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EPISTASIS

EPISTASIS. Dr. S. Ramgopal Rao. 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

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EPISTASIS

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  1. EPISTASIS Dr. S. Ramgopal Rao

  2. 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

  3. Definition • Epistasis is a form of gene interaction in which one gene masks the phenotypic expression of another. • There are no new phenotypes produced by this type of gene interaction.

  4. Epistatic versus Hypostatic • The alleles that are masking the effect are called epistatic alleles • The alleles whose effect is being masked are called the hypostatic alleles.

  5. Recessive or Dominant? • Epistasis can be described as either recessive epistasis or dominant epistasis. • Let’s look at an example of recessive epistasis….

  6. Labrador Retrievers • Fur color in Labrador Retrievers is controlled by two separate genes. • Fur color is a polygenic trait! Gene 1: Represented by B : Controls color Gene 2: Represented by E : Controls expression of B

  7. Labrador Retrievers • If a Labrador retriever has a dominant B allele, they will have black fur. • If they have two recessive alleles (bb) they will have brown fur.

  8. Labrador Retrievers • If a retriever receives at least one dominant “E” allele, they will remain the color that the “B” allele coded for. • Either black of brown • However, if a dog receives a pair of homozygous recessive “e” alleles, they will be golden regardless of their “B” alleles!

  9. Labrador Retrievers • BBEE and BbEe --> Black retrievers • bbEE and bbEe --> Brown retrievers • BBee, Bbee, or bbee --> Golden retrievers

  10. Try this cross… • You have decided to cross your golden retriever (bbee) with the neighbor’s chocolate retriever (bbEe). What color pups will they have?

  11. bbee x bbEe • gametes: be • Gametes: bE or be • Genotypes of F1 generation: bbEe and bbee • Pups phenotypes:Brown and golden

  12. Dominant Epistasis • Let’s have a look at dominant epistasis… • Squash fruit color is controlled by two genes. • Gene 1 is represented by a W • Gene 2 is represented by a G

  13. Squash Fruit Color • Genotypes and Phenotypes: • W-/G- white • W-/gg white • ww/G- green • ww/gg yellow

  14. Squash Fruit Color • Which allele is epistatic in squash color? • How do you know? The dominant W allele is epistatic Because every time a dominant W allele shows up in a squash genotype, the squash fruit color is white.

  15. Try this cross…. • Cross a green squash (wwGg) with a white squash (Wwgg). • What color are the offspring?

  16. Wwgg x wwGg • gametes: Wg or wg • gametes: wG or wg • F1 generation genotypes: • Phenotypes:

  17. 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

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

  19. 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

  20. 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

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