspring offspring GeneticsGenetics - study of - study of heredity heredity


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Types of Genetic Types of Genetic CrossesCrossesMonohybrid cross Monohybrid cross - - cross cross

involving a single traitinvolving a single traite.g. flower color e.g. flower color Dihybrid crossDihybrid cross - - cross cross involving two traits involving two traits e.g. flower color & plant e.g. flower color & plant heightheight


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Punnett SquarePunnett Square

Used to help Used to help solve solve genetics genetics problemsproblems


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Designer Designer ““GenesGenes””

AllelesAlleles - - two forms of a two forms of a gene gene (dominant & recessive)(dominant & recessive) DominantDominant - - stronger of two genes stronger of two genes expressed in the hybrid; expressed in the hybrid; represented byrepresented by aa capital letter (R)capital letter (R) RecessiveRecessive - - gene that shows up gene that shows up less often in a cross; represented less often in a cross; represented by aby a lowercase letter (r)lowercase letter (r)


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More TerminologyMore Terminology

GenotypeGenotype - - gene gene combination for a traitcombination for a trait (e.g. RR, Rr, rr) (e.g. RR, Rr, rr) PhenotypePhenotype - - the physical the physical feature resulting from a feature resulting from a genotypegenotype (e.g. red, white) (e.g. red, white)


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Genotype & Phenotype in Genotype & Phenotype in FlowersFlowers

Genotype of alleles:Genotype of alleles:RR = red flower= red flowerrr = yellow flower= yellow flower

All genes occur in pairs, so All genes occur in pairs, so 22 allelesalleles affect a characteristic affect a characteristic

Possible combinations are:Possible combinations are:

GenotypesGenotypes RRRR RRrr rrrr

PhenotypesPhenotypesRED RED RED RED YELLOWYELLOW


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GenotypesGenotypesHomozygousHomozygous genotype - gene genotype - gene

combination involving 2 combination involving 2 dominant or 2 recessive genes dominant or 2 recessive genes (e.g. RR or rr);(e.g. RR or rr); also called also called pure pure HeterozygousHeterozygous genotype - gene genotype - gene combination of one dominant & combination of one dominant & one recessive allele (one recessive allele (e.g. Rr);e.g. Rr); also calledalso called hybridhybrid


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Genes and Environment Genes and Environment Determine CharacteristicsDetermine Characteristics


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MendelMendel’’s Pea s Pea Plant Plant

ExperimentsExperiments


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Why peas,Why peas, Pisum Pisum sativumsativum??Can be grown in a Can be grown in a

small areasmall area Produce Produce lots of lots of offspring offspring Produce Produce purepure plants when allowed plants when allowed to to self-pollinateself-pollinate several generations several generations Can be Can be artificially artificially cross-pollinatedcross-pollinated


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Reproduction in Flowering Reproduction in Flowering PlantsPlants

Pollen contains spermPollen contains spermProduced by the stamenProduced by the stamen

Ovary contains eggsOvary contains eggsFound inside the flowerFound inside the flower

Pollen carries sperm to Pollen carries sperm to the eggs for fertilizationthe eggs for fertilization

Self-fertilizationSelf-fertilization can can occur in the same occur in the same flowerflower

Cross-fertilizationCross-fertilization can can occur between occur between flowersflowers


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MendelMendel’’s s Experimental Experimental

MethodsMethodsMendel Mendel hand-pollinatedhand-pollinated flowers using a flowers using a paintbrushpaintbrush

He could He could snip the snip the stamensstamens to prevent to prevent self-pollinationself-pollination

Covered each flower Covered each flower with a cloth bagwith a cloth bag

He traced traits He traced traits through the through the several several generationsgenerations


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How Mendel BeganHow Mendel BeganMendel Mendel produced produced purepure strains by strains by allowing allowing the plants the plants to to self-self-pollinatepollinate for for several several generatiogenerationsns copyright cmassengale

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Eight Pea Plant TraitsEight Pea Plant Traits

Seed shapeSeed shape --- Round --- Round (R)(R) or Wrinkled or Wrinkled (r)(r)

Seed ColorSeed Color ---- Yellow ---- Yellow (Y)(Y) or Green or Green ((yy))

Pod ShapePod Shape --- Smooth --- Smooth (S)(S) or wrinkled or wrinkled ((ss))Pod ColorPod Color --- Green --- Green (G)(G) or Yellow or Yellow (g)(g)Seed Coat ColorSeed Coat Color ---Gray ---Gray (G)(G) or White or White (g)(g)Flower positionFlower position---Axial ---Axial (A)(A) or Terminal or Terminal (a)(a)Plant HeightPlant Height --- Tall --- Tall (T)(T) or Short or Short (t)(t)

Flower color Flower color --- --- Purple Purple (P)(P) or white or white ((pp))


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MendelMendel’’s Experimental s Experimental ResultsResults


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Did the observed ratio match the theoretical ratio?Did the observed ratio match the theoretical ratio?

