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TheChromosomalBasisofInheritanceBIOL100
Ch.8
Overview:Loca8ngGenesAlongChromosomes• Mendel’s“hereditaryfactors”weregenes
• Notknownatthe>me
• Todaywecanshowthatgenesarelocatedonchromosomes
• Loca>onofapar>culargene
• Canbeseenbytaggingisolatedchromosomeswithafluorescentdyethathighlightsthegene
Fig.15-2
P Generation Yellow-round seeds (YYRR)
Y
F1 Generation
Y
R R
R Y
× r r
r
y
y
y
Meiosis
Fertilization Gametes
Green-wrinkled seeds ( yyrr)
All F1 plants produce yellow-round seeds (YyRr)
R R
Y Y r r
y y
Meiosis
R R
Y Y
r r
y y
Metaphase I
Y Y
R R r r
y yAnaphase I
r r
y Y
Metaphase II R
Y
R
y
yyy
R R
Y Y r r r r
yY Y R R
yRYryrYR1/4 1/4 1/4 1/4
F2 Generation
Gametes
An F1 × F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.
LAW OF SEGREGATION The two alleles for each gene separate during gamete formation.
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2
3 3
2
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• Inmammals
• twokindsofsexchromosomes-largerX,smallerYchromosome
• RegionsonendsofYchromosomearehomologouswiththeXchromosome
• TheSRYgeneontheYchromosomecodesforthedevelopmentoftestes
• Someotheranimalshavedifferentmethodsofsexdetermina>on
• Birds,fish,somecrustaceans,someinsects
• Female:ZW;MaleZZ
Sex-linkedinheritance
InheritanceofSex-LinkedGenes• Sexchromosomes
• Havegenesformanycharactersunrelatedtosex
• Sex-linkedgene
• Agenelocatedoneithersexchromosome
• Inhumans,sex-linkedusuallyreferstoageneonthelargerXchromosome
• CalledX-linked
• Sex-linkedgenesfollowspecificpaVernsofinheritance
• Forarecessivesex-linkedtraittobeexpressed
• Afemaleneedstwocopiesoftheallele
• Amaleneedsonlyonecopyoftheallele
• Sex-linkedrecessivedisorders
• Aremuchmorecommoninmalesthaninfemales
Sex-linkedinheritance
(a) (b) (c)
XNXN XnY XNXn × × XNY XNXn × XnY
Y Xn Sperm Y XN Sperm Y Xn Sperm
XNXn Eggs XN
XN XNXn
XNY
XNY
Eggs XN
Xn
XNXN
XnXN
XNY
XnY
Eggs XN
Xn
XNXn
XnXn
XNY
XnY
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• CommonX-linkedcondi>ons-Duetorecessivealleles
• Colorblindness
• Red/greenmostcommon
• Mayinclude19chromosomesand56genes
• Duchennemusculardystrophy
• MutantDystrophingeneXp21
• Hemophilia-Failedbloodclo]ng
• Threetypes
• TwofoundonXchromosome
Sex-linkedinheritance
XInac8va8oninFemaleMammals• Mammalianfemales
• OneofthetwoXchromosomesineachcellisrandomlyinac>vatedduringembryonicdevelopment
• Barrbody
• Thecondensedinac>veX
• Ifafemaleisheterozygousforapar>culargenelocatedontheXchromosome
• Shewillbeamosaicforthatcharacter
X chromosomes
Early embryo:
Allele for orange fur
Allele for black fur
Cell division and X chromosome inactivation Two cell
populations in adult cat:
Active X Active X
Inactive X
Black fur Orange fur
• Eachchromosome
• hashundredsorthousandsofgenes
• linkedgenes
• Geneslocatedonthesamechromosomethattendtobeinheritedtogetherarecalled
LinkedGenes
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HowLinkageAffectsInheritance• ThomasHuntMorgandidexperimentswithfruitflies
• Toseehowlinkageaffectsinheritanceoftwocharacters
• Crossedflies
• Thatdifferedintraitsofbodycolorandwingsize
b+ vg+
Parents in testcross
Most offspring
b+ vg+
b vg
b vg
b vg
b vg
b vg
b vg
×
or
Fig.15-9-4
EXPERIMENT P Generation (homozygous)
RESULTS
Wild type (gray body, normal wings)
Double mutant (black body, vestigial wings) ×
b b vg vg
b b vg vg
Double mutant TESTCROSS
×
b+ b+ vg+ vg+
F1 dihybrid (wild type)
b+ b vg+ vg
Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+
Black- normal
Gray- vestigial
Black- vestigial
Wild type (gray-normal)
b vg
Sperm
