Epistasis

Definition. А pair at genes at one locus may prevent the expression of а pair of genes at another locus. Such genes are called inhibiting, or epistatic genes (G. epi = over, stasis = standing). The prevention of the expression of one pair of genes by another pair of genes is known as epistasis in contrast to dominance in which the expression of one allele is prevented by the other allele of the same gene. In other words, epistasis is the interallelic gene suppression and dominance is the intraallelic gene suppression. The basic genes, the expression of which is prevented by epistatic genes, are termed hypostatic genes. (G. hypo = under, stasin=standing). Epistases reduces the number of phenotypes shown by the individuals of the P2 generation of а dihybrid cross.

Example. Interaction of inhibiting and basic genes occurs in the white fowls. There are two

varieties of white fowls: white Leghorn and white Plymouth Rock or Wyandotte. А cross between, а homozygous dominant white, Leghorn CCII and а homozygous recessive white Plymouth Rock CCii produces dihybrid white- feathered fowls with CCii genotype in Ft. When the F₁ white fowls are inbred, F₂ includes white and coloured fowls in the ratio of 13: 3.

Explanation. The basic gene С produces colour in the feathers but the inhibiting gene 1 prevents the development of colour. In other words, the gene 1 interacts with the gene С so as tо suppress its expression. Due to this interaction, the Leghorn fowl is white CCII. The Plymouth Rock is white because it has recessive genes ccii. In F_2, the genotypes having the inhibiting gene 1 along with the basic colour- producing gene С, produce white fowls. The genotypes, which do not have 1 gene, give rise to coloured fowls.

Table. Monogenic versus Polygenic Inheritance

Monogenic Inheritance

Polygenic Inheritance

1. It deals with qualitative traits such as form, colour and structure. 2. It is controlled by а single pair of contrasting genes (alleles). 3. It produces two distinct phenotypes that show discontinuous variation. 4. No intermediate phenotypes are formed. 5. A single dominant gene expresses its trait fully. 6. No additive effect оf dominant genes. 7. F1 individuals resemble the dominant parent. 8. F2 individuals resemble the dominant and recessive grandparents in the ratio of 3: 1. 9. Phenotypic expression of the genotype is not affected by environment. 10. Progeny analyzed by counts and ratios.

1. It deals with quantitative traits such as height, weight and number. 2. It is controlled by more than one pair of non allelic genes (polygenes). 3. It produces а wide range of phenotypes that show continuous variation. 4. Intermediate phenotypes are formed. 5. One dominant gene expresses its trait partly. 6. Dominant genes add up their effect to produce а phenotype. 7. F1 individuals are intermediate between thе two рагеnts. 8. Only 2/64 F2 individuals resemble the grandparents if 3 pairs of genes control а trait. 9. Phenotypic expression of polygenes is affected by environment. 10. Progeny analyzed by statistical methods.