The theoretical or expected ratio of The theoretical or expected ratio of plants producing round or wrinkled plants producing round or wrinkled seeds is seeds is 3 round :1 wrinkled3 round :1 wrinkled

Mendel’s observed ratio was 2.96:1Mendel’s observed ratio was 2.96:1

The discrepancy is due to The discrepancy is due to statistical statistical errorerror

The The larger the samplelarger the sample the more the more nearly the results approximate to nearly the results approximate to the theoretical ratiothe theoretical ratio


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Generation Generation ““GapGap””Parental PParental P11 Generation Generation = the parental = the parental

generation in a breeding experimentgeneration in a breeding experiment..

FF11 generation generation = the first-generation = the first-generation offspring in a breeding experiment. offspring in a breeding experiment. (1st (1st filial generation)filial generation)

From breeding individuals from the PFrom breeding individuals from the P11 generationgeneration

FF22 generation generation = the second-generation = the second-generation offspring in a breeding experiment. offspring in a breeding experiment. (2nd filial generation)(2nd filial generation)

From breeding individuals from the FFrom breeding individuals from the F11 generationgeneration


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Following the Following the GenerationsGenerations

Cross 2 Cross 2 Pure Pure

PlantsPlantsTT x ttTT x tt

Results Results in all in all

HybridsHybridsTtTt

Cross 2 Cross 2 HybridsHybrids

getget3 Tall & 1 3 Tall & 1

ShortShortTT, Tt, ttTT, Tt, tt


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Monohybrid Monohybrid CrossesCrosses


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Trait: Seed ShapeTrait: Seed Shape

Alleles: Alleles: RR – Round – Round rr – Wrinkled – Wrinkled

Cross: Cross: RoundRound seedsseeds xx Wrinkled Wrinkled seedsseeds

RRRR xx rr rr

PP11 Monohybrid Cross Monohybrid Cross

R

R

rr

Rr

RrRr

Rr

Genotype:Genotype: RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: All alikeAll alike

PhenotypicPhenotypicRatio:Ratio: All alike All alikecopyright cmassengale

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PP11 Monohybrid Cross Monohybrid Cross ReviewReviewHomozygous dominant x Homozygous dominant x

Homozygous recessiveHomozygous recessiveOffspringOffspring allall HeterozygousHeterozygous (hybrids)(hybrids)Offspring calledOffspring called FF11 generation generation

Genotypic & Phenotypic ratio Genotypic & Phenotypic ratio isis ALL ALIKEALL ALIKE


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Cross: Cross: RoundRound seeds seeds xx Round Round seedsseeds

RrRr xx Rr Rr

FF11 Monohybrid Cross Monohybrid Cross

R

r

rR

RR

rrRr

Rr

Genotype:Genotype: RR, Rr, RR, Rr, rrrr

PhenotypePhenotype: Round Round && wrinkled wrinkled

G.Ratio:G.Ratio: 1:2:11:2:1

P.Ratio:P.Ratio: 3:1 3:1copyright cmassengale

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FF11 Monohybrid Cross Monohybrid Cross ReviewReviewHeterozygous x heterozygousHeterozygous x heterozygousOffspring:Offspring:

25% Homozygous dominant25% Homozygous dominant RRRR50% Heterozygous50% Heterozygous RrRr25% Homozygous Recessive25% Homozygous Recessive rrrrOffspring calledOffspring called FF22 generation generationGenotypic ratio isGenotypic ratio is 1:2:11:2:1Phenotypic RatioPhenotypic Ratio is 3:1 is 3:1


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What Do the Peas Look What Do the Peas Look Like?Like?


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……And Now the Test And Now the Test CrossCross

Mendel then crossed a Mendel then crossed a purepure & a & a hybridhybrid from his from his FF2 2 generationgeneration

This is known as an This is known as an FF22 or test cross or test cross

There are There are twotwo possible testcrosses: possible testcrosses:Homozygous dominant x HybridHomozygous dominant x HybridHomozygous recessive x HybridHomozygous recessive x Hybrid


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Cross: Cross: RoundRound seedsseeds xx Round Round seedsseeds

RRRR xx Rr Rr

FF22 Monohybrid Cross (1 Monohybrid Cross (1stst))

