This document discusses gene interactions and epistasis. It provides several examples of gene interactions that result in ratios other than the expected 9:3:3:1 ratio for dihybrid crosses. These include complementary gene action between two enzymes that produce a product, duplicate gene action where two genes encode redundant enzymes, and different forms of epistasis where one gene is masked by the other. Specific examples discussed include interactions governing pigment production in fruit flies and comb morphology in chickens.

1.  Gene interactions occur when two or more different
genes influence the outcome of a single trait
 Most morphological traits (height, weight, color) are
affected by multiple genes
 Epistasis describes situation between various alleles
of two genes
 Quantitative loci is a term to describe those loci
controlling quantitatively measurable traits
 Pleiotropy describes situations where one gene affects
multiple traits
Epistatic Gene Interactions

2.  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 Interactions

3. A Cross Producing a 9:7 ratio
Figure 4.18
9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp
purple white

4. Epistatic Gene Interaction
 Complementary gene action
 Enzyme C and enzyme P cooperate to
make a product, therefore they
complement one another
Enzyme C Enzyme P
Purple
pigment
Colorless
intermediate
Colorless
precursor

5.  Epistasis describes the situation in which a gene masks the
phenotypic effects of another gene
 Epistatic interactions arise because the two genes encode
proteins that participate in sequence in a biochemical pathway
 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

6.  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

7. 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

8. Figure 4.19
Testing the Hypothesis
cream allele is
recessive to +

9. 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 eyes
47 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

10. 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 eyes
47 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

11. 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 does not
 C = Normal allele
 Does not modify the eosin phenotype
 ca
= Cream allele

Modifies the eosin color to cream, does not effect wt or white
allele of white gene.

12. Male gametes
CY
CCXw+
Xw+
CCXw+
Y ca
ca
Xw+
Xw+
Cca
Xw+
YCXw+
CXw+
ca
Xw+
ca
Y
CXw-e
ca
Xw+
ca
Xw-e
CCXw+
Xw-e
CCXw-e
Y Cca
Xw+
Xw-e
Cca
Xw-e
Y
Cca
Xw+
Xw+
Cca
Xw+
Y ca
ca
Xw+
Xw+
ca
ca
Xw+
Y
Cca
Xw+
Xw-e
Cca
Xw-e
Y ca
ca
Xw+
Xw-e
ca
ca
Xw-e
Y
Femalegametes
Putative genotypes in a cross
P 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

13.  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

14. Figure 4.17b
The crosses of Bateson and Punnett

15.  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

16. Gene Interaction
 Duplicate gene action
 Enzyme 1 and enzyme 2
are redundant
 They both make product C,
therefore they duplicate
each other

17. Duplicate Gene
Action Epistasis
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
Triangular
ttvv
Ovate
TtVv
All triangular
F1 (TtVv) x F1 (TtVv)
x
F1 generation
15:1 ratio results

18. Bombay Phenotype

19. Bombay Phenotype

20. Bombay Phenotype

21. Categories of Inheritance Paterns

22. Complementaryaction
Duplicateaction
EpistasisofaaoverB-
EpistasisofA-overbb
Generation of Epistatic Ratios