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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
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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
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A Cross Producing a 9:7 ratio
Figure 4.18 9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp purple white
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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
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Epistatic Gene Interaction
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 Colorless precursor intermediate Purple pigment Enzyme C Enzyme P genotype pp
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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
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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
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Testing the Hypothesis
cream allele is recessive to + Figure 4.19
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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
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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 – eye: 44 we eye: 14 ca eye a 12 : 3 : 1 ratio
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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.
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Modeling the Data 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+/_ 9/16 C/_ ; + 3/16 ca/ca; + 3/16 C/_ ; we 1/16 ca/ca; we Male gametes CY CCXw+Xw+ CCXw+Y cacaXw+Xw+ CcaXw+Y CXw+ caXw+ caY CXw-e caXw-e CCXw+Xw-e CCXw-eY CcaXw+Xw-e CcaXw-eY CcaXw+Xw+ cacaXw+Y cacaXw+Xw-e cacaXw-eY Female gametes red eosin cream 12:3:1
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Inheritance of comb morphology in chicken
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
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Figure 4.17b The crosses of Bateson and Punnett
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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
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Gene Interaction Duplicate gene action
Enzyme 1 and enzyme 2 are redundant They both make product C, therefore they duplicate each other
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Duplicate Gene Action Epistasis
x Duplicate Gene Action Epistasis TTVV Triangular ttvv Ovate F1 generation TtVv All triangular F1 (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
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Bombay Phenotype Figure: 04-02 Caption:
Partial pedigree of woman displaying the Bombay phenotype.
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Bombay Phenotype Figure: 04-05 Caption:
Outcome of mating btw individuals heterozygous at 2 genes determining blood type.
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Bombay Phenotype Figure: 04-05 Caption:
Outcome of mating btw individuals heterozygous at 2 genes determining blood type.
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Categories of Inheritance Paterns
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Generation of Epistatic Ratios
Complementary action Epistasis of aa over B- Epistasis of A- over bb Duplicate action Figure: 04-06 Caption: Generation of various modified dihybrid ratios from 9 unique genotypes.
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Figure: 04-07 Caption: Basis of modified dihybrid F2 phenotypic ratios.
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Figure: 04-08 Caption: Summer squash exhibiting various fruit-shape phenotypes.
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