7. Inter-Allelic Interactions

8/10/2019 7. Inter-Allelic Interactions

1/28

G E N E T I C S-1

MODULE 5

EXTENSION OF MENDELIAN INHERITANCE-2

INTER-ALLELIC INTERACTIONS


2/28

Describe that inter-allelic interactions stem from the

relationship between alleles of different genes.

Describe the different classes of epistasis (complementary,

inhibitory and duplicate) and the types of epistasis within each

of the classes.

Outline and explain the various inter-allelic relationships and

the underlying biochemical basis for inhibitory and duplicated

epistases, clearly showing in each case how they modify

expected Mendelian phenotypic ratios.

LEARNING OUTCOMES

G E N E T I C S-I


3/28

INTERALLELIC INTERACTIONS (Epistasis)

Mendelian dihybrid ratios of 9:3:3:1 in the F2and 1:1:1:1 ratio in the

test cross are altered by epistatic interactions.

Inter-allelic interactions or epistasisoccur when two or more

genes specify enzymes which catalyze steps in a common

biochemical pathway.

G E N E T I C S-I


4/28

In epistasis, the interacting genes produce enzymes that function

in the same biochemical pathway.

Each enzyme acts at a different step in the biochemical pathway.

PRECURSOR END PRODUCTINTERMEDIATE

Gene 1 Gene 2

Enzyme 1 Enzyme 2

G E N E T I C S-I



5/28

INTER-ALLELIC INTERACTIONS

The interacting genes may have an apparent dominant/recessive

type relationship.

The alleles at one gene locus appear to suppress the alleles at

another locus that affects the same phenotype.

A gene that suppresses another is referred to as being epistatic.

The gene that is suppressed is referred to as being hypostatic.

G E N E T I C S-I


6/28

Epistasis can fall into one of THREEmain categories:

1. Complementary epistasis

Two genes complement each other to produce one finalphenotype.

2. Inhibitory epistasis

An inhibitor or repressor is involved.

3. Duplicate epistasis

Two duplicate pathways lead to the same end product.

G E N E T I C S-I



7/28

Assume two independent genes with complete dominance

governs coat colour in mice.

One of the two genes in the homozygous recessive form is

epistatic over the other.

Lets say the epistatic gene is A

and the hypostatic gene is B.

Then aa (homozygous recessive state) is epistaticover B

regardless of the form it is in (BB, Bb, bb).

1. RECESSIVE EPISTASIS

Complementary epistasis

G E N E T I C S-I


8/28

Recessive epistasis: Coat colour in mice

AGOUTI BLACK ALBINO

G E N E T I C S-I


9/28

RECESSIVE EPISTASIS

Recessive epistasis

Agouti Albino

AABB x aabb

AaBb (Agouti) self

F2Genetic Explanation

9 A-B- = Agouti 9

3 A-bb = Black 3

3 aaB- =

1 aabb =Albino 4

G E N E T I C S-I


10/28

Test cross for Recessive Epistasis

Recessive epistasis

Agouti Albino

AABB x aabb

AaBb (Agouti) F1

Test cross: AaBb (Agouti) x aabb (albino, double recessive)

Testcross Genetic Explanation

1 AaBb = Agouti 1

1 Aabb = Black 1

1 aaBb =

1 aabb = Albino 2

AB Ab aB ab

ab AaBb Aabb aaBb aabb

G E N E T I C S-I


11/28

Recessive Epistasis: A Biochemical Basis

PRECURSOR(Albino) END PRODUCT(Agouti)INTERMEDIATE(Black)

GeneA- Gene B-

EnzymeA EnzymeB

NOTE: Both genes A and B complement each

other to generate the final product (agouti).

COMPLEMENTARY EPISTASIS

9 A-B- = 9 Agouti

3 A-bb = 3 Black

3 aaB- =

1 aabb =4 Albino

G E N E T I C S-I


12/28

G E N E T I C SRECESSIVE EPISTASIS

Regulatory gene - produces a

regulatory protein (or activator),

which is required to transcriptionallyactivate a structural gene.

Recessive epistasis occurs:

two gene loci produce enzymes

which are part of the same

biochemical pathway (or)

when one gene transcriptionally

activates or regulates another gene


13/28

Assume TWO independent genes with complete dominance control

the production of hydrocyanic acid in clover.