b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg PREDICTED RATIOS
If genes are located on different chromosomes:
If genes are located on the same chromosome and parental alleles are always inherited together:
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1
1
1
1 1
0 0
965 944 206 185
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:
:
:
:
:
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• Morganfoundthat
• Bodycolorandwingsize
• Areusuallyinheritedtogetherinspecificcombina>ons
• FailedLawofSegrega>on
• Reasonedthattheywereonthesamechromosome
• Thereforelinked
• Theclosertogetheronchromosome
• Morelikelytobeinheritedtogether
Linkedinheritance
RESULTS
Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+
Black- normal
Gray- vestigial
Black- vestigial
Wild type (gray-normal)
b vg
Sperm
b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg
PREDICTED RATIOS If genes are located on different chromosomes:
If genes are located on the same chromosome and parental alleles are always inherited together:
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1
1
1
1 1
0 0
965 944 206 185
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• Afewnon-parentalphenotypeswerealsoproduced
• Gene8crecombina8on
• Theproduc>onofoffspringwithcombina>onsoftraits
• Differingfromeitherparent
• Ienotgray/normalorblack/ves>gial
• Butgray/ves>gialorblack/normal
Linkedinheritance
RESULTS
Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+
Black- normal
Gray- vestigial
Black- vestigial
Wild type (gray-normal)
b vg
Sperm
b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg
PREDICTED RATIOS If genes are located on different chromosomes:
If genes are located on the same chromosome and parental alleles are always inherited together:
1
1
1
1
1 1
0 0
965 944 206 185
:
:
:
:
:
:
:
:
:
IndependentAssortmentofChromosomes• Mendel-Combina>onsoftraitsinsomeoffspringdifferfromeitherparent
• Parentaltypes
• Offspringwithaphenotypematchingoneoftheparentalphenotypes
• Recombinants
• Offspringwithnonparentalphenotypes
• newcombina>onsoftraits
• A50%frequencyofrecombina>onobservedforanytwogenesondiffchromosomes
• Dueen>relytoindependentassortmentinmetaphase1
YyRr
Gametes from green- wrinkled homozygous recessive parent ( yyrr)
Gametes from yellow-round heterozygous parent (YyRr)
Parental- type
offspring
Recombinant offspring
yr
yyrr Yyrr yyRr
YR yr Yr yR
Recombina8onofLinkedGenes:CrossingOver• Morgandiscoveredthatgenescanbelinked
• Butthelinkagewasincomplete
• Evidentfromrecombinantphenotypes
• Proposedthatsomeprocessmustsome>mesbreakthephysicalconnec>onbetweengenesonthesamechromosome
• crossingoverofhomologouschromosomeswasthatmechanism
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MappingtheDistanceBetweenGenes• AlfredSturtevant
• StudentofMorgan
• Constructedagene8cmap
• anorderedlistofthegene>cloci
alongapar>cularchromosome
• Predictedthat
• Thefartheraparttwogenesare
• thehighertheprobabilitythatacrossoverwilloccurbetweenthem
• andthereforethehighertherecombina;onfrequency
Recombination frequencies
Chromosome 9% 9.5%
17%
b cn vg
• Linkagemap
• gene>cmapofachromosomebasedonrecombina>onfrequencies
• Mapunits
• Expressionofdistancesbetweengenes
• onemapunit,orcen>morgan,representsa1%recombina>onfrequency
• Mapunitsindicaterela>vedistanceandorder,notpreciseloca>onsofgenes
LinkageMap
Meiosis I
Nondisjunction
(a) Nondisjunction of homologous chromosomes in meiosis I
(b) Nondisjunction of sister chromatids in meiosis II
Meiosis II
Nondisjunction
Gametes
Number of chromosomes
n + 1 n + 1 n + 1 n – 1 n – 1 n – 1 n n
Altera8onsofchromosomenumberorstructure• Large-scalechromosomalaltera>ons
