Post-Mendelian Genetics

This presentation offers an interaction about Post-Mendelian genetics overview with special refrence4 to Mono-hybrid interaction.

1. GENETICS
POST-MENDELIAN GENETICS AND
FACTOR HYPOTHESIS
PRESENTED BY
N. Sannigrahi, Associate Professor
Department of Botany, Nistarini College,
D. B. Road, Purulia (W.B) 723101 , India

2. INTRODUCTION
Mendelwasabletoreachtotheconclusionofthelawsof
inheritancepattern&itstransmissionlikelawofsegregation
andlawofindependentassortmentduetohismathematical
interpretationalongwiththevirtueoftheexperimental
material,Peaplantforitsuniquegeneticnature.Theseven
contrastingcharacterschosenbyhimwaslocatedinseven
differentnon-homologouschromosomethathelpedhimto
drawtheconclusion.ButafterMendel’discovery,theother
successorsinthefieldofgeneticstriedtoexplorebutgot
differentresultwhichwasnotatpertheMendelianfindings
irrespectiveoftheMonohybridordihybridexperiments
conductedontheotherexperimentalplantsmaterials.These
findingsofsuccessorsgeneticistspublishedaspost-
MendelianhypothesisorFactorhypothesis.

3. INCOMPLETEDOMINANCE
Mendelhadshownthateachcharacterwasgovernedbya
singlegeneandonlyonegenewillexpressirrespectiveits
homozygousorheterozygousinnature.InpresenceofTt,pea
plantwillbetall.Butdeviationswerediscoveredsoonafterthe
experimentsconductedonanotherplant,four-o’clock,
Mirabilisjalapa.Whenapureredcoloredflowerplant
(dominant)wascrossedwithwhite(dwarf)one,itbecome
pink,neitherrednorwhite,theintermediatebetweenthetwo
coloranditwasduetoexpressionoftheboththefactorsor
alleles.ItwastreatedasIncompletedominance.Theallelesof
onegenecaninteractamongthemselvesatthefunctionallevel
resultinginvariationsintheexpressionofdominanceand
markedphenotypesoccurindifferentalleliccombinations.

5. MULTIPLE ALLELES & CODOMINANCE
Generally,agenehastwoalternativeformscalledalleles.
Usually,oneofthetwoallelesofagenedominantoverthe
other.Thetwoallelesofagenedeterminethetwocontrasting
formsofasinglecharactere.g.,roundandwrinkledseedshapes
inPea.Butinmanycases,severalallelesofasinglegene
determineareknowneachgoverningadistinctformofthe
concernedtrait.Suchasituationisknownasmultipleallelism,
andthemanyallelesofasinglegenearecalledmultiplealleles.
Inthecaseofmultiplealleles,onlythemonohybridratiois
observedintheF
2
generation.Manygenesbothprokaryotesand
eukaryotes(bothplants&animals)showmultiplealleles.Some
examplesofsuchgenesareHumanABO,Rh,Otherblood
groups,humanhemoglobin,selfincompatibilityinplants,
notchedwing,whiteeye,colorgenesofdrosophila,waxygene
ofmaize,.

6. MULTIPLE ALLELES & CODOMINANCE
Bloodgroupsofhumangenerallydeterminedbytheantigenic
propertiesremainintheerythrocytesmoleculesofRBC.An
antigenisamoleculewhichspecificallyinteractswithan
antibody,akindofproteinformsinpresenceofantigen.Most
antigensareimmunogenic.Animmunogenicisamolecule
whichwhenintroducedintothesystemofanimalinducesthe
productionofaclassofproteinscalledantibodies.Antigenand
antibodyareverymuchspecifictoeachother.Whenmolecules
ofanantigencomeincontactwiththemoleculesofantibody
specifictothisantigen,severalantigenandantibodymolecules
bindtogetherandundergoesdifferentinteraction-agglutination,
precipitation,adherence,lyses,neutralizationetc.This
interactionishighlyspecificandformsaverysensitiveand
potenttoolinbiologicalstudy.Suchprocessiscalled
serologicaltests.

