Test Bank For Human Genetics, 13th Edition All Chapters
b b b b b b b b b
-
Lectureb1:b-bbbANSWERbb
-bDNAbisbpackagedbasbchromatin,bchromosomesbbecomebvisiblebduringbmitosis.b-
bbbANSWERbb
Homologues-b(maternalbandbpaternalbformsbofbsamebchromosome)b-bbbANSWERbb
Meiosis:bG1,bS,bG2,bM.bBeforebreplicationbphase,bchromosomesbhavebonebchromatidba
ndbafterbreplication,bchromosomesbhaveb2bsisterbchromatids,bheldbtogetherbatbthebcentr
omere.bEndsbwithb4bdaughterbcellsbwithb1bchromatidbeach.b-bbbANSWERbb
Keybdifferencesbfrombmitosis:bHomologsbpairb(ProphasebI),bSisterbcentromeresbactbasba
bsinglebcentromereb(MetaphasebI),bSisterbchromatidsbremainbattachedb(AnaphasebI),bM
eiosisbIbisbabreductionbdivision-
bmeaningbstartbmeiosisbIbwithb46bunitsbandbendbmeiosisbIbwithb23bunitsb(b2bchromatidsbinb
eachbhaploidbdaughterbcells),bMeiosisbIIbisbanbequationalbdivisionb(identicalbtobmitosis).b-
bbbANSWERbb
RecombinationboccursbinbProphasebI.bCrossingboverbandbrecombbisbExchangebofbhomo
logousbsegmentsbbetweenbnon-sisterbchromatids.b-bbbANSWERbb
HomologuesbmovebapartbduringbanaphasebI-
disjunction.b223bpossiblebcombinationsbofbchromosomes.bInbcytokinesisbI,bCellsbdividebi
ntobtwobhaploidbdaughterbcells.bOnebcellbreceivesbmostbofbthebcytoplasmbandbthebotherb
becomesbthebfirstbpolarbbodybwhichbdoesn'tbgobthroughbMeiosisbII.bMeiosisbIbendsbafterb
ovulation.bNonbdysjunctionbinbmeiosisbI:bcanbcausebproblemsblikebTrisomyb21.b-
bbbANSWERbb
-
Briefbinterphasebbetweenbfirstbandbsecondbmeioticbdivisions.bNobSbphaseboccursbinbMei
osisbII.b-bbbANSWERbb
-
Inbthebsecondbmeioticbdivisionbabsecondbpolarbbodybformsbduringboogenesis.bInboogene
sis,beggbisbarrestedbinbmetaphasebIIbuntilbfertilization.b-bbbANSWERbb
AfterbMeiosisbtherebisbabReductionbofbchromosomebnumberb2nn.b(diploidbvs.bhaploidbcel
ls)-
bNotationb'n'bhasbtobdobwithbamountbofbDNAb(#of bnucleotides),bnotbthebnecessarilybtheb#b
ofbchromosomes.b-bbbANSWERbb
,Lectureb2:bPatternsbofbInheritanceb-bbbANSWERbb
MedelianbInheritance-
bdeterminedbbybabsingle bmajorbgene.bBasedbonbindependentbassortment.bDihybridbcros
sb(2bgenotypes)bwithbry,bRy,brY,bRybsetbupbonbbothbsidesphenotypicbratiobofb9:3:3:1-
yellowbround,bgreenbround,byellowbwrinkled,bgreenbwrinkled,bIfbgetb9:3:3:1byoubknowbtha
tbsegregationbisbindependent.b-bbbANSWERbb
Multifactorialbinheritance-multiplebgeneticbandbnon-geneticbfactorsbinvolvedb-
bbbANSWERbb
Compoundbheterozygous:bhasbtwobdifferentbmutantballelesbforbabcharacterb-
bbbANSWERbb
Linkage-
2bgenesbphysicallybnearbeachbotherbonbabchromosomebwillbnotbassortbrandomlybinbmeio
sis.bTightlyblinked:bwillbgetb2btypesbofbgametesbex.bPLbandbpl.bUnlinked:bwillbgetb4btypesb
