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Epistasis

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martyynyyte

This document discusses gene interactions and epistasis. It provides several examples of gene interactions that result in ratios other than the expected 9:3:3:1 ratio for dihybrid crosses. These include complementary gene action between two enzymes that produce a product, duplicate gene action where two genes encode redundant enzymes, and different forms of epistasis where one gene is masked by the other. Specific examples discussed include interactions governing pigment production in fruit flies and comb morphology in chickens.

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 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 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 Epistatic Gene Interactions
A Cross Producing a 9:7 ratio Figure 4.18 9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp purple white
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 intermediate Colorless precursor
 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 Colorless precursor Colorless intermediate Purple pigment Enzyme C Enzyme P Epistatic Gene Interaction genotype cc genotype pp Colorless precursor Colorless intermediate Purple pigment Enzyme C Enzyme P
 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 Epistasis of Involving Sex-linked Genes
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
Figure 4.19 Testing the Hypothesis cream allele is recessive to +
Interpreting the Data Cross Outcome P cross: Cream-eyed male X wild-type female F1: all red eyes F1 cross: F1 brother X F1 sister F2: 104 females with red eyes 47 males with red eyes 44 males with eosin eyes 14 males with cream eyes F2 generation contains males with eosin eyes This indicates that the cream allele is not in the same gene as the eosin allele
Interpreting the Data Cross Outcome P cross: Cream-eyed male X wild-type female F1: all red eyes F1 cross: F1 brother X F1 sister F2: 104 females with red eyes 47 males with red eyes 44 males with eosin eyes 14 males with cream eyes F2 generation contains – 151 + eye: 44 we eye: 14 ca eye a 12 : 3 : 1 ratio
Modeling the 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.
Male gametes CY CCXw+ Xw+ CCXw+ Y ca ca Xw+ Xw+ Cca Xw+ YCXw+ CXw+ ca Xw+ ca Y CXw-e ca Xw+ ca Xw-e CCXw+ Xw-e CCXw-e Y Cca Xw+ Xw-e Cca Xw-e Y Cca Xw+ Xw+ Cca Xw+ Y ca ca Xw+ Xw+ ca ca Xw+ Y Cca Xw+ Xw-e Cca Xw-e Y ca ca Xw+ Xw-e ca ca Xw-e Y Femalegametes 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+ /_ ¼ we /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 Modeling the Data red eosin cream 12:3:1
 Inheritance of comb morphology in chicken  First example of gene interaction  William Bateson and Reginald Punnett in 1906  Four different comb morphologies A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio
Figure 4.17b The crosses of Bateson and Punnett
 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
Gene Interaction  Duplicate gene action  Enzyme 1 and enzyme 2 are redundant  They both make product C, therefore they duplicate each other
Duplicate Gene Action Epistasis TV TV Tv Tv tV tV tv tv TTVV TTVv TtVV TtVv TTVv TTvv TtVv Ttvv TtVV TtVv ttVV ttVv TtVv Ttvv ttVv ttvv (b) The crosses of Shull TTVV Triangular ttvv Ovate TtVv All triangular F1 (TtVv) x F1 (TtVv) x F1 generation 15:1 ratio results
Bombay Phenotype
Bombay Phenotype
Bombay Phenotype
Categories of Inheritance Paterns
Complementaryaction Duplicateaction EpistasisofaaoverB- EpistasisofA-overbb Generation of Epistatic Ratios
Epistasis
Epistasis

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Epistasis

  • 1.  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 Epistatic Gene Interactions
  • 2.  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 Epistatic Gene Interactions
  • 3. A Cross Producing a 9:7 ratio Figure 4.18 9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp purple white
  • 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 intermediate Colorless precursor
  • 5.  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 Colorless precursor Colorless intermediate Purple pigment Enzyme C Enzyme P Epistatic Gene Interaction genotype cc genotype pp Colorless precursor Colorless intermediate Purple pigment Enzyme C Enzyme P
  • 6.  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 Epistasis of Involving Sex-linked Genes
  • 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. Figure 4.19 Testing the Hypothesis cream allele is recessive to +
  • 9. Interpreting the Data Cross Outcome P cross: Cream-eyed male X wild-type female F1: all red eyes F1 cross: F1 brother X F1 sister F2: 104 females with red eyes 47 males with red eyes 44 males with eosin eyes 14 males with cream eyes 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 Cross Outcome P cross: Cream-eyed male X wild-type female F1: all red eyes F1 cross: F1 brother X F1 sister F2: 104 females with red eyes 47 males with red eyes 44 males with eosin eyes 14 males with cream eyes F2 generation contains – 151 + eye: 44 we eye: 14 ca eye a 12 : 3 : 1 ratio
  • 11. Modeling the 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. Male gametes CY CCXw+ Xw+ CCXw+ Y ca ca Xw+ Xw+ Cca Xw+ YCXw+ CXw+ ca Xw+ ca Y CXw-e ca Xw+ ca Xw-e CCXw+ Xw-e CCXw-e Y Cca Xw+ Xw-e Cca Xw-e Y Cca Xw+ Xw+ Cca Xw+ Y ca ca Xw+ Xw+ ca ca Xw+ Y Cca Xw+ Xw-e Cca Xw-e Y ca ca Xw+ Xw-e ca ca Xw-e Y Femalegametes 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+ /_ ¼ we /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 Modeling the Data red eosin cream 12:3:1
  • 13.  Inheritance of comb morphology in chicken  First example of gene interaction  William Bateson and Reginald Punnett in 1906  Four different comb morphologies A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio
  • 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 TV TV Tv Tv tV tV tv tv TTVV TTVv TtVV TtVv TTVv TTvv TtVv Ttvv TtVV TtVv ttVV ttVv TtVv Ttvv ttVv ttvv (b) The crosses of Shull TTVV Triangular ttvv Ovate TtVv All triangular F1 (TtVv) x F1 (TtVv) x F1 generation 15:1 ratio results

Editor's Notes

  1. Figure: 04-02 Caption: Partial pedigree of woman displaying the Bombay phenotype.
  2. Figure: 04-05 Caption: Outcome of mating btw individuals heterozygous at 2 genes determining blood type.
  3. Figure: 04-05 Caption: Outcome of mating btw individuals heterozygous at 2 genes determining blood type.
  4. Figure: 04-06 Caption: Generation of various modified dihybrid ratios from 9 unique genotypes.
  5. Figure: 04-07 Caption: Basis of modified dihybrid F2 phenotypic ratios.
  6. Figure: 04-08 Caption: Summer squash exhibiting various fruit-shape phenotypes.