7. ABO ANTIGEN & BLOOD GROUPS
Basedupontheabilityofantigenstocarryouttheirfunctions,
antigensareoftwotypes:completeantigensandincomplete
antigens(haptens).Acompleteantigenisabletoinduceantibody
formationandproduceaspecificandobservablereactionwith
theantibodysoproduced.Haptens(Gr.haptentograsp;partial
antigens)aresubstanceswhichareincapableofinducing
antibodyformationbythemselves,butcanbecapableofinducing
antibodiesoncombiningwithlargermolecules(normally
proteins)whichserveascarriers.Antigenswhicharepresenton
thebody’sowncellsarecalledtheauto-antigensorselfantigens.
Theantigensonthenon-selfcellsareknownasforeignantigens
ornon-selfantigens.RedbloodcorpusclesofallABOblood
groupspossessacommonantigen,theHantigen,whichisa
precursorfortheformationofAandВantigens.Duetouniversal
distribution,Hantigenisnotordinarilyimportantingroupingor
bloodtransfusion.

8. CODOMINANCE
Mendeldidnotanticipatetheexpressionofthetwofactors
simultaneously.Butincertaincases,boththeallelesofagene
expressthemselvesintheheterozygote.Thisiscalledco
dominance.Asaconsequence,heterozygoteforsuchgenes
possessthephenotypesproducedbyboththeconcernedalleles.
Bloodgroupantigensofmanpresentexcellentexamplesofco
dominance.Oneofthemostwidelyknown,andtheearliest
recognized,humanbloodgroupistheABObloodgroup.These
bloodgroupariseduetothepresenceofantigenontheinner
surfaceofRBC.Theseantigensareproducedbyagenei.One
dominantalleleofthisgeneviz.I
A
alleleproducestheantigen
A,turnsbloodgroupA,anotherdominantalleleisI
B
that
producestheantigenBandthebloodgroupisB.IfboththeI
a
&
I
b
present,thebloodgroupbecomesABandtheabsenceofboth
makesthebloodgroupO.

9. CODOMINANCE
MarriagesbetweenheterozygousI
A
I
B
,havingbloodgroup
AB,producesthreetypesofprogeny.Onefourthoftheir
progenyarehomozygousforthealleleI
a
(I
a
I
A
)andhaveA
typeofbloodgroup,whileanotheronefourthhaveB
bloodgroupsincetheyarehomozygousfortheIB
allele(I
B
I
B
).Theremainingonehalfoftheprogenyhave
ABbloodgroupastheyhaveheterozygous(I
A
I
B
)and
possessboththeantigens,A&B.Theco-dominancealso
produces1:2:1ratioinF
2generation.Sicklecellanemiais
akindofhumandiseaseanditisalsotheoutcomeofthe
co-dominanceoftwodifferentallelesinthisregard.The
ladybirdbeetle,Harmoniaaxyridis,differinthe
pigmentationpatternfoundintheelytraorwingcover.
Somehaveblackdotsonayellowbackgroundwhereasthe
othershavebroadblackbandsonayellowblackcover.It
isalsoduetoco-dominance.

11. LETHAL FACTORS
Alethalfactorcausedeathofalltheindividualscarryingthis
geneintheappropriategenotypebeforetheyreachadulthood.
Itmaybeeitherdominantorrecessivelethal.Ithasbeen
observedthatallgenesorgeneticfactorsarenotusefultothe
organism.Therearesomegeneticfactorsorgenes,when
presentinanyorganismcauseitsdeathduringearlystageof
development.Theymayevencausedeathoftheindividual
eitherinhomozygousdominantorhomozygousrecessive
condition.E.Baur(1907)observedlethalgenein
Snapdragon(Antirrhinum)andfoundthatitischaracterized
byvariegatedleaves.The“golden”varietyonselfinggives
riseto2typesofoffspring,goldenandgreenintheratioof2:1
insteadof3:1.Thegoldenonesareheterozygousandthe
greenonesbreedtruebeingrecessivehomozygous.

12. LETHAL FACTOR
Inanimalworld,suchtypeofinteractionisalsoobserved.A
FrenchgeneticistL.Cuenot(1905)reportedonthe
inheritanceofmousebodycolor.Hefoundthat“yellow”
bodycolorwasdominantovernormal“brown”colorand
wasgovernedbysinglegene“Y”.Itwasobservedthat
yellowmicecouldneverbeobtainedinhomozygous
condition.Whenyellowcoatedmicewascrossedwith
anotheryellowcoatedmice,segregationforyellowand
brownbodycolorwasobtainedin2:1ratio.Thebrown
individualswerepureandhomozygouswhereasyellow
individualswereheterozygous.Theseresultsmaybe
explainedontheassumptionthatthedominantallelefor
yellowbodycolorislethalinhomozygouscondition.Lethal
genesmaybeof5types-Recessive,dominant,Conditional,
Balanced&Gameticlethal.