ofbgametesbPL,bPl,bpL,bpl.b-bbbANSWERbb
Thebfrequencybofbrecombinationbbetweenbtwobgenesbisbproportionalbtobthebdistancebbet
weenbthebgenes.bThebcloserbthebgenesbarebonbthebchromosomebtheblessblikelybcrossingb
overboccurs.bLinkagebmap:b1%brecombinationb=b1bmapbunitb=b1bcentiMorganb(cM),bMap
bdistancesbarebadditive.b-bbbANSWERbb
Thebnon-
randombassociationbbetweenballelesbatbtwoblocationsbonbabchromosomebisbcalledblinkag
ebdisequilibrium.bIfbthebfrequencybofbchromosomesbwithbAB=Ab=aB=abbthenbthebgenes
barebinbequilibrium.bIfbfrequencybofb1ballelebisbseenbmoreb(AbmorebthanbBbforbex)bthenbge
nesbarebinblinkagebdisequilibrium.b-bbbANSWERbb
AutosomalbdominantbinheritancebExamples:b-bbbANSWERbb
Achondroplasia-
bFGFR3bmutations,bAlwaysbfullbpenetrancebwithbachondroplasiab(sobnormalbparentsbha
vebabchildbwithbaplasiabthenbit'sbabnewbmutation).bHeterozygousbb/cbhomozygotesbusual
lybdiebinbuterob-bbbANSWERbb
Neurofibromatosis-bNF1b(neurofibromin)bandbNF2b(merlin)bmutations.b-bbbANSWERbb
Incompletebdominance-
bmixedbphenotype.bInbcasesbofbdisease,bDominantbdisordersbarebmorebseverebinbhomoz
ygotesbthenbinbheterozygotesb(termedbalsob"semidominant")bIe.bFamilialbHypercholeste
rolemia.b-bbbANSWERbb
Co-dominance-
bphenotypicbexpressionbofbtwobdifferentballelesbforbablocusbie.bBloodbtype.b-bbbANSWERbb
Autosomalbrecessivebinheritance:bExamples:bCysticbfibrosis,bTay-
Sachsbdisease,bSickle-cellbdiseaseb-bbbANSWERbb
,Pseudodominance:bthebinheritancebofbanbautosomalbrecessivebtraitbmimicsbanbautoso
malbdominantbpatternb-bbbANSWERbb
MalesbarebhemizygousbwithbrespectbtobX-linkedbgenes.b-bbbANSWERbb
X-
linkedbdominantbinheritance:bAffectedbfemalesbarebtwicebasbcommonbasbaffectedbmales
bbutbmalesbusuallybmorebseverelybaffectedborbthebdisorderbmaybbeblethalbinbmalesb(Rettb
syndrome).b-bbbANSWERbb
X-
linkedbrecessive:bincidencebisbmuchbhigherbinbmalesbandbaffectedbmalesbdobnotbusuallyb
transmitbthebdisorderbunlessbmotherbisbabcarrier.bHeterozygotebfemalesbarebusuallybuna
ffected,bbutbsomebmaybexpressbthebconditionbwithbvariablebseveritybasbdeterminedbbybth
ebpatternbofbXbinactivation.bAbsignificantbproportionbofbisolatedbcasesbarebduebtobnewbmu
tationb(Duchennebmuscularbdystrophy-DMD).b-bbbANSWERbb
Pseudoautosomalbinheritance-
bgroupbofbgenesbonbthebinactivebxbchromosomebarebNOTbinactivated.bDiseasesbassocia
tedbwithbthesebgenesbarebinheritedbsimilarbtobautosomalbinheritance.b-bbbANSWERbb
SamebamountbofbX-
linkedbgenebproductsbbetweenbmalesbandbfemalesbachievedbthroughbdosagebcompens
ation.bLyonbHypothesisbstatesbthatbthebinactivebXbisbNOTbrandomlybchosenbinbeachbcellb
