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examine cases involving 2 loci (genes) that each have 2 alleles

Crosses performed can be illustrated in general by

 AaBb X AaBb

Where A is dominant to a and B is dominant to b

If these two genes govern two different traits

 A 9:3:3:1 ratio is predicted among the offspring

simple Mendelian dihybrid inheritance pattern

If these two genes do affect the same trait the 9:3:3:1 ratio may

be altered

9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1

epistatic ratios 

Epistatic Gene Interactionshttp://faculty.mwsu.edu/ biology/ jon.scales/ Courses/ Genetics

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 A Cross Producing a 9:7 ratio

Figure 4.18

9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp

purple white

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Epistatic Gene Interaction

Complementary gene action

Enzyme C and enzyme P cooperate tomake a product, therefore theycomplement one another

Enzyme C Enzyme P

Purple

pigmentColorless

intermediate

Colorless

precursor

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Epistasis describes the situation in which a gene masks thephenotypic effects of another gene

Epistatic interactions arise because the two genes encodeproteins that participate in sequence in a biochemicalpathway

If either loci is homozygous for a null mutation, none of that

enzyme will be made and the pathway is blocked

Colorless

precursor

Colorless

intermediate

Purple

pigment

Enzyme C Enzyme P

Epistatic Gene Interaction

genotype cc

genotype pp

Colorless

precursor

Colorless

intermediate

Purple

pigment

Enzyme C

Enzyme P

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Inheritance of the Cream-Eye allele in

Drosophila

a rare fly with cream-colored eyes identified in a

true-breeding culture of flies with eosin eyes

possible explanations 1. Mutation of the eosin allele into a cream allele

2. Mutation of a 2nd gene that modifies expression of the

eosin allele

Epistasis of Involving Sex-linked Genes

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The Hypothesis

Cream-colored eyes in fruit flies are due to the

effect of a second gene that modifies the

expression of the eosin allele

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Figure 4.19

Testing the Hypothesis

cream allele is recessiveto +

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Interpreting the Data 

Cross Outcome

P cross:

Cream-eyed male X

wild-type female

F1: all red eyes

F1 cross:

F1 brother X F1 sister F2: 104 females with red eyes47 males with red eyes

44 males with eosin eyes

14 males with cream eyes

F2 generation contains males with eosin eyes

This indicates that the cream allele is

not in the same gene as the eosin allele

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Interpreting the Data 

Cross Outcome

P cross:

Cream-eyed male X

wild-type female

F1: all red eyes

F1 cross:

F1 brother X F1 sister F2: 104 females with red eyes47 males with red eyes

44 males with eosin eyes

14 males with cream eyes

F2 generation contains  – 

151 + eye: 44 we eye: 14 ca eye

a 12 : 3 : 1 ratio

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Modeling the Data 

Cream phenotype is recessive therefore the

cream allele is recessive allele (either sex-linked or autosomal)

The mutated allele of the cream gene modifies

the we allele, while the wt cream allele doesnot 

C  = Normal allele

Does not modify the eosin phenotype c a = Cream allele

Modifies the eosin color to cream, does not effect wt or

white allele of white gene.

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Male gametes

C  Y

CC Xw+ Xw+ CC Xw+  Y  c a c a Xw+ Xw+ Cc a Xw+  Y C Xw+

C Xw+ c a Xw+ c a  Y 

C Xw-e

c a Xw+

c a Xw-e

CC Xw+ Xw-e CC Xw-e  Y  Cc a Xw+ Xw-e Cc a Xw-e  Y 

Cc a Xw+ Xw+ Cc a Xw+  Y  c a c a Xw+ Xw+ c a c a Xw+  Y 

Cc a Xw+ Xw-e Cc a Xw-e  Y c a c a Xw+ Xw-e c a c a Xw-e  Y 

   F  e  m  a

   l  e  g  a  m  e   t  e  s

Putative genotypes in a crossP w+ / w+; C/C x we /Y; ca /ca

F1 w+ / we; C/ca  & w+ /Y; C/ca

F2 ¾ C/_ x ¾ w+ /_

¼ we /Y

¼ ca /ca x ¾ w+ /_

¼ we /Y

9/16 C/_ ; +3/16 ca /ca; +3/16 C/_ ; we 1/16 ca /ca; we

Modeling the Data

red

eosin

cream

12:3:1

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Inheritance of comb morphology in chicken

First example of gene interaction

William Bateson and Reginald Punnett in 1906

Four different comb morphologies

 A Cross Involving a Two-Gene Interaction Can

Still Produce a 9:3:3:1 ratio 

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Figure 4.17b

The crosses of Bateson and Punnett

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F2 generation consisted of chickens with four

types of combs

9 walnut : 3 rose : 3 pea : 1 single Bateson and Punnett reasoned that comb

morphology is determined by two different

genes R  (rose comb) is dominant to r

P  (pea comb) is dominant to p 

R and P are codominant (walnut comb) rrpp produces single comb

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Gene Interaction

Duplicate gene action

Enzyme 1 and enzyme 2 areredundant

They both make product C,

therefore they duplicateeach other

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Duplicate Gene ActionEpistasis

TV

TV

Tv

Tv

tV

tV

tv

tv

TTVV TTVv TtVV TtVv

TTVv TTvv TtVv Ttvv

TtVV TtVv ttVV ttVv

TtVv Ttvv ttVv ttvv

(b) The crosses of Shull

TTVV

Triangulart tvv

Ovate

TtVv

All triangular

F1 (TtVv ) x F1 (TtVv )

x

F1 generation

15:1 ratio results

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Bombay Phenotype

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Bombay Phenotype

B b Ph t

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Bombay Phenotype

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Categories of Inheritance Paterns

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   C  o  m  p   l  e  m  e  n   t  a  r

  y  a  c   t   i  o  n

   D  u  p   l   i  c  a   t  e  a  c   t   i  o

  n

   E  p   i  s   t  a  s   i  s  o   f  a  a  o  v  e  r   B  -

   E  p   i  s   t  a  s   i  s  o   f   A  -

  o  v  e  r   b   b

Generation of Epistatic Ratios

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Epistasis

Interaction betweentwo gene products

B = black is dominant

to b = brown E = hair pigment

ee = no pigment

 X

F 1 puppiesare all BbEe  

bbee  BBEE  

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Epistasis

Cross puppies from twoseparate litter which areknown to be BbEe

BBEE  

BBEe  

BbEE  

BbEe   Bbee  

BbEe  

Bbee  

BBEe   BbEE   BbEe  

BBee   BbEe  

bbee  bbEe  

bbEE   bbEe  

BE   Be   bE   be  

BE  

Be  

bE  

be  

black

brown

 yellow

F 2 puppies