13. LETHAL FACTOR
Onthebasisofsurvivality,thegenesmaybeof5types-Vital
genes,Lethalgenes,Sub-lethalgenes,Sub-vitalgenes&super-
vitalgenes.
Vitalgenes-Thegeneswhichdonotaffectthesurvivalofthe
individualsinwhichtheyarepresentaresaidasvitalgenes.It
doesnotmeanthatthesegenesarenecessaryforthesurvivalof
theconcernedindividual.Wildtypeallelesofallthegenesofan
organismaresaidasvitalgenes.Inotherwords,thesurvivalof
theorganismisnotinfluencedbythevitalgenes,whethermay
bepresentinhomoorheterozygouscondition.
Sub-lethalgenes-Suchgenesdonotleadtheorganismtothe
deaththatcarrytheminappropriategenotype.90%ofthe
individualsdie,however,onlylessthan10%oftheindividuals
survive.SomeXanthamutantsofseveralplantsaresub-lethalor
semi-lethalinthehomozygousstate.

14. LETHAL GENES
Mostofthemutantgenesincreasethesurvivalofsuch
individualswhichcarrytheminappropriategenotypeas
comparedtothatofwildtypeallele.Suchgenesarecalledas
super-vitalgenes.Supervitalgenesprotecttheindividuals
carryingthemagainstthevariousdiseasethusincreasingthe
chanceoftheirsurvival.Likewise,thegenesproviding
resistanceortolerancetodifferentenvironmentalpressureor
strainlikehighandlowtemperature,lowandhighlight
intensity,drought,salinity,alkanityetc.Mostofthemutant
genesreducetheviabilityofindividualshavingthemin
appropriategenotypeascomparedtothatofnormal
individuals.Mostofthemutantgenesaresub-vitalintheir
effectandkilllessthan90%oftheindividualswhichcarry
them.Theexamplesaresomevirdismutantsofbarley,
miniaturewingsinDrosophila.

15. PLEIOTROPY
AsperMendel’srule,onegeneaffectsasinglecharacter.But
genesareknowntoaffectmorethanonecharacter;Suchgenes
areknownaspleiotropicgenesandtheconditionisknownas
Pleiotropy.Themostcommonexampleisfoundinhuman
beings.GenesorHb
s
whichproducessicklecellanemiainthe
sshomozygouscondition.Morethan50%ofindividualsfor
thisgene(ss)diebeforetheageof20years.Theprimaryeffect
ofthisgeneisthesubstitutionofavalinemoleculefora
glutamicacidmoleculeattheposition6oftheβ-chainof
hemoglobin.Themutant(Sicklecell)hemoglobinremainsin
solutionaslongasthesolutionconc.ofoxygenisveryhigh.
However,atlowoxygenconcentration,filamentousaggregates
ofthesicklecellhemoglobinprecipitatecausingtheredblood
cellassumethecharacteristicsickleshaped.

16. PllEIOTROPY
Inadditiontothemorphologicalchangeofshape,theperson
sufferfromthisgeneticissuesuffermultipleproblemslike
dilationofheartandheartfailure,poorphysicaldevelopment,
impairedmentalfunction,pneumoniaduetolungdamage,
rheumatism,kidneydamageanditsdefectivedamage.Different
othersyndromeslikeHunter’ssyndrome,Huntington’schorea,
cysticfibrosisetcalsohappenandallthesehealthissuesarevery
seriousconcerninthisfield.Genesshowingpleiotropyproduce
asinglepolypeptidejustlikeothernonpleiotropicgenes.But
theirpolypeptidesgovernsuchabiochemicalreaction,whichis
basictomanybiochemicalevents.Asaresult,theimpairmentof
thisfunctioninterfereswithanumberofdevelopmentalevents,
whichinturnleadstothepleiotropicexpressionofthegene.

18. EXPRESSIVITY & PENETRANCE
Mostofthegenesproduceidenticalphenotypesinallthe
individualsthatarepresentintheappropriategenotype.For
example,alltheseedshavingwallele,governingseedshape
inpea,inthehomozygousstate(ww)haveuniformlywrinkle
shape,similarlythoseseedshaveeitherWWorWw
genotypewillbeuniformlyround.Theabilityofageneto
producethesamephenotypeinallindividualscarryingitin
theappropriategenotypeisknownasuniformexpressivity.As
opposedtothis,.anygeneshavevariableexpressivityinthat
theyproducevariablephenotypesintheindividualsthathave
themintheappropriategenotype.Anexampleofvariable
e3xpressivityisfurnishedbytherecessivegeneproducing
partialdeficiencyinthecotyledonaryleavesoflimabeans.
Cotyledonaryleavesofseedlingshomozygousforthisgene
showvariabledegreesofchlorophylldeficiency-eitheritis
absentintheentireleavesorvariablepart.Asinglegenewith