Definition of duplicate recessive epistasis: Either one of the twogenes in the homozygous recessive state is epistatic over the other

gene, i.e.:

Gene Ain its recessive form aa is epistatic over gene Bregardlessof its form (BB, Bb or bb)

and

Gene Bin its recessive form bb is epistatic over gene Aregardless

of its form (AA, Aa or aa).

2. DUPLICATE RECESSIVE EPISTASIS

G E N E T I C S-I


14/28

Hydrocyanic Acid (HCN)

production in clover

P: AABB aabb

High [HCN] no [HCN]

F1: AaBb

High [HCN]

Self

F2: A-B- 9 High [HCN]

A-bb 3

aaB- 3 No [HCN]

aabb 1

Modified F2ratio- 9:7

Test-cross: AaBb x aabb

AaBb 1 High [HCN]

Aabb 1

aaBb 1 No [HCN]

aabb 1

Modified Test-cross ratio- 1:3

X

G E N E T I C S-I


15/28

Duplicate Recessive Epistasis: A Biochemical Basis

PRECURSOR(No HCN) END PRODUCT(HCN)INTERMEDIATE(Cyanic glucoside)

GeneA- Gene B-

EnzymeA EnzymeB

NOTE: Both genes A and B complement

each other to generate the final product

(high [HCN])

COMPLEMENTARY EPISTASIS

9 A-B- = 9 HCN

3 A-bb =

3 aaB- =

1 aabb =7 [No HCN]

G E N E T I C S-I


16/28

Assume two independent genes with complete dominance.

One gene in the dominant state is epistatic over the other gene locusregardless of the form it is in (homozygous dominant, heterozygous or

homozygous recessive).

e.g. Consider genes Aand B that control a single phenotype.

If the gene Alocus is epistatic and the gene Blocus is hypostatic, thenAAand Aaare epistaticover either BB, Bb or bb.

3. DOMINANT EPISTASISINHIBITORY EPISTASIS

G E N E T I C S-I


17/28

Fruit colour inSummer SquashP: AABB aabb

White Green

F1: AaBb

White

F2:A-B- 9 White

A-bb 3

aaB- 3 Yellow

aabb 1 Green

Modified F2ratio- 12:3:1

Test-cross: AaBb x aabbAaBb 1 White

Aabb 1

aaBb 1 Yellow

aabb 1 Green

Modified Test-cross ratio- 2:1:1

X

G E N E T I C S-I

DOMINANT EPISTASIS


18/28

Dominant Epistasis: A Biochemical Basis

PRECURSOR 1

(White)

END PRODUCT

(Yellow)

INTERMEDIATE

(Green)

GeneA-

Gene B-

EnzymeA

EnzymeB

NOTE: Gene A inhibits the production of the

final product (yellow squash colour).

INHIBITORY EPISTASIS

PRECURSOR 2 INHIBITOR

9 A-B- =

3 A-bb =

3 aaB- = 3 Yellow

1 aabb = 1 Green

12 white

G E N E T I C S-I


19/28

Assume two independent genes with complete dominance.

One gene in the dominant state is epistatic over the other gene locus

regardless of the form it is inAND

The other gene in its homozygous recessive form is epistatic over the

previous gene locus regardless of the form it is in.

e.g. Consider genes Aand Bthat control a single phenotype

Then AAand Aaare epistaticover either BB, Bb or bb

And bbis epistaticover either AA, Aa or aa.

4. DOMINANT AND RECESSIVE INTERACTION

G E N E T I C S-I


20/28

Flower colour

(malvidin) in

Primula

P: AABB aabb

White White

F1: AaBb

White

F2:

A-B- 9 White

A-bb 3

aaB- 3 Blue

aabb 1 White



AaBb 1 White

Aabb 1

aaBb 1 Blue

aabb 1 White


X

G E N E T I C S-I

4. DOMINANT AND RECESSIVE INTERACTION


21/28

Dominant and Recessive Interaction: A Biochemical Basis

PRECURSOR 1

(White)

END PRODUCT

(Blue)

INTERMEDIATE

(White)

GeneA-

Gene B-

EnzymeA

EnzymeB

NOTE: Gene A inhibits the production of

the final product (blue flower colour).