R

R

rR

RR

RrRR

Rr

Genotype:Genotype: RR, RR, RrRr

PhenotypePhenotype: RoundRound

GenotypicGenotypicRatio:Ratio: 1:11:1

PhenotypicPhenotypicRatio:Ratio: All alike All alike


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Cross: Cross: WrinkledWrinkled seedsseeds xx Round Round seedsseeds

rrrr xx Rr Rr

FF22 Monohybrid Cross (2nd) Monohybrid Cross (2nd)

r

r

rR

Rr

rrRr

rr

Genotype:Genotype: Rr, rrRr, rr

PhenotypePhenotype: Round & Round & WrinkledWrinkled

G. Ratio:G. Ratio: 1:11:1

P.Ratio:P.Ratio: 1:1 1:1copyright cmassengale

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FF22 Monohybrid Cross Monohybrid Cross ReviewReviewHomozygous x Homozygous x

heterozygous(hybrid)heterozygous(hybrid)Offspring:Offspring:50% Homozygous 50% Homozygous RR or rrRR or rr50% Heterozygous50% Heterozygous RrRrPhenotypic RatioPhenotypic Ratio is 1:1 is 1:1Called Called Test CrossTest Cross because the because the offspring have offspring have SAMESAME genotype genotype as parentsas parents


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Practice Your CrossesPractice Your Crosses

Work the PWork the P11, F, F11, and , and both Fboth F22 Crosses for Crosses for each of the other each of the other

Seven Pea Plant TraitsSeven Pea Plant Traits


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MendelMendel’’s Lawss Laws


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Results of Monohybrid Results of Monohybrid CrossesCrosses

InheritableInheritable factors or genesfactors or genes are are responsible for all heritable responsible for all heritable characteristics characteristics

PhenotypePhenotype is based on is based on GenotypeGenotype Each traitEach trait is based onis based on two genestwo genes, ,

one from the mother and the one from the mother and the other from the fatherother from the father

True-breeding individuals are True-breeding individuals are homozygous ( both alleles)homozygous ( both alleles) are are the samethe same


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Law of DominanceLaw of Dominance

In a cross of parents that are In a cross of parents that are pure for contrasting traitspure for contrasting traits, , only one form of the trait will only one form of the trait will appear in the next generation.appear in the next generation.

All the offspring will be All the offspring will be heterozygous and express heterozygous and express only the only the dominant trait.dominant trait.

RR x rr RR x rr yieldsyields all Rr (round all Rr (round seeds)seeds)


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Law of DominanceLaw of Dominance


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Law of SegregationLaw of Segregation

During the During the formation of formation of gametesgametes (eggs or sperm), the (eggs or sperm), the two allelestwo alleles responsible for a responsible for a trait trait separateseparate from each other. from each other.

Alleles for a trait are then Alleles for a trait are then "recombined" at fertilization"recombined" at fertilization, , producing the genotype for producing the genotype for the traits of the offspringthe traits of the offspring.


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Applying the Law of Applying the Law of SegregationSegregation


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Law of Independent Law of Independent AssortmentAssortment

Alleles for Alleles for differentdifferent traits are traits are distributed to sex cells (& distributed to sex cells (& offspring) independently of offspring) independently of one another.one another.

This law can be illustrated This law can be illustrated using using dihybrid crossesdihybrid crosses. .


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Dihybrid CrossDihybrid Cross

A breeding experiment that tracks A breeding experiment that tracks the the inheritance of two traitsinheritance of two traits..

Mendel’s Mendel’s “Law of Independent “Law of Independent Assortment”Assortment”

a. Each pair of alleles segregates a. Each pair of alleles segregates independentlyindependently during gamete during gamete formationformation

b. Formula: 2b. Formula: 2nn (n = # of heterozygotes) (n = # of heterozygotes)


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Question:Question:How many gametes will be produced How many gametes will be produced

for the following allele for the following allele arrangements?arrangements?

Remember:Remember: 22nn (n = # of heterozygotes) (n = # of heterozygotes)

1.1. RrYyRrYy

2.2. AaBbCCDdAaBbCCDd

3.3. MmNnOoPPQQRrssTtQqMmNnOoPPQQRrssTtQq


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Answer:Answer:1. RrYy: 21. RrYy: 2nn = 2 = 222 = 4 gametes = 4 gametes

RY Ry rY ryRY Ry rY ry

2. AaBbCCDd: 22. AaBbCCDd: 2nn = 2 = 233 = 8 gametes = 8 gametes

ABCD ABCd AbCD AbCdABCD ABCd AbCD AbCd

aBCD aBCd abCD abCDaBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 23. MmNnOoPPQQRrssTtQq: 2nn = 2 = 266 = = 64 gametes64 gametes


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Dihybrid CrossDihybrid CrossTraits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles:Alleles: R round

r wrinkled Y yellow y green

RrYy x RrYy

RY Ry rY ryRY Ry rY ry RY Ry rY ryRY Ry rY ry

All possible gamete combinationsAll possible gamete combinationscopyright cmassengale

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RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry


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RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry


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Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1


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Test CrossTest CrossA mating between an individual of A mating between an individual of

unknown genotype unknown genotype and a and a homozygous homozygous recessiverecessive individual. individual.

