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Dihybrid Cross
• Inheritance of two traits that are coded by
genes located on different chromosomes
The phenotypic ratio in F2 is :
9A__: !A_bb : !aa_ : "aabb
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SY
SY
SSYY S sYY
ssYY
ssy Y
S sy Y
SSY y S sY y
S sY y
ssyy
S syy SS yy
sS y Y sS yy
sSYY sSY y
SS y Y
sY
sY
sy
sy
S y
S y
eggs
self-fertilize
ssY y
14
14
14
14
1
4
14
14
14
s
p e r m
1
16
116
116
116
1
16
116
116
116
1
16
116
116
116
116
116
116
116
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#henotypic ratio
• " $$%%& 2$s%%& 2$$%y& '$s%y ( 9
smooth& yellow
• " $$yy& 2 $syy ) ! smooth& green
• " ss%%& 2 ss%y ) ! wrin*led& yellow
• " ss yy ) " wrin*led& green
• +r: 9$_%_ : !ss%_ : ! $_yy : " ssyy
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Dihybrid cross
• According to the r,les of probability& if pairs of
characters are ________________ in a dihybrid
cross& then the F2 from F"-F" cross will gi.e a9:!:!:" ratio of the fo,r possible phenotypes
• $,ch a ratio is the res,lt of the ______________
of two gene pairs into the gametes and of the
__________________________ of those gametes
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ranching Diagram
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Law of independent
assortment.• The independent assortment of each pair of alleles
d,ring gamete formation is now called /endel0s
law of independent assortment• /endel1s post,late states that traits assort
independently d,ring gamete formation and that
all possible combinations of gametes will form
with e,al fre,ency3
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8/21Figure 3-8 Copyright © 2006 Pearson Prentice Hall, Inc.
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9/21Figure 3-9 Copyright © 2006 Pearson Prentice Hall, Inc.
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#rinciples of #robability
• #robability ) the ratio of the n,mber of
times a partic,lar e.ent is e-pected to occ,r
to the n,mber of trials d,ring which thee.ent co,ld happen
• #robability of pic*ing a heart 4"! total5
from a dec* of 62 cards:
"!762 ( "7'
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• /endel0s laws of segregation and independentassortment reflect the same laws of probabilitythat apply to tossing coins or rolling dice3
• The probability scale ranged from 8ero 4an e.entwith no chance of occ,rring5 to one 4an e.ent thatis certain to occ,r53 ) The probability of tossing heads with a normal coin is
"723
) The probability of rolling a ! with a si-sided die is"7& and the probability of rolling any other n,mber is673
Mendelian Inheritance Reflects the
Rules of Probability
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• ;hen tossing a coin& the o,tcome of one toss
has no impact on the o,tcome of the ne-t toss3
• i*e a coin toss& each o.,m
from a hetero8ygo,s parenthas a "72 chance of carrying
the dominant allele and a
"72 chance of carrying the
recessi.e allele3 ) The same odds apply to
the sperm3
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• The r,le of addition also applies to genetic problems3
• ?nder the r,le of addition& the probability of ane.ent that can occ,r two or more different waysis the s,m of the separate probabilities of thoseways3
) For e-ample& there are two ways that F" gametescan combine to form a hetero8ygote3• The dominant allele co,ld come from the sperm and the
recessi.e from the o.,m 4probability ( "7'53
• +r& the dominant allele co,ld come from the o.,m and
the recessi.e from the sperm 4probability ( "7'53• The probability of a hetero8ygote is "7' @ "7' ( "723
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>aws of Chance
• ;hat is the chance that the co,ple first
child will be a boy or a girl
)
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The #rod,ct B,le
• The probability of two independent e.entsocc,rring sim,ltaneo,sly is the prod,ct of
of each of their indi.id,al probabilities ) The probability that both children in a family
with two *ids are girls is
- (
Ea.ing three boys in a row
- - ( "7
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The $,m B,le
• The probability of occ,rrence of any of the
se.eral m,t,ally e-cl,si.e e.ents is the s,m
of the probabilities of the indi.id,al e.ents ) The probability of ha.ing two boys and two
girls in a family is:
@ (
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Chis,are• An important ,estion to answer in any genetic e-periment
is how can we decide if o,r data fits any of the /endelian
ratios we ha.e disc,ssed3 A statistical test that can test o,t
ratios is the Chi$,are or Goodness of Fit test3
• Chi-Square Formula
H ( 4obser.ed .al,e ) e-pected .al,e527 e-pected.al,e
• e!rees of freedom "df# ( n" where n is the n,mber of
classes obser.ed
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19/21Table 3-3 Copyright © 2006 Pearson Prentice Hall, Inc.
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