biol 3301 - genetics ch2b - dihybrid cross st

8/18/2019 BIOL 3301 - Genetics Ch2B - Dihybrid Cross St

1/21

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


2/21


3/21

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


4/21

#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


5/21

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


6/21

ranching Diagram


7/21

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


10/21


11/21

#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'


12/21

• /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


13/21

• ;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


14/21

• 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


15/21

>aws of Chance

• ;hat is the chance that the co,ple first

child will be a boy or a girl

)


16/21

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


17/21

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:

@ (


18/21

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


20/21


21/21

biol 3301 - genetics ch2b - dihybrid cross st

Documents