Example:Example: bbC__ bbC__ x x bbccbbcc

BB = brown eyesBB = brown eyesBb = brown eyesBb = brown eyesbb = blue eyesbb = blue eyes

CC = curly hairCC = curly hairCc = curly hairCc = curly haircc = straight haircc = straight hair

bCbC b___b___

bcbc


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Test CrossTest Cross

Possible results:Possible results:

bCbC b___b___

bcbc bbCc bbCc

C bCbC b___b___

bcbc bbCc bbccor

c


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Summary of Mendel’s lawsSummary of Mendel’s laws

LAWLAW PARENT PARENT CROSSCROSS OFFSPRINGOFFSPRING

DOMINANCEDOMINANCE TT x ttTT x tt tall x shorttall x short

100% Tt 100% Tt talltall

SEGREGATIONSEGREGATION Tt x TtTt x Tt tall x talltall x tall

75% tall 75% tall 25% short25% short

INDEPENDENT INDEPENDENT ASSORTMENTASSORTMENT

RrGg x RrGgRrGg x RrGg round & round & green x green x round & round & greengreen

9/16 round seeds & green 9/16 round seeds & green pods pods

3/16 round seeds & yellow 3/16 round seeds & yellow pods pods

3/16 wrinkled seeds & 3/16 wrinkled seeds & green pods green pods

1/16 wrinkled seeds & 1/16 wrinkled seeds & yellow podsyellow pods


Single-gene Inheritance in Humans1. A Mendelian trait is caused by a single

gene.2. Mendelian human conditions are

considered more rare than multifactorial ones

3. Mendelian inheritance in humans is more complex than in model organisms such as pea plants.


Eye color

The inheritance of eye color is an example of a single gene trait


Modes of Inheritance

Modes of inheritance – rules explaining common patterns of inheritance

Genes may be autosomal or on a sex chromosome

Alleles can be dominant or recessvie


Criteria for an autosomal dominant trait:1. Males & females can be affected; male to male transmission can occur2. Both transmit the trait with equal frequency3. Successive generations are affected.4. Transmission stops after a generation where no one is affected.


Criteria for an autosomal recessive trait1. Males & females can be affected2. Both can transmit the gene, unless it causes death before reproductive age3. The trait can skip generations4. Parents of an affected individual are heterozygous or have the trait.


Modes of inheritance, cont.

1. Blood relatives that have children together have a much higher risk of having a child with a rare recessive disorder.

2. Marriage between relatives introduces consanguinity, literally meaning “shared blood” However genes are not passed in blood

3. Punnett squares apply Mendel’s first law to predict recurrence risks of inherited disorders or traits.

4. A Mendelian trait applies anew to each child.


On the Meaning of Dominance & Recessiveness

At the biochemical level, recessive disorders often result from alleles that cause a loss of function or loss of a normal protein

Dominant disorders can result from production of an abnormal protein that interferes with the function of a normal protein or imparts a gain of function.


Recessive disorders

Recessive orders tend to be more severe producing symptoms earlier than dominant disorders

Disease causing recessive alleles remain in populations because heterozygotes pass them to future generations.


Dominant disordersIf a dominant mutation that harms early in life

arises people who have the allele are too ill or don’t survive long enough to reproduce

The allele becomes rare in the population unless it arises anew by mutation

Dominant disorders that do not appear until adulthood or drastically disrupt health, remain in the population until after a person has reproduced.


Pedigree AnalysisPedigree charts depict family relationships

and transmission of inherited traits.Squares represent males and circles

represent females.Horizontal lines indicate parents, vertical

lines show generations, and elevated lines depict siblings.

Symbols for heterozygotes are half-shaded, and for an individual with a particular phenotype, completely shaded


Pedigrees Then and Now

Pedigrees have been in use since antiquity.

They have been used throughout history to show family relationships, particularly royal bloodlines.


Pedigrees Display Mendel’s Laws

Pedigrees can reveal mode of inheritance, and can suggest molecular information, carrier status, and input from other genes and the environment.

Interpretation of pedigrees can be inconclusive when more than one mode of inheritance can explain the pattern seen.


Solving Genetics ProblemsList genotypes and phenotypes

for the traitDetermine the genotypes of the

parentsPossible gametesPossible genotypes of offspringRepeat for successive

generations

Independent Events

The probability of simultaneous independent events = the product of the probability of each event

Example: If both parents are heterozygous (Bb)) what is

the probability that they will produce a BB child?

Probability of a sperm with B allele = ½Probability of a ova with B allele = ½Probability of a BB child is ½ X ½ = ¼

Dependent Events

The probability of dependent events = the sum of probability of each event

Example•Parents are heterozygous for a trait, R.

•What is the chance that their child carries at least one dominant R allele?

•Probability of child carrying RR = ¼•Probability of child carrying Rr = ½

•Probability of child carrying R_ = ¼ + ½ = ¾

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