Ex.bAbstructurallybabnormalbXbisbpreferentiallybinactivated.bInactivationbisbNOTbcomplet
e-
bsomebgenesbcanbescapebinactivationb(ie.bThosebwithbabfunctionalbhomologbonbthebY).bI
nactivationbisbNOTbpermanent-
breversedbinbdevelopmentbofbgermbcellsb(notbpassedbonbtobgametes).b-bbbANSWERbb
ThebkeybplayerbisbthebX-
linkedbgenebXIST→bXb(inactive)bspecificbtranscript.bXISTbisbtranscribedbtobproducebabno
n-codingbRNAbthatb"coats"bthebX-
chromosomebandbinactivatesbit.bXISTbisbonlybexpressedbfrombthebinactivebX.bThebhiston
esbonbthebcoatedbXbundergobmethylationbwhichbcausesbthebchromosomebtobcondenseb(
heterochromatin),bformingbabBarrbbody.b-bbbANSWERbb
-
SomebgenesbdobnotbhavebYbhomologuebandbdobnotbundergobinactivationb(e.g.bsteroidbs
ulfatasebgene)b-bbbANSWERbb
-Random/skewedbinactivationbmaybresultbinbaffected/totallybhealthybheterozygotes.b-
bbbANSWERbb
VariablebexpressionbofbX-
inactivation:bOnbbothbextremes,babheterozygousbfemalebwithbrecessivebx-
linkedbdiseasebcouldbmanifestbthebdisease.bInbabcasebwithbabdominantbX-
linkedbtrait,binbwhichbalmostballbofbabfemalesbXbchromosomesbwithbmutationbisbinactive,b
mightbnotbmanifestbthisbdisease.bIdenticalbtwinsbcouldbevenbhavebdiffbphenotypesbduebt
obskewedbXbinactivation.b-bbbANSWERbb
, Mosaicism-
bXbchromosomebinactivationboccursbrandomlybandbinactivationbpatternbisbpassedbtobcellb
progeny.bResult:bfunctionalbmosaicismbinbwhichbfemalebisbabmosaicbwithbrespectbtobexp
ressionbofbgenesbonbXbchrom.bEx.bCalicobCat,bBb-bdominantborange,bbb-
brecessivebgenebblack,bGenesbforbwhite:bautosomal.b-bbbANSWERbb
GeneticbHeterogeneitybcanbbebthebresultbofbLocusbHeterogeneityb,bAllelicbHeterogeneit
y,bandbModifierbLoci.b-bbbANSWERbb
LocusbHeterogeneity-
babsinglebdisorder,btrait,borbpatternbofbtraitsbcausedbbybmutationsbinbgenesbatbdifferentbch
romosomalbloci.bEx.bretinitisbpigmentosabhasbautosomalbdominant,bautosomalbrecessiv
e,bandbX-
linkedborigins.bHowever,bonlybonebmutantblocusbisbneededbforbthebphenotypebtobmanifes
tb-bbbANSWERbb
Allelicbheterogeneityb-
bManybgeneticblocibpossessbmorebthanbonebmutantballele.bInbthebCFTRbgene,bnearlyb14
00bmutationsbfound.bSomebmutationsbcausebclassicalbCFb+bpancreaticbinsufficiencyb+bc
ongenitalbabsencebofbvasbdeferens,bothersbcauseblungbdiseasebwithbnormalbpancreaticb
fxn.bOtherbcausebonlybmalebsterility.b-bbbANSWERbb
Modifierbgene:bAbgenebthatbaffectsbthebphenotypicbexpressionbofbanotherbgene.bSpecifi
cballelesbofboneborbmorebgenesb(modifierbgenes)bcanbsometimesbdramaticallybmodifybth
ebclinicalbseveritybofbthebphenotypebproducedbbybmutationsbinbabdisease-
