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Gene Interaction-PART 2
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Todays lecture (0122)
Mendels Dihybrid Cross - the 9:3:3:1 ratio
Revisited Concept of genetic interaction between alleles at
two dierent loci!
"he 9:3:3:1 ratio no gene interaction
#pistasis "he 9:$ ratio genes in the sa%e pathway
&'ppressors( %o)diers and &ynthetic lethals!
*enetrance and e+pressivity
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Mendels dihybrid cross
Mendels dihybrid cross:
- two dierent genes ,R,ror Y(y
- each gene had an independent phenotypiceect: seed shape ,ro'nd or wrin.led
and seed color ,yellow or green- i!e! / genes( / phenotypes and genes do not
interact ,Yoryaect only seed color and do notaect seed shape and vice versa for the Rand r
alleles for seed shape
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Dihybrid ross
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Dihybrid ross
R/-0 Y/-: ro'nd( yellow 9R/-0y/y: ro'nd( green 3r/r0 Y/-: wrin.led( yellow 3r/r0y/y: wrin.led( green 1
ll of the 12 genotypes of the /fall
in these 4 genotype categories:
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Dihybrid ross
5n general( for a dihybrid cross between two do'ble
hetero6ygotes ,hetero6ygo's at two genetic loci:
7a 0 87b 7a 0 87b
enotype Categories:7- 0 87- : 97- 0 b7b : 3a7a 0 87- : 3
a7a 0 b7b : 1
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Interactions bet!een alleles o" t!odi#erent $enes enes %ay not always act independently in their
phenotypic e+pression
enes at one loc's can aect genes at another loc'sand prod'cts of genes at dierent loci co%bine toprod'ce a new phenotype ; ene interaction
ene interactions change Mendelian phenotypic
ratios ,do not get 9:3:3:1 ratio
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Interactions bet!een alleles o" t!odi#erent $enes
The %&'&'&1 ratio& no $ene interaction
#g: s.in coloration in Corn sna.es:
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Interactions bet!een alleles o" t!odi#erent $enes
>range-blac. ,ca%o'=age : o+7-0 b+7-
8lac. ,lac.s orange : o 7o; b+7-
>range ,lac.s blac. : o+7-0 b 7b
lbino ,no blac.( no orange: o 7o 0 b 7b
o+7o+; b7b ,orange x o7o ;b+/b,blac.
1: o+/o ; b+/b ,ca%o'=aged
&elf 1 o+/o ; b+/b x o+/o ; b+/b
/: 9 o+7-0 b+7- ,ca%o'=aged
3 o+7-0 b7b ,orange
3o7o; b+7- ,blac.
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Interactions bet!een alleles o" t!odi#erent $enes The %& ratio+ $enes in sa.e ,ath!ay:
#g: 8l'e-white =ower color in harebell plants:
- ?ild-type =ower color is bl'e and %'tant =ower color is white
- 8l'e color is d'e to pig%ent called anthocyanin!
- nthocyanin absorbs light of all wavelengths e+cept bl'e and
re=ects bl'e which is the color of =ower seen!
- nthocyanin %ade fro% prec'rsors byen6y%es
- 5f en6y%e for anthocyanin is defective:
o anthocyanin not f'nctional(
o prec'rsors do not absorb light b't re=ect itbac.
o =ower appears white
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Interactions bet!een alleles o" t!odi#erent $enes "he 9:$ ratio( genes in sa%e pathway:
- 8ioche%ical pathway for synthesi6ing anthocyanin:
gene w1@ gene w2@
prec'rsor 1 en6y%e 1 prec'rsor / en6y%e / anthocyanin,bl'e
- M'tation in either w1@ or w2@;
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Interactions bet!een alleles o" t!odi#erent $enes "he 9:$ ratio( genes in sa%e pathway:
* w17w10 w2@7w2@,white w1@7w1@ 0 w27w2,white
a%etes w1;w2@ w1@0 w2
1 w1@7w10 w2@7w2 ,bl'e
&elf 1 w1@7w10 w2@7w2,bl'e w1@7w10 w2@7w2,bl'e
a%etes w1@0w2+ w1@0w2 w1;w2@ w1;w2 w1@0w2+ w1@0w2 w1;w2@ w1;w2
/ 9712 w1@
7-
0 w2@
7-,bl'e 9 ?ild-type
3712 w1@7-0 w27w2,white &ingle %'tant
3712 w17w10 w2@7-,white $ &ingle %'tant
1712 w17w10 w27w2 ,white Do'ble
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Interactions bet!een alleles o" t!odi#erent $enes
The %&'& ratio& Recessi3e 4,istasis
- abrador dogs can be blac.( brown or yellow!
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Interactions bet!een alleles o" t!odi#erent $enes
"he 9:3:4 ratio: Recessive #pistasis
- Coat color deter%ined by interaction between / gene loci!
