EPISTASIS

AQSA . HAMID .SHIRGAONKAR. MSC PART 1 PAPER 3 SINDHU SWADHYAY SANSTHA. KALINA CAMPUS . 2014-2015

A basic physical and functional unit of heredity.

Each person has two copies of each gene. Alleles are the forms of the same gene

contributing to each person’s unique physical features.◦DOMINANT◦RECESSIVE

WHAT IS A GENE???

Gregor Johann Mendel gave the LAW OF DOMINANCE where he stated that,

“in a cross between two homozygous organisms differing in a single pair of contrasting characters, the characters which expresses itself in F1 generation is called dominant character and the unexpressed one is called recessive character.”

Dominance and recessiveness determine the inheritance patterns of certain traits.

DOMINANCE AND RECESSIVENESS

2 types: Intra-allelic interaction: Takes place

between alleles at the same locus.◦ May involve dominance relationship or multiple

alleles. Inter-allelic interaction: Takes place

between alleles at different loci.◦ May involve non-epistatic or epistatic interactions.

INTERACTION OF GENES

Intra –allelic interactions :- TYPES OF DOMINANCE:

A

A A

A

A

B

B

BB

B

B

B

BA

AACOMPLETE DOMINANCE

INCOMPLETE DOMINANCE CODOMINANCE

Non-epistatic interactions:

◦ When two independently working gene loci affect the same trait, present together in dominant heterozygous or homozygous condition, produce a different (new) phenotype, then it is said to be a non-epistatic interaction.

◦ Eg. Shape of comb in fowl.

INTER-ALLELIC INTERACTIONS

Epistasis is “When one gene masks the effect of another gene.”

Term first used by Bateson in 1909. Epistatic Genes: They are the genes that suppress the other

gene’s expression. Hypostatic Genes: They are the ones whose action is

suppressed. Epistasis occurs in all of the following scenarios:

◦ Whenever two or more loci interact to create new phenotype.

◦ Whenever an allele at one locus masks the effects of alleles at one or more other loci

◦ Whenever an allele at one locus modifies the effects of alleles at one or more other loci

EPISTATIC INTERACTIONS/EPISTASIS

Feather colour in chickens: Gene C codes for colored feathers. Another non-allelic gene A

is epistatic to C. In the presence of A, C cannot produce coloured feathers. Therefore, feathers are white.

Only when A is absent (aa), C is effective. C_A_ = White (A epistatic to C), ccA_ = White (no C gene),

ccaa= White C_aa = Coloured. A cross between White (CCAA) and white (ccaa) produces all

White feathers in F1 (CcAa). In F2, White and coloured are obtained in 13:3 ratio.

EXAMPLE FOR EPISTASIS

Recessive epistasis: ◦ When the recessive allele of one gene masks the effects of either allele

of the second gene. Eg. Coat colour in mice Black AlbinoParents AAbb x aaBBGametes Ab aB

F1 Generation AaBb (Agouti)

TYPES OF EPISTASIS

F2 Generation AaBb x AaBb

Phenotypic ratio: Agouti : Black : White 9 : 3 : 4

The ratio obtained is a modified mendelian ratio (9:3:3:1 =

3+1= 4). So the ratio here gets modified to 9:3:4

FEMALE GAMETES

MALEGAMETE

S

AB Ab aB ab

AB AABB AABb AaBB AaBb

Ab AABb AAbb AaBb Aabb

aB AaBB AaBb aaBB aaBb

ab AaBb Aabb aaBb aabb

If both gene loci have homozygous recessive alleles and both of them produce identical phenotype, the F2 ratio 9:3:3:1 gets modified to 9:7.

Both dominant alleles when present together, complement each other and produce a different phenotype (aka Complementary gene action)

E.g. Inheritance of deafness in humans Deaf DeafParents aaBB x AAbbGametes aB Ab

F1 Generation AaBb (Normal)

DUPLICATE RECESSIVE EPISTASIS

F2 Generation AaBb x AaBb

Phenotypic ratio : Normal : Deaf 9 : 7

Here, the ratio gets modified from 9:3:3:1 to 9:7 (3+3+1=7)

FEMALE GAMETES

MALEGAMETE

S

AB Ab aB ab

AB AABB AABb AaBB AaBb

Ab AABb AAbb AaBb Aabb

aB AaBB AaBb aaBB aaBb

ab AaBb Aabb aaBb aabb

A dominant allele of one allelic pair masks the effect of the pairs of alleles present on another locus.

Eg. Summer squash- Common fruit colours are White, Green and Yellow.

White is dominant over yellow and green, Yellow is dominant over green, but yellow is recessive to white.

White GreenParents WWYY x wwyy Gametes WY wy

F1 Generation WwYy (White)

DOMINANT EPISTASIS

F2 Generation WwYy x WwYy

Phenotypic ratio: White : Yellow : Green 12 : 3 : 1

The ratio gets modified to 12:3:1 (9:3:3:1 = 9+3=12).

FEAMLE GAMETES

MALEGAMETE

S

WY Wy wY wy

WY WWYY WWYy WwYY WwYy

Wy WWYy Wwyy WwYy Wwyy

wY WwYY WwYy wwYY wwYy

wy WwYy Wwyy wwYy wwyy

When two alleles on different loci in the dominant form have an identical effect on the phenotype, similar to that produced by each dominant gene alone, the phenomena is said to be Duplicate dominant epistasis.

Eg. Seed capsule of shepherd’s purse (Capsella) Triangular OvoidParents AABB x aabbGametes AB ab

F1 Generation AaBb (Triangular)

DUPLICATE DOMINANT EPISTASIS

F2 Generation AaBb x AaBb

Phenotypic ratio: Triangular : Ovoid 15 :

1

Here, the ratio gets modified from 9:3:3:1 to 15:1 (9+3+3= 15)

FEMALE GAMETES

MALEGAMETE

S

AB Ab aB ab

AB AABB AABb AaBB AaBb

Ab AABb AAbb AaBb Aabb

aB AaBB AaBb aaBB aaBb

ab AaBb Aabb aaBb aabb

When the dominant alleles of one gene locus in homozygous or heterozygous condition AA or Aa and homozygous recessive alleles bb of another gene locus produce the same phenotype, the phenomena is said to be Dominant- Recessive epistasis.

Eg. Feather colour of cock Coloured WhiteParents CCii x ccIIGametes Ci cI

F1 Generation CcIi (White)

DOMINANT-RECESSIVE EPISTASIS

F2 Generation CcIi x CcIi

Phenotypic ratio: White : Coloured 13 : 3

Here, the ratio gets modified from 9:3:3:1 to 13:3 (9+3+1=13)

FEMALE GAMETES

MALEGAMETE

S

CI Ci cI ci

CI CCII CCIi CcII CcIi

Ci CCIi Ccii CcIi Ccii

cI CcII CcIi ccII ccIi

ci CcIi Ccii ccIi ccii

http://www.nature.com/scitable/topicpage/epistasis-gene-interaction-and-phenotype-effects-460

http://www.yourarticlelibrary.com/biology/6-most-important-kinds-of-epistasis-biology/6436/

Principles of Genetics by Robert Tamarin. Advanced Genetics by G.S. Miglani. Concepts of Genetics by William S. Kluge, M.R.

Cummings. F.Y.B.Sc Botany.

Reference:

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