causingbgene.Thisbsourcebofbclinicalbheterogeneitybisboftenbreferredbtobasbgeneticbback
ground.bEx.bTwinsbwithbthebsamebmutationbinbcysticbfibrosisbbutbonebisbseverelybsickban
dbonebonlybmoderatelybsick-bexplainedbbybmodifierbgenes.b-bbbANSWERbb
CandidateblocibactingbasbmodifiersbforbCFTRbmutations:TGFB1b(cytokinebtransformingb
growthbfactorbbbandbMLB2b(Mannose-
bindingblectin).bDifferentballelesbofbthesebgenesbmodifybthebseveritybofbCFb-bbbANSWERbb
Clinical/phenotypicbheterogeneity:bmultiplebphenotypesbofbonebsinglebgenebex.bUSH2A
bgene,bcodesbforbproteinbimportantbinbdevelopmentbofbthebinnerbearbandbretina.bCanbbeb
blindbandbdeafb(typebIIbUsherbsyndrome)borbjustbblindb(RetinitisbPigmentosa).b-
bbbANSWERbb
Otherbfactorsbaffectionbphenotypebincludebenvironmentalbfactorsbie.bG6PDbmutationbO
RbsoilbpHbandbhydrangeabcolor.bAlsobSex-
relatedbfactorsbiebHemochromatosis,bAR,bcausedbbybironboverload,bisblessbcommonbinbf
emalesbduebtoblowerbironbintake/menstrualbironbloss.b-bbbANSWERbb
G6PDb(glucose-6-phosphatebdehydrogenase)bDeficiencyb(X-linked)-
bCanbleadbtobhemolyticbanemia,bmanybpplbwithbthisbdisorderbneverbexperiencebanybsym
ptoms.bHemolyticbanemiabcanbbebtriggeredbbybenvironmentalbfactorsbsuchbasbbacterial/
viralbinfections,bdrugb(sulfa,bmalariabmedications),beatingbfavabbeansborbinhalingbfavabb
eanbplantbpollen.b-bbbANSWERbb
b b b b b b b b b
-
Lectureb1:b-bbbANSWERbb
-bDNAbisbpackagedbasbchromatin,bchromosomesbbecomebvisiblebduringbmitosis.b-
bbbANSWERbb
Homologues-b(maternalbandbpaternalbformsbofbsamebchromosome)b-bbbANSWERbb
Meiosis:bG1,bS,bG2,bM.bBeforebreplicationbphase,bchromosomesbhavebonebchromatidba
ndbafterbreplication,bchromosomesbhaveb2bsisterbchromatids,bheldbtogetherbatbthebcentr
omere.bEndsbwithb4bdaughterbcellsbwithb1bchromatidbeach.b-bbbANSWERbb
Keybdifferencesbfrombmitosis:bHomologsbpairb(ProphasebI),bSisterbcentromeresbactbasba
bsinglebcentromereb(MetaphasebI),bSisterbchromatidsbremainbattachedb(AnaphasebI),bM
eiosisbIbisbabreductionbdivision-
bmeaningbstartbmeiosisbIbwithb46bunitsbandbendbmeiosisbIbwithb23bunitsb(b2bchromatidsbinb
eachbhaploidbdaughterbcells),bMeiosisbIIbisbanbequationalbdivisionb(identicalbtobmitosis).b-
bbbANSWERbb
RecombinationboccursbinbProphasebI.bCrossingboverbandbrecombbisbExchangebofbhomo
logousbsegmentsbbetweenbnon-sisterbchromatids.b-bbbANSWERbb
HomologuesbmovebapartbduringbanaphasebI-
disjunction.b223bpossiblebcombinationsbofbchromosomes.bInbcytokinesisbI,bCellsbdividebi
ntobtwobhaploidbdaughterbcells.bOnebcellbreceivesbmostbofbthebcytoplasmbandbthebotherb
becomesbthebfirstbpolarbbodybwhichbdoesn'tbgobthroughbMeiosisbII.bMeiosisbIbendsbafterb
