lecture 16

Recessive epistasis (9:3:4) Duplicate and additive epistasis (9:6:1). Duplicate dominant epistasis (15:1)

Recessive epistasis: (9:3:4) (supplementary factor):

• The recessive allele at one locus masks the expression of both dominant and recessive alleles at another locus resulting in 9: 3 :4 ratio

• Also called as Supplementary Epistasis.

• Example: Grain colour in Maize  

• Three colours in maize - Purple, Red and White 58

• Purple colour - in the presence of 2 dominant genes (R and P)

• Red colour - in the presence of 1 dominant gene (R)

• White colour – homozygous recessive condition (rrpp)

• The allele r is recessive to R but epistatic to alleles P and p

In F2, plants with genotypes

R-P- (9/16) – Purple colour grains

R-pp (3/16) – Red colour grains

rrP- (3/16)) – White colour grains

rrpp (1/16) – White colour grains

The dihybrid 9:3:3:1 ratio is modified to 9:3:4 ratio. The epistatic allele is r – produce white colour under homozygous recessive condition.

Duplicate and additive epistasis: (9:6:1)

• In this gene action, the two genes controlling a character produce identical phenotypes when they are alone.

• But when the genes are present together, their phenotypic effect is enhanced as if the effect of the two genes were cumulative (or) additive.

• E.g. In barley, two completely dominant genes A and B affect the length of awns. Gene A or B alone gives rise to awns of medium length. But when both the genes A and B are present together, they produce long awns indicating that the effects of A and B are added together.

Duplicate dominant epistasis (15: 1):

• The dominant alleles at either of the two loci mask the expression of recessive alleles at the two loci, resulting in 15: 1 ratio

• Also called as Duplicate gene action

• Example: Awn character in Rice

• Development of Awn in Rice is controlled by 2 dominant duplicate genes (A and B) · Presence of any one of these 2 dominant genes will produce awn (A or B)

• The awn less condition develops when both these genes are in homozygous recessive state (aabb)

• A cross was made between

Selfing of F1

15 Awned plants and 1 Awn less plant in F2

In F2, plants with genotypes

A-B- (9/16), A-bb (3/16) and aaB- (3/16) – develop awn

aabb (1/16) double recessive condition – develop awn less