biol 222 ch 14 mendelian inheritance part 2 - science...

1

DihybridCross

• Crossinvolvingtwotraits• Mendel

– Observedthatthegenesforthedifferentcharactershestudiedwerepassedonindependentlyofoneanother

– Genesforthesecharactersresidedonseparate,non‐homologouschromosomes

– Foundthephysicalbasisforindependentassortmentofchromosomepairsduringmeiosis.

DihybridCross

• DeterminePgeneraDon– Diploid

• Determinegametes– Haploid

• Crossgametesinsquaretoyieldoffspring– Nowneed16squaresinsteadoffour– UseFOILforcalculaDnggametes

• First,outer,inner,last

• Summarize

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

IncompleteDominance

• NotallinheritanceworksthroughtheprinciplesMendelperceivedinhispeas.

• Incompletedominance– neitheralleleforagivengeneiscompletelydominant

– heterozygousgenotypescanyieldanintermediatephenotype• Pinksnapdragons• Wavyhair

2

IncompleteDominance

Figure 11.10

RRred

rrwhite

P generation

The starting plants are a snapdragon homozygous for red color (RR) and snapdragon homozygous for white color (rr).

F1 generation

When these plants are crossed, the resulting Rr genotype yields only enough pigment to produce a flower that is pink—the only phenotype in the F1 generation.

F2 generation

In the F2 generation, alleles combine to produce red, pink, and white phenotypes.

Rr100% pink

sperm r

r

rr

R

RR Rr

Rr

R

egg

1red

: : 1white

2pink

1.

2.

3.

Codominance

• Codominance– Neitheralleleisrecessive

– Insomeinstances,differingallelesofthesamegenewillhaveindependenteffectsinasingleorganism.

• ABObloodgroups– SuchisthecasewiththegenethatcodesforthetypeAandBglycolipidsthatextendfromthesurfaceofhumanredbloodcells.

Codominance• ABObloodgroups

– MulDplealleles

• AnindividualwhohasoneAandoneBallelewillhavetypeABblood– InsuchasituaDon,neitheralleleisdominant;rather,eachishavingaseparatephenotypiceffect

– ButtheOalleleisrecessivetobothAandB

• AA=typeA• BB=typeB• AO=typeA• BO=typeB• OO=typeO

Codominance

Figure 11.11

no surface glycolipids

Surface glycolipids on red blood cells

Blood type(phenotype) . . .

. . . has these surfaceglycolipids . . .

A AA or AO

BB or BO

AB

OO

B

AB

O

. . . and isproduced bythese genotypes

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PolygenicInheritance

• Humanbeingsandmanyotherspeciescanhavenomorethantwoallelesforagivengene,eachalleleresidingonaseparate,homologouschromosome.– Becausewe’rediploid

• BUT,manyallelicvariantsofagenecanexistinapopulaDon– Withonlytwoofthosepossessedbyanyoneindividual


• Mosttraitsinlivingthingsaregovernedbymanygenes.

• ThesegenesoXenhaveseveralallelicvariants.


• Polygenicinheritance– AddiDveeffectof2ormoregenesonasingletrait

– Skin/hair/eyecolorsandheightinhumans


• Polygenicinheritance– tendstoproduceconDnuousvariaDoninphenotypes• therearenofixedincrementsofdifferencebetweenindividuals.

• Humanskin– comesinarangeofcolorsinwhichonecolorshadesimpercepDblyintothenext.

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• Polygenictraitstendtomanifestinbell‐curvedistribuDons– mostindividualsdisplaynearaveragetraitvalues

• ratherthanextremetraitvalues

P generation

1–8

F1 generation

F2 generation

Frac

tion

of p

opul

atio

n

Skin color

Eggs

Sperm1–8

1–8

1–8

1–8

1–81–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

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


Figure 11.13

(a) Continuous variation in human height (b) The bell curve

Beak depth (mm)

Num

ber o

f ind

ivid

uals

beak depth

5 6 7 8 9 10 11 12 13 140

20

40

60

80


• GeneinteracDonsandgene–environmentinteracDonsaresocomplexinpolygenicinheritancethatpredicDonsaboutphenotypesareama\erofprobability,notcertainty.

5

GenesandEnvironment

• Natureversusnurture– Theeffectsofgenescanvarygreatlyinaccordancewiththeenvironmentinwhichthegenesareexpressed

• Genotypeandenvironmentinteract– producethatorganism’sphenotype

GenesandEnvironment

Figure 11.14

Pleiotropy

• Pleiotropyisaphenomenoninwhichonegenehasmanyeffects.

• Genesworkinaninterrelatedfashion,suchthatasinglegeneislikelytohavemulDpleeffects.– PKU‐phenolketonuria– Sickelcellanemia

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

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Genesaffec;nggenes

• Epistasis– ageneatonelocusaltersthephenotypicexpressionofageneatasecondlocus

• Forexample,inmiceandmanyothermammals,coatcolordependsontwogenes– Onegenedeterminesthepigmentcolor(withallelesBforblackandbforbrown)

– Theothergene(withallelesCforcolorandcfornocolor)determineswhetherthepigmentwillbedepositedinthehair

BbCc BbCc

Sperm

EggsBC bC Bc bc

BC

bC

Bc

bc

BBCC

1/4 1/4 1/4 1/4

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

×

PedigreeAnalysis

• Pedigree– familytreethatdescribestheinterrelaDonshipsofparentsandchildrenacrossgeneraDons

– Inheritancepa\ernsofparDculartraitscanbetracedanddescribedusingpedigrees

Fig. 14-15a

KeyMale

Female

AffectedmaleAffectedfemale

Mating

Offspring, inbirth order(first-born on left)

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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

Youshouldnowbeableto:

1. Definethefollowingterms:truebreeding,hybridizaDon,monohybridcross,PgeneraDon,F1generaDon,F2generaDon

2. DisDnguishbetweenthefollowingpairsofterms:dominantandrecessive;heterozygousandhomozygous;genotypeandphenotype

3. UseaPunne\squaretopredicttheresultsofacrossandtostatethephenotypicandgenotypicraDosoftheF2generaDon

4. Explainhowphenotypicexpressionintheheterozygotedifferswithcompletedominance,incompletedominance,andcodominance

5. Defineandgiveexamplesofpleiotropyandepistasis

6. Explainwhylethaldominantgenesaremuchrarerthanlethalrecessivegenes

biol 222 ch 14 mendelian inheritance part 2 - science...

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