ovulation.bNonbdysjunctionbinbmeiosisbI:bcanbcausebproblemsblikebTrisomyb21.b-
bbbANSWERbb
-
Briefbinterphasebbetweenbfirstbandbsecondbmeioticbdivisions.bNobSbphaseboccursbinbMei
osisbII.b-bbbANSWERbb
-
Inbthebsecondbmeioticbdivisionbabsecondbpolarbbodybformsbduringboogenesis.bInboogene
sis,beggbisbarrestedbinbmetaphasebIIbuntilbfertilization.b-bbbANSWERbb
AfterbMeiosisbtherebisbabReductionbofbchromosomebnumberb2nn.b(diploidbvs.bhaploidbcel
ls)-
bNotationb'n'bhasbtobdobwithbamountbofbDNAb(#of bnucleotides),bnotbthebnecessarilybtheb#b
ofbchromosomes.b-bbbANSWERbb
,Lectureb2:bPatternsbofbInheritanceb-bbbANSWERbb
MedelianbInheritance-
bdeterminedbbybabsingle bmajorbgene.bBasedbonbindependentbassortment.bDihybridbcros
sb(2bgenotypes)bwithbry,bRy,brY,bRybsetbupbonbbothbsidesphenotypicbratiobofb9:3:3:1-
yellowbround,bgreenbround,byellowbwrinkled,bgreenbwrinkled,bIfbgetb9:3:3:1byoubknowbtha
tbsegregationbisbindependent.b-bbbANSWERbb
Multifactorialbinheritance-multiplebgeneticbandbnon-geneticbfactorsbinvolvedb-
bbbANSWERbb
Compoundbheterozygous:bhasbtwobdifferentbmutantballelesbforbabcharacterb-
bbbANSWERbb
Linkage-
2bgenesbphysicallybnearbeachbotherbonbabchromosomebwillbnotbassortbrandomlybinbmeio
sis.bTightlyblinked:bwillbgetb2btypesbofbgametesbex.bPLbandbpl.bUnlinked:bwillbgetb4btypesb
ofbgametesbPL,bPl,bpL,bpl.b-bbbANSWERbb
Thebfrequencybofbrecombinationbbetweenbtwobgenesbisbproportionalbtobthebdistancebbet
weenbthebgenes.bThebcloserbthebgenesbarebonbthebchromosomebtheblessblikelybcrossingb
overboccurs.bLinkagebmap:b1%brecombinationb=b1bmapbunitb=b1bcentiMorganb(cM),bMap
bdistancesbarebadditive.b-bbbANSWERbb
Thebnon-
randombassociationbbetweenballelesbatbtwoblocationsbonbabchromosomebisbcalledblinkag
ebdisequilibrium.bIfbthebfrequencybofbchromosomesbwithbAB=Ab=aB=abbthenbthebgenes
barebinbequilibrium.bIfbfrequencybofb1ballelebisbseenbmoreb(AbmorebthanbBbforbex)bthenbge
nesbarebinblinkagebdisequilibrium.b-bbbANSWERbb
AutosomalbdominantbinheritancebExamples:b-bbbANSWERbb
Achondroplasia-
bFGFR3bmutations,bAlwaysbfullbpenetrancebwithbachondroplasiab(sobnormalbparentsbha
vebabchildbwithbaplasiabthenbit'sbabnewbmutation).bHeterozygousbb/cbhomozygotesbusual
lybdiebinbuterob-bbbANSWERbb
Neurofibromatosis-bNF1b(neurofibromin)bandbNF2b(merlin)bmutations.b-bbbANSWERbb
Incompletebdominance-
bmixedbphenotype.bInbcasesbofbdisease,bDominantbdisordersbarebmorebseverebinbhomoz
ygotesbthenbinbheterozygotesb(termedbalsob"semidominant")bIe.bFamilialbHypercholeste
rolemia.b-bbbANSWERbb
Co-dominance-
bphenotypicbexpressionbofbtwobdifferentballelesbforbablocusbie.bBloodbtype.b-bbbANSWERbb
