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COURSE Principles of inheritance and variations Lesson 13 Epistasis Dr. Praveen Kumar Agrawal M.Sc., Ph.D., CSIR NET URF), SRF, GATIE 21 Years of Pre-medical teaching experience
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EPISTASIS According to Mendel's 'Principle of independent assortment' two genes (which have separate loci) do not influence each other and are inherited independently. Mendel obtained always a ration of 9:3:3:1 in all his dihybrid experiments. But later on several examples were noticed which show interaction between two or more genes. In such cases the classical dihybrid ratio (9:3:3:1) gets modified. Some of these examples are epistasis, complementary genes etc. Epistasis is the interaction between two (non-allelic) genes, where one gene inhibits or suppresses the expression of other gene. The gene, which suppresses the other gene, is called epistatic gene, while other gene (which gets suppressed) is called hypostatic gene Difference between epistasis and dominance Epistasis looks similar to the phenomenon of dominance and recessiveness, however the two phenomena are completely different. In dominance, the two genes are alleles of each other and have similar loci on homologous chromosomes. Hence dominance is an inter-allelic (or intra-genic) phenomenon. On the other hand, genes involved in epistasis are non-allelic and have separate loci on homologous chromosomes. Hence epistasis is an extra-allelic or inter-genic phenomenon.
Epistasis may be of two types 1. Dominant epistasis and 2. Recessive epistasis. 1. Dominant epistasis In this case, the epistatic gene shows its effect only in dominant condition. In recessive condition, the gene is perfectly normal and does not show any inhibiting effect For example appears, while in recessive condition (b) brown colour appears. and white colour (albino) appears. So R acts as a dominant epistatic gene. In this way Suppose, in dog, the coat colour is governed by a gene B. In dominant condition (B), black colour Another non-allelic gene R, when present in dominant condition (RR or Rr), inhibits the expression of B But when this epistatic gene is in recessive condition (rr), the B and b show their normal expression BBrr, Bbrr Black, bbrr- Brown, BBRR, BBRr, BbRR, BbRr, bbbRR, bbRr-Albino.
Pure Albino BBRR bbrr Pure brown BR BR br br F1 BbR BbR BbRYBR All individuals are albino
Selfing of F1 BbRr BbRr Male Female BR bR BbRR Albino BBRR BBRr BbRr BR Albino Albino Albino BBRr BBrr BbRr Bbrr Albino Black Albino Black BbRR BbRr bbRR bbRr bR Albino Albino Albino Albino BbRr Bbrr bbRr bbrr Albino Black Albino Brown
Results Phenotypic ratio 12:3 1 (Albino 12, Black 3, Brown 1)
Recessive epistasis In this case the epistatic gene shows its effect only in recessive condition. In dominant condition the gene is perfectly normal and does not show inhibiting effect. For example Suppose, in mice, the gene A codes for agouti colour in dominant condition (AA and Aa). In recessive Another non-allelic gene B is epistatic in recessive condition. Whenever it is in recessive (bb) condition, it inhibits the expression of A, resulting in the production of albino mice. (But in dominant condition, BB or Bb) condition black colour is produced it does not cause inhibition effect. In this way, AABB, AaBB, AABb, AaBb - Agouti, .AAbb, Aabb, aabb-Albino, aaBB, aaBb Black
Pure Agouti AABBv aabb Pure Albino AB AB ab ab Fb AaBb Aab AaBbAll individuals are agouti
Selfing of F1 AaBb AaBb Male Female AB AABB AABb AaBB AaBb AB Agouti Agouti Agouti Agouti AABb AAbb AaBb Aabb Ab Agouti Albino Agouti Albino AaBB AaBb aaBB aaBb Agouti Agouti Black Black AaBb Aabb aaBb aabb ab Agouti Albino Black Albino
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