19. EXPRESSIVITY
Withvariableexpressivityproducesanumberofphenotypes
oftenproducingaquantitativevariationintheconcerned
characters.Variableexpressivitymaybeduetothefollowing
reasons:Modifyingeffectofgeneticbackgroundormodifying
genes,andtheeffectofvariousenvironmentalfactorsongene
expression.Inmostcases,boththesefactorsarelikelytobe
involved.Ingeneral,onlymutantallelesshowvariable
expressivity,whilethewildtype(normal)allelesarebuffered
againstsuchvariations.Manygeneticconditionsareidentified
asasetofcharacteristics,occurringtogether.Agroupof
recognizablecharacteristics,whichoccurduetoacommon
causeiscalledasyndrome.Geneticsyndromesarequite
variableintheirexpressivity.ThoughthemutationofMarfan
syndromeoccursinFBN1,thecharacteristicsofthesyndrome
widely vary among people.

20. PENETRANCE
Penetranceisthepercentageofindividualswithagiven
phenotype,whoexhibitthephenotypeassociatedwiththat
particulargenotype.Inotherwords,itexplainstheextenttowhich
aparticulargeneorsetofgenesisexpressedinthephenotypesof
individualscarryingit,measuredbytheproportionofcarriers
showingthecharacteristicphenotype.Itisaquantitative
measurement,describingtheexpressionvariationsonthelevelof
phenotype.Ingeneral,geneexpressthemselvesinallindividuals
inwhichtheyarepresentintheappropriategenotype;thisis
knownascompletepenetrance.Butgenesdonotproducethe
concernedphenotypesinallindividuals,whichcarrythem
appropriategenotype;suchasituationisknownasincomplete
penetrance.Forexample,therecessivegeneproducingpartial
chlorophylldeficiencyinthecotyledonaryleavesoflimabeans
showsincompletepenetranceaswell;itexpressesitselfonlyof

21. PENETRANCE
10%individualoftheindividuals.Whenageneispresentinthe
appropriategenotype,thepercentindividualsinwhichitisable
toexpressitselfisameasureofpenetrance.Thuschlorophyll
deficiencygeneinlimabeanshasapenetranceof10%.Almost
allgenesshowingincompletepenetranceexhibitvariable
expressivityaswell.Thus,incompletepenetranceis,infact,an
expressionofthevariableexpressivityinthatsomeindividuals
showsuchasmallexpressionofthegenethatthetrait
producedbyitselfhaslimitedchanceofdetectionthrough
phenotypesexpression.Penetranceandexpressivitydescribe
therelationshipbetweenadominancegenotypeandits
associatedphenotype.TheexpressionofBRCA1geneand
BRCA2genesdevelopcancersinsomeindividualsbutnotin
othersasaresultofpenetrance.

22. Penetranceistheproportionofaparticulargenotype,expressing
itsassociatedphenotype.Itisaquantitativemeasurementofthe
amountoftheexpressionofaparticulargene.Expressivity
describesthevariationsingeneexpressionofaparticular
genotype.Itisaqualitativemeasurement,whichcorrelateswith
theextentofgeneexpression.Therefore,themaindifference
betweenpenetranceandexpressivityisintheirparameters.
Thepre-Mendeliangeneticswasmainlybasedontheassumption
ofallelesthatstatestheexpressionofphenotypebythesingle
geneorfactorinteractionanddominantwillalwayssuppressedthe
recessiveone.Noconceptofincompletedominance,lethality,co
dominanceandmultiplealleleswereadvocated.Butlaterresearch
confirmedtheaboveconceptandhasbeenstrengthenbythe
conceptoflinkage,crossingoverandmaskingeffectofgeneby
others.
CONCLUSION

23. THANKS FOR YOUR VISIT
References:
1.Googleforimages,
2.PrinciplesofGenetics-Basu&Hossain,
3.AtextbookofBotany(VolIII)Ghosh,Bhattacharya,Hait
4.FundamentalsofGenetics-B.D.Singh,
5.ATextbookofgenetics-AjoyPaul
DISCLAIMER:
Thispresentationhasbeenmadetoenrichopensourceof
informationwithoutanyfinancialinterest.Thepresenter
acknowledgesGoogleforimagesandotheropensourcesof
knowledgetodevelopthisPPT.