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Epistatic Gene Interactions

Epistatic Gene Interactions. Gene interactions occur when two or more different genes influence the outcome of a single trait Most morphological traits (height, weight, color) are affected by multiple genes Epistasis describes situation between various alleles of two genes

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Epistatic Gene Interactions

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  1. Epistatic Gene Interactions • Gene interactions occur when two or more different genes influence the outcome of a single trait • Most morphological traits (height, weight, color) are affected by multiple genes • Epistasis describes situation between various alleles of two genes • Quantitative loci is a term to describe those loci controlling quantitatively measurable traits • Pleiotropy describes situations where one gene affects multiple traits

  2. Epistatic Gene Interactions • examine cases involving 2 loci (genes) that each have 2 alleles • Crosses performed can be illustrated in general by • AaBb X AaBb • Where A is dominant to a and B is dominant to b • If these two genes govern two different traits • A 9:3:3:1 ratio is predicted among the offspring • simple Mendelian dihybrid inheritance pattern • If these two genes do affect the same trait the 9:3:3:1 ratio may be altered • 9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1 • epistatic ratios

  3. A Cross Producing a 9:7 ratio purple white Figure 4.18 9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp

  4. Epistatic Gene Interaction • Complementary gene action • Enzyme C and enzyme P cooperate to make a product, therefore they complement one another Enzyme C Enzyme P Purple pigment Colorless precursor Colorless intermediate

  5. Epistatic Gene Interaction Enzyme C Enzyme P Colorless precursor Colorless intermediate Purple pigment • Epistasis describes the situation in which a gene masks the phenotypic effects of another gene • Epistatic interactions arise because the two genes encode proteins that participate in sequence in a biochemical pathway • If either loci is homozygous for a null mutation, none of that enzyme will be made and the pathway is blocked Enzyme C Enzyme P Colorless precursor Colorless intermediate Purple pigment genotype cc genotype pp

  6. Epistasis of Involving Sex-linked Genes • Inheritance of the Cream-Eye allele in Drosophila • a rare fly with cream-colored eyes identified in a true-breeding culture of flies with eosin eyes • possible explanations • 1. Mutation of the eosin allele into a cream allele • 2. Mutation of a 2nd gene that modifies expression of the eosin allele

  7. The Hypothesis • Cream-colored eyes in fruit flies are due to the effect of a second gene that modifies the expression of the eosin allele

  8. Testing the Hypothesis cream allele is recessive to + Figure 4.19

  9. Interpreting the Data F2 generation contains males with eosin eyes This indicates that the cream allele is not in the same gene as the eosin allele

  10. Interpreting the Data F2 generation contains – 151 + eye: 44 we eye: 14 ca eye a 12 : 3 : 1 ratio

  11. Modelingthe Data • Cream phenotype is recessive therefore the cream allele is recessive allele (either sex-linked or autosomal) • The mutated allele of the cream gene modifies the we allele, while the wt cream allele does not • C = Normal allele • Does not modify the eosin phenotype • ca = Cream allele • Modifies the eosin color to cream, does not effect wt or white allele of white gene.

  12. Modeling the Data Male gametes CXw+ caXw+ caY CY CXw+ CCXw+Xw+ CCXw+Y cacaXw+Xw+ CcaXw+Y CXw-e CCXw+Xw-e CCXw-eY CcaXw+Xw-e CcaXw-eY red Female gametes caXw+ CcaXw+Xw+ CcaXw+Y cacaXw+Xw+ cacaXw+Y caXw-e cacaXw+Xw-e cacaXw-eY CcaXw+Xw-e CcaXw-eY Putative genotypes in a cross P w+/ w+; C/C x we/Y; ca/ca F1 w+/ we; C/ca & w+/Y; C/ca F2 ¾ C/_ x ¾ w+/_ ¼we/Y ¼ ca/ca x ¾ w+/_ ¼ we/Y 9/16 C/_ ; + 3/16 ca/ca; + 3/16 C/_ ; we 1/16 ca/ca; we eosin cream 12:3:1

  13. A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio • Inheritance of comb morphology in chicken • First example of gene interaction • William Bateson and Reginald Punnett in 1906 • Four different comb morphologies

  14. Figure 4.17b The crosses of Bateson and Punnett

  15. F2 generation consisted of chickens with four types of combs • 9 walnut : 3 rose : 3 pea : 1 single • Bateson and Punnett reasoned that comb morphology is determined by two different genes • R (rose comb) is dominant to r • P (pea comb) is dominant to p • R and P are codominant(walnut comb) • rrpp produces single comb

  16. Gene Interaction • Duplicate gene action • Enzyme 1 and enzyme 2 are redundant • They both make product C, therefore they duplicate each other

  17. Duplicate Gene Action Epistasis x TTVVTriangular ttvv Ovate F1 generation TtVvAll triangularF1 (TtVv) x F1 (TtVv) 15:1 ratio results TV Tv tV tv TTVV TTVv TtVV TtVv TV TTVv TTvv TtVv Ttvv Tv TtVV TtVv ttVV ttVv tV TtVv Ttvv ttVv ttvv tv (b) The crosses of Shull

  18. Bombay Phenotype

  19. Bombay Phenotype

  20. Bombay Phenotype

  21. Categories of Inheritance Paterns

  22. Generation of Epistatic Ratios Complementary action Epistasis of A- over bb Epistasis of aa over B- Duplicate action

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