Epistasis, Pleiotropy

 EPISTASIS 

The phenomena where the effect of one gene depends on the presence of one or more gene, is known as epistasis. The phenotypic effect of one gene is masked by another gene. The gene which masks the effect of another gene is known as epistatic gene. The epistatic genes can be dominant or recessive in their effects and the gene whose effect is masked by the epistatic gene is known as hypostatic gene.

By the definition, confusion arises between the dominance and epistasis but dominance involves intra-allelic gene interaction and one allele hides the effect of other allele at the same gene pair, whereas in epistasis it involves inter-allelic gene interaction i.e. one gene hides the effect of other gene at other gene loci.

For the study of linkage association analysis, epistasis plays an important role. Epistatic mutations therefore have different effects on their own than when they occur together. Epistasis has a great influence on the evolvability of phenotypic traits.

 

TYPES OF EPISTASIS

 Dominant Epistasis When a dominant allele hides the effect of allele of another gene and expresses and itself phenotypically, is known as the dominant epistasis. The hypostatic allele will only get expressed when the gene locus contains two recessive alleles. The expression of one dominant or recessive allele is masked by another dominant gene. This is also referred to as simple epistasis.

An example of dominant epistasis is found for fruit colour in summer squash. There are three types of fruit colors - white, yellow and green. White colour is controlled by dominant gene W and yellow colour by dominant gene G. White is dominant over both yellow and green. The green fruits are produced in recessive condition (wwgg). A cross between plants having white and yellow fruits produced F1 with white fruits. Intermating of F1 plants produced plants with white, yellow and green coloured fruits in F2 in 12:3:1 ratio.

FIG- The figure explains the dominant epistasis for fruit colour in summer squash. The normal dihybrid modified to12:3:1 in F2 generation. Here W is dominant to w and epistatic to alleles G and g. Hence it will mask the expression of G/g alleles. Hence in F2, plants with W-G-(9/16) and W-gg (3/16) genotypes will produce white fruits; plants with wwG-(3/16) will produce yellow fruits and those with wwgg (1/16) genotype will produce green fruits. Thus the normal dihybrid ratio 9:3:3:1 is modified to 12:3: 1 ratio in F2 generation.

Similar type of gene interaction has been reported for skin color in mice and seed coat color in barley.

 Recessive Epistasis- It is also known as Supplementary interactions. The recessive allele of one gene locus hides the effect of another gene locus and expresses itself phenotypically. When recessive alleles at one locus mask the expression of both (dominant and recessive) alleles at another locus, it is known as recessive epistasis. A good example of such gene interaction is found for grain colour in maize. There are three colours of grain in maize, viz., purple, red and white. The purple colour develops in the presence of two dominant genes (R and P), red colour in the presence of a dominant gene R, and white in homozygous recessive condition (rrpp). A cross between purple (RRPP) and white (rrpp) grain colour strains of maize produced plants with purple colour in F1. Inter-mating of these F1 plants produced progeny with purple, red and white grains in F2 in the ratio of 9:3:4.

Fig:- Recessive epistasis for grain colour in maize.

The normal dihybrid segregation ratio 9:3:3:1 is modified to 9:3:4 in F2 generation. Here allele r is recessive to R, but epistatic to alleles P and p. In F2, all plants with R-P-(9/16) will have purple grains and those with R-pp genotypes (3/16) have red grain color. The epistatic allele r in homozygous condition will produce plants with white grains from rrP-(3/16) and rrpp (1/16) genotypes. Thus, the normal segregation ratio of 9:3:3:1 is modified to 9:3:4 in F2 generation.

 Such type of gene interaction is also found for coat color in mice, bulb color in onion and for certain characters in many other organisms.