BIOL 222 Ch 14 Mendelian Inheritance part 2 - Science...
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Transcript of 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
3
PolygenicInheritance
• Humanbeingsandmanyotherspeciescanhavenomorethantwoallelesforagivengene,eachalleleresidingonaseparate,homologouschromosome.– Becausewe’rediploid
• BUT,manyallelicvariantsofagenecanexistinapopulaDon– Withonlytwoofthosepossessedbyanyoneindividual
PolygenicInheritance
• Mosttraitsinlivingthingsaregovernedbymanygenes.
• ThesegenesoXenhaveseveralallelicvariants.
PolygenicInheritance
• Polygenicinheritance– AddiDveeffectof2ormoregenesonasingletrait
– Skin/hair/eyecolorsandheightinhumans
PolygenicInheritance
• Polygenicinheritance– tendstoproduceconDnuousvariaDoninphenotypes• therearenofixedincrementsofdifferencebetweenindividuals.
• Humanskin– comesinarangeofcolorsinwhichonecolorshadesimpercepDblyintothenext.
4
PolygenicInheritance
• 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
PolygenicInheritance
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
PolygenicInheritance
• 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
6
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)
7
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