Autosomalbrecessivebinheritance:bExamples:bCysticbfibrosis,bTay-
Sachsbdisease,bSickle-cellbdiseaseb-bbbANSWERbb
,Pseudodominance:bthebinheritancebofbanbautosomalbrecessivebtraitbmimicsbanbautoso
malbdominantbpatternb-bbbANSWERbb
MalesbarebhemizygousbwithbrespectbtobX-linkedbgenes.b-bbbANSWERbb
X-
linkedbdominantbinheritance:bAffectedbfemalesbarebtwicebasbcommonbasbaffectedbmales
bbutbmalesbusuallybmorebseverelybaffectedborbthebdisorderbmaybbeblethalbinbmalesb(Rettb
syndrome).b-bbbANSWERbb
X-
linkedbrecessive:bincidencebisbmuchbhigherbinbmalesbandbaffectedbmalesbdobnotbusuallyb
transmitbthebdisorderbunlessbmotherbisbabcarrier.bHeterozygotebfemalesbarebusuallybuna
ffected,bbutbsomebmaybexpressbthebconditionbwithbvariablebseveritybasbdeterminedbbybth
ebpatternbofbXbinactivation.bAbsignificantbproportionbofbisolatedbcasesbarebduebtobnewbmu
tationb(Duchennebmuscularbdystrophy-DMD).b-bbbANSWERbb
Pseudoautosomalbinheritance-
bgroupbofbgenesbonbthebinactivebxbchromosomebarebNOTbinactivated.bDiseasesbassocia
tedbwithbthesebgenesbarebinheritedbsimilarbtobautosomalbinheritance.b-bbbANSWERbb
SamebamountbofbX-
linkedbgenebproductsbbetweenbmalesbandbfemalesbachievedbthroughbdosagebcompens
ation.bLyonbHypothesisbstatesbthatbthebinactivebXbisbNOTbrandomlybchosenbinbeachbcellb
Ex.bAbstructurallybabnormalbXbisbpreferentiallybinactivated.bInactivationbisbNOTbcomplet
e-
bsomebgenesbcanbescapebinactivationb(ie.bThosebwithbabfunctionalbhomologbonbthebY).bI
nactivationbisbNOTbpermanent-
breversedbinbdevelopmentbofbgermbcellsb(notbpassedbonbtobgametes).b-bbbANSWERbb
ThebkeybplayerbisbthebX-
linkedbgenebXIST→bXb(inactive)bspecificbtranscript.bXISTbisbtranscribedbtobproducebabno
n-codingbRNAbthatb"coats"bthebX-
chromosomebandbinactivatesbit.bXISTbisbonlybexpressedbfrombthebinactivebX.bThebhiston
esbonbthebcoatedbXbundergobmethylationbwhichbcausesbthebchromosomebtobcondenseb(
heterochromatin),bformingbabBarrbbody.b-bbbANSWERbb
-
SomebgenesbdobnotbhavebYbhomologuebandbdobnotbundergobinactivationb(e.g.bsteroidbs
ulfatasebgene)b-bbbANSWERbb
-Random/skewedbinactivationbmaybresultbinbaffected/totallybhealthybheterozygotes.b-
bbbANSWERbb
VariablebexpressionbofbX-
inactivation:bOnbbothbextremes,babheterozygousbfemalebwithbrecessivebx-
linkedbdiseasebcouldbmanifestbthebdisease.bInbabcasebwithbabdominantbX-
linkedbtrait,binbwhichbalmostballbofbabfemalesbXbchromosomesbwithbmutationbisbinactive,b
mightbnotbmanifestbthisbdisease.bIdenticalbtwinsbcouldbevenbhavebdiffbphenotypesbduebt
obskewedbXbinactivation.b-bbbANSWERbb
, Mosaicism-
bXbchromosomebinactivationboccursbrandomlybandbinactivationbpatternbisbpassedbtobcellb