- ene loc's 1: type of dar. s.in pig%ent:
allele B: blac. pig%ent( do%inant
allele b:brown pig%ent( recessive
- ene loc's /: deposition of the pig%ent in the shaft of the hair:
allele E:deposition of pig%ent ,blac. or brown(do%inant
allele e:prevents deposition of pig%ent ,yellow(
recessive
- enotype eeat the second loc's %as.s e+pression of the blac.and brown alleles at the )rst loc's! ,i!e! -/-( e/e; yellow
- #pistasis is an eect of gene interaction s'ch that onegene
%as.s ,hides the eect of another gene at a dierent loc's!
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Interactions bet!een alleles o" t!odi#erent $enes
"he 9:3:4 ratio: Recessive #pistasis ,coat color of abrador dogs
Cross a blac. labrador ho%o6ygo's for the do%inant alleles at bothgene loci with a yellow labrador ho%o6ygo's for the recessivealleles at both gene loci
* B/B; E/E,8lac. + b/b; e/e ,Bellow
Gametes B;E b;e1 B/b; E/e ,8lac.
&elf 1 B/b;E/e ,8lac. + B/b;E/e ,8lac.
/ 9712 B7-0E7- ,8lac. 9
3712 b7b0E7- ,8rown 3
3712 B7-0e7e ,yellow
1712 b7b0e7e ,yellow 4 ,-7-0e/e
Phenoty,ic ratio lac*& ro!n& 5ello! %&'&
R4AD A6T P4TA7 PIGM48T 958T:49I9 I8 764-454D
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Interactions bet!een alleles o" t!odi#erent $enes
"he 9:3:4 ratio: Recessive #pistasis
- enotype eeat the second loc's %as.s e+pression of the blac.and brown alleles at the )rst loc's!
- #pistasisis an eect of gene interaction s'ch that onegene%as.s ,hides the eect of another gene at a dierent loc's!
-ene that does the %as.ing ; epistaticgene ,pig%entdeposition
- ene whose eect is %as.ed ; hypostaticgene ,coat colorpig%ent
- Recessiveepistasis: Recessive allele at second loc's ,no pig%ent
deposition overrides pig%ent color phenotype deter%ined by)rst loc's!
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Interactions bet!een alleles o" t!odi#erent $enes
"he 1/:3:1 ratio: Do%inant #pistasis:
#g: *etal coloration in o+gloves
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Interactions bet!een alleles o" t!odi#erent $enes
"he 1/:3:1 ratio: Do%inant #pistasis:
#g: *etal coloration in o+gloves- *etal coloration deter%ined by interaction between / gene loci!
- ene loc's 1: intensity of red pig%ent in petal:
allele d: light red color( recessive ,seen
nat'rallyallele D:dar. red color( do%inant ,%'tant
- ene loc's /: distrib'tion of pig%ent in petals ,light or dar. red:
allele w:pig%ent synthesi6ed thro'gho't petal(
recessiveallele W:white petal with red throat spots(
do%inant
- enotype W/-at the second loc's %as.s distrib'tion of the redpig%ent thro'gho't petal!,i!e! -7-0 W/-; white petals with spots
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Interactions bet!een alleles o" t!odi#erent $enes
"he 1/:3:1 ratio: Do%inant #pistasis ,*etal coloration ino+gloves
Cross =owers that have white petals and red throat spots(ho%o6ygo's for the do%inant alleles at both gene loci with a =owerwith light red petals( ho%o6ygo's for the recessive alleles at bothgene loci
* D/D; W/W,?hite with spots + d/d; w/w ,ightRed
Gametes D;W d;w
1 D/d;W/w ,?hite with spots
D/d;W/w ,?hite with spots + D/d;W/w ,?hite with spots
/ 9712 D7-0W7- ,?hite with spots 1/,-7-0W7W 3712 d7d0W7- ,?hite with spots
3712 D7-0w7w ,Dar* red 3
1712 d7d0w/w ,ight red 1
Phenoty,ic ratio hite !ith s,ots& Dar* red& ight red12&'&1
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Interactions bet!een alleles o" t!odi#erent $enes
&'ppressors-
A .utant allele o" one $ene that re3erses the e#ect o" a.utation at another $ene+ resultin$ in !ild-ty,e,henoty,e
- &'ppressor gene and target gene %'st interact at f'nctionallevel in wild-type state
#g: Drosophiaeye color
ene loc's 1: #ye color:
allelepd: p'rple color( recessive ,%'tant
allelepd+
:red color( do%inant ,wild-type ene loc's /: &'ppressor ,no independent phenotype:
allele s!:s'ppressespd( recessive
allele s!+:
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Interactions bet!een alleles o" t!odi#erent $enes
&'ppressors: ,Drosophiaeye color
Cross a ho%o6ygo's p'rple-eyed =y with a ho%o6ygo's red-eyed
=y carrying the s'ppressor
* pd/pd; s!+/s!+,*'rple + pd+/pd+; s!/s! ,Red
Gametes pd; s!+ pd+; s!