• Odenleadtospontaneousabor>ons(miscarriages)
• Orcauseavarietyofdevelopmentaldisorders
• Nondisjunc8on
• Pairsofhomologouschromosomesorsisterchroma;dsdonotseparatenormallyduringmeiosis
• Asaresultonegametereceivestwoofthesametypeofchromosome
• Andanothergametereceivesnocopy
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• Aneuploidy
• Resultsfromthefer>liza>onofgametesinwhichnondisjunc>onoccurred
• Offspringwiththiscondi>onhaveanabnormalnumberofapar>cularchromosome
AbnormalChromosomeNumber
• Monosomiczygote
• hasonlyonecopyofapar>cularchromosome
• Trisomiczygote
• hasthreecopiesofapar>cularchromosome
AbnormalChromosomeNumber
• Polyploidy
• condi>oninwhichanorganismhasmorethantwocompletesetsofchromosomes
• Triploidy(3n)isthreesetsofchromosomes
• Tetraploidy(4n)isfoursetsofchromosomes
• commoninplants
• butnotanimals
• Polyploidsaremorenormalinappearancethananeuploids
ChromosomeNumber
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Altera8onsofChromosomeStructure• Breakageofachromosomecanleadtofourtypesofchangesinchromosome
structure:
• Dele8onremovesachromosomalsegment
• Duplica8onrepeatsasegment
• Inversionreversesasegmentwithinachromosome
• Transloca8onmovesasegmentfromonechromosometoanother
Deletion A B C D E F G H A B C E F G H (a)
(b)
(c)
(d)
Duplication
Inversion
Reciprocal translocation
A B C D E F G H
A B C D E F G H
A B C D E F G H
A B C B C D E F G H
A D C B E F G H
M N O C D E F G H
M N O P Q R A B P Q R
AneuploidyDownSyndrome(Trisomy21)• Downsyndrome
• Aneuploidcondi>onthatresultsfromthreecopiesofchromosome21
• Affectsaboutoneoutofevery700children
• IntheUnitedStates
• ThefrequencyofDownsyndromeincreaseswiththeageofthemother
• Acorrela>onthathasnotbeenexplained
AneuploidyofSexChromosomes• Nondisjunc>onofsexchromosomes
• Producesavarietyofaneuploidcondi>ons
• Klinefeltersyndrome
• XXYmale
• Sterile,smalltestes,somefemalesecondarysexualcharacteris>cs
• MonosomyX,Turnersyndrome
• X0female
• sterile
• itistheonlyknownviablemonosomyinhumans
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StructurallyAlteredChromosomes• criduchat(“cryofthecat”)
• specificdele>oninchromosome5
• mentalretarda>on
• catlikecry
• usuallydieininfancyorearlychildhood
• Transloca>onsofchromosomes
• Certaincancers,includingchronic
myelogenousleukemia(CML)
Normal chromosome 9
Normal chromosome 22
Reciprocal translocation Translocated chromosome 9
Translocated chromosome 22 (Philadelphia chromosome)
InheritanceofOrganelleGenes• Extranucleargenes(orcytoplasmicgenes)
• genesfoundinorganellesinthecytoplasm
• Mitochondria,chloroplasts,andotherplantplas>dscarrysmallcircularDNAmolecules
• Extranucleargenes
• inheritedmaternally
• zygote’scytoplasmcomesfromtheegg
• Thefirstevidenceofextranucleargenes
• camefromstudiesontheinheritanceofyelloworwhitepatchesonleaves
• Variega>on
• Somedefectsinmitochondrialgenes
• PreventcellsfrommakingenoughATP
• Andresultindiseasesthataffectthemuscularandnervoussystems
• Forexample,mitochondrialmyopathyandLeber’shereditaryop>cneuropathy
• Failureofoxida>vephosphoryla>on
InheritanceofOrganelleGenes
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Youshouldnowbeableto:
1. Explainthechromosomaltheoryofinheritanceanditsdiscovery
2. Explainwhysex-linkeddiseasesaremorecommoninhumanmalesthanfemales
3. Dis>nguishbetweensex-linkedgenesandlinkedgenes
4. Explainhowmeiosisaccountsforrecombinantphenotypes
6. Explainhownondisjunc>oncanleadtoaneuploidy
7. Definetrisomy,triploidy,andpolyploidy
8. Dis>nguishamongdele>ons,duplica>ons,inversions,andtransloca>ons
9. ExplainwhyextranucleargenesarenotinheritedinaMendelianfashion