INHIBITORY EPISTASIS

PRECURSOR 2 INHIBITOR

9 A-B- =

3 A-bb =

3 aaB- = 3 Blue

1 aabb = 1 white

12 white

G E N E T I C S-I


22/28

Assume two independent genes with complete dominance

Either of the two genes in the dominant state is epistatic over the other

gene locus regardless of the form it is in.

e.g. Consider genes Aand Bthat control a single phenotype

Then AAandAaare both epistaticover either BB, Bb or bb

And both BBandBbare epistaticover either AA, Aa or aa.

5. DUPLICATE DOMINANT EPISTASIS

Duplicated Epistasis

G E N E T I C S-I


23/28

Feathered

Chicken Shank

P: FFSS ffss

Feathered Shank Clean Shank

F1: FfSs

Feathered Shank

F2:

F-S- 9

F-ss 3

ffS- 3

ffss 1 Clean


Test cross: FfSs x ffss

Ff Ss 1

Ff ss 1

ff Ss 1

ff ss 1 Clean


X

Feathered Feathered

G E N E T I C S-I

Duplicated Epistasis
http://www.google.tt/url?sa=i&rct=j&q=feathered+shank&source=images&cd=&cad=rja&docid=5GKxbIx7EZ7J6M&tbnid=WqbG8l9G0hIQAM:&ved=0CAUQjRw&url=http://www.backyardchickens.com/t/299038/icelandic-chickens/16750&ei=yFwRUZztOuW90QGewYHYBA&bvm=bv.41867550,d.dmQ&psig=AFQjCNHPy78sDYNbspB3k4SoYnmTgtWdiQ&ust=1360178716811376


24/28

6. Duplicate DOMINANT EPISTASIS: A Biochemical Basis

PRECURSOR 1

(Clean)

END PRODUCT

(Feathered)

Gene S-

Gene F-

EnzymeS

EnzymeF

NOTE: Gene F and S produce the same end

product (feathered chicken shank).

DUPLICATED EPISTASIS

PRECURSOR 2(Clean) 9 F-S- =

3 F-ss =

3 ffS- =

1 ffss = 1 Clean shank

15 Feathered

shank

G E N E T I C S-I


25/28

Each of the two genes in the dominant state (homozygous dominant

or heterozygous) produces the same phenotype. When both genes arepresent in the dominant state a novel phenotype is generated.

e.g.

Consider genesA and Bthat control a single phenotype

Then AA /AaORBB/Bbwill produce the same phenotype.

When both AA /AaANDBB /Bbare present, a novel phenotype is

generated.

7. DUPLICATE INTERACTION

G E N E T I C S-I


26/28

Fruit shape

in squashP: AABB aabb

Disc Pyriform (long)

F1: AaBb

Disc

F2

:

A-B- 9 Disc

A-bb 3

aaB- 3

aabb 1 Pyriform

Modified F2ratio- 9:6:1


AaBb 1 Disc

Aabb 1

aaBb 1

aabb 1 Pyriform

Modified Test-cross ratio- 1:2:1

X

SphericalSpherical

G E N E T I C S-I

7. DUPLICATE INTERACTION


27/28

Duplicate Interaction: A Biochemical Basis

PRECURSOR 1

(Pyriform)

Spherical

Spherical

GeneA-

Gene B-

EnzymeA

EnzymeB

NOTE: Gene A and B produce the same end

product (spherical shape).

PRECURSOR 2(Pyriform)

Disc

9 A-B- = 9 Disc

3 A-bb =

3 aaBb =

1 aabb = 1 pyriform

6 Spherical

G E N E T I C S-I


28/28

Epistasis is interactions between alleles of different genes.

There are three types of epistasis

Complementary epistasistwo genes complement each other Inhibitory epistasisan inhibitor or a repressor is involved

Duplicate epistasistwo duplicate pathways lead to same product.

Epistasis ONLYmodify Mendelian phenotypic ratios.

Epistasis causes many different genotypes to fall into one phenotypic

category. (Just like dominance)

SUMMARY

G E N E T I C S-I

Documents

7. Inter-Allelic Interactions