1 pd+/pd; s!+/s! ,Red
pd+/pd; s!+/s! ,Red+ pd+/pd; s!+/s! ,Red
/ 9712pd+7-0s!+7- ,Red
3712pd+7-0s!7s!,Red 13
1712pd7pd0s!7s! ,Red do'ble%'tant
3712pd7pd0 s!+7- ,p'rple 3
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Interactions bet!een alleles o" t!odi#erent $enes &'ppressors: Molec'lar Mechanis%
1! 5f %'tation bloc.s a %etabolic pathway( s'ppressor bypassesthe bloc.
,eg: by rero'ting into inter%ediates si%ilar to those in thepathway
beyond the bloc.
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Interactions bet!een alleles o" t!odi#erent $enes
&'ppressors: Molec'lar %echanis%
/:
%@
7%! s@
7s + %@
7%!s@7s
%@7- ! s@7-: active 9%7%! s@7s: active/ 14%@7% ! s7s: active/
%7% ! s7s: active
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Interactions bet!een alleles o" t!odi#erent $enes Modi)ers:
- 5nstead of one gene %as.ing the phenotypic eect of anothergene( it %ay %odify the e+pression of the second gene!
#g: Coat color in %ice:ene oc's 1: ene for Coat color:
llele B; blac. coatllele b; brown coat
ene oc's /: 5ntensity of coat color:llele D; f'll color ,blac. or brownllele d; dil'ted e+pression of Bor balleles
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Interactions bet!een alleles o" t!odi#erent $enes Modi)ers:
1 B/b; D/d ,8lac.
&elf 1 B/b;D/d ,8lac. + B/b;D/d,8lac.
/ 9712 B7-0 D7- ,8lac.
3712 b7b0 D7- ,8rown
3712 B7-0 d7d ,dil'te blac.
1712 b7b0 d7d ,dil'te brown
Phenoty,ic ratio lac*& ro!n& dilute blac*& dilutebro!n %&'&'&1
llele at loc's for intensity of coat color doesnt %as.( b't%odi)es( the eect on coat color
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Interactions bet!een alleles o" t!odi#erent $enes 9ynthetic 7ethals:
1! Crossing two single %'tants res'lts in do'ble %'tants ,eg: aand bare
%'tant alleles at / dierent gene loci that are lethal:
self the 1: a+7a 0 b+7b a+7a 0 b+7b
/: a+7- 0 b+7- : 9
a+7- 0 b7b: 3
a7a0 b+7- : 3
a7a0 b7b: 1 ethal
Ratio ; 9:3:3
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Interactions bet!een alleles o" t!odi#erent $enes &ynthetic ethals: Molec'lar Mechanis%
1! D'plicate syste%s in cells as a Ebac.'pF! 5f n'll %'tations inboth
syste%s -G
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Interactions bet!een alleles o" t!odi#erent $enes
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Penetrance and 4;,ressi3ity *enetrance ; H of individ'als with a given allele that e+press
the phenotype associated with that allele
5f 1II individ'als have an allele for bl'e eye color and all havebl'e eyes( the allele is 1IIH penetrant into the phenotype
5f not( the allele is inco%pletely penetrant
Reasons:
- #nviron%ent
- 5n='ence of other interacting genes
- 5f it is a %'tant allele( the eects on
phenotype %ay be s'btle
do%inantallele is not
f'lly penetrant
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Penetrance and 4;,ressi3ity #+pressivity: Meas'res the degree7e+tent of phenotypic
e+pression of a given allele ,i!e! intensity of phenotype
#g: piebald spotting in beagles! #ach has allele for piebaldspotting b't there is variation in e+tent of spotting!
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Penetrance and 4;,ressi3ity Penetrance 3s< e;,ressi3ity
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7earnin$ utco.esBo' sho'ld be able to:
#+plain the concept of gene interaction with respect to
interactions between alleles at two dierent gene loci! &olve genetic proble%s based on genetic interaction
between two genetic loci!
#+plain the dierence between Recessive and
Do%inant epistasis! Describe the %olec'lar %echanis%s 'nderlying the
eects of %odi)er and s'ppressor alleles!
#+plain the dierence between penetrance and
e+pressivity!
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7earnin$ utco.esBo' sho'ld be able to:
Describe the %olec'lar %echanis%s 'nderlying the
eects of %odi)er and s'ppressor alleles! #+plain the dierence between penetrance and
e+pressivity!
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