progeny.bResult:bfunctionalbmosaicismbinbwhichbfemalebisbabmosaicbwithbrespectbtobexp
ressionbofbgenesbonbXbchrom.bEx.bCalicobCat,bBb-bdominantborange,bbb-
brecessivebgenebblack,bGenesbforbwhite:bautosomal.b-bbbANSWERbb
GeneticbHeterogeneitybcanbbebthebresultbofbLocusbHeterogeneityb,bAllelicbHeterogeneit
y,bandbModifierbLoci.b-bbbANSWERbb
LocusbHeterogeneity-
babsinglebdisorder,btrait,borbpatternbofbtraitsbcausedbbybmutationsbinbgenesbatbdifferentbch
romosomalbloci.bEx.bretinitisbpigmentosabhasbautosomalbdominant,bautosomalbrecessiv
e,bandbX-
linkedborigins.bHowever,bonlybonebmutantblocusbisbneededbforbthebphenotypebtobmanifes
tb-bbbANSWERbb
Allelicbheterogeneityb-
bManybgeneticblocibpossessbmorebthanbonebmutantballele.bInbthebCFTRbgene,bnearlyb14
00bmutationsbfound.bSomebmutationsbcausebclassicalbCFb+bpancreaticbinsufficiencyb+bc
ongenitalbabsencebofbvasbdeferens,bothersbcauseblungbdiseasebwithbnormalbpancreaticb
fxn.bOtherbcausebonlybmalebsterility.b-bbbANSWERbb
Modifierbgene:bAbgenebthatbaffectsbthebphenotypicbexpressionbofbanotherbgene.bSpecifi
cballelesbofboneborbmorebgenesb(modifierbgenes)bcanbsometimesbdramaticallybmodifybth
ebclinicalbseveritybofbthebphenotypebproducedbbybmutationsbinbabdisease-
causingbgene.Thisbsourcebofbclinicalbheterogeneitybisboftenbreferredbtobasbgeneticbback
ground.bEx.bTwinsbwithbthebsamebmutationbinbcysticbfibrosisbbutbonebisbseverelybsickban
dbonebonlybmoderatelybsick-bexplainedbbybmodifierbgenes.b-bbbANSWERbb
CandidateblocibactingbasbmodifiersbforbCFTRbmutations:TGFB1b(cytokinebtransformingb
growthbfactorbbbandbMLB2b(Mannose-
bindingblectin).bDifferentballelesbofbthesebgenesbmodifybthebseveritybofbCFb-bbbANSWERbb
Clinical/phenotypicbheterogeneity:bmultiplebphenotypesbofbonebsinglebgenebex.bUSH2A
bgene,bcodesbforbproteinbimportantbinbdevelopmentbofbthebinnerbearbandbretina.bCanbbeb
blindbandbdeafb(typebIIbUsherbsyndrome)borbjustbblindb(RetinitisbPigmentosa).b-
bbbANSWERbb
Otherbfactorsbaffectionbphenotypebincludebenvironmentalbfactorsbie.bG6PDbmutationbO
RbsoilbpHbandbhydrangeabcolor.bAlsobSex-
relatedbfactorsbiebHemochromatosis,bAR,bcausedbbybironboverload,bisblessbcommonbinbf
emalesbduebtoblowerbironbintake/menstrualbironbloss.b-bbbANSWERbb
G6PDb(glucose-6-phosphatebdehydrogenase)bDeficiencyb(X-linked)-
bCanbleadbtobhemolyticbanemia,bmanybpplbwithbthisbdisorderbneverbexperiencebanybsym
ptoms.bHemolyticbanemiabcanbbebtriggeredbbybenvironmentalbfactorsbsuchbasbbacterial/
viralbinfections,bdrugb(sulfa,bmalariabmedications),beatingbfavabbeansborbinhalingbfavabb
eanbplantbpollen.b-bbbANSWERbb
