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7. PLEIOTROPI, GEN MODIFIER, PENETRASI DAN EKSPRESIVITAS

7. PLEIOTROPI, GEN MODIFIER, PENETRASI DAN EKSPRESIVITAS. Pleiotrophy. One gene has effect(s) on multiple phenotypes Many examples Cystic fibrosis Marfan syndrome Porphyria variegata Cannot metabolize porphyrin, deep red urine

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7. PLEIOTROPI, GEN MODIFIER, PENETRASI DAN EKSPRESIVITAS

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  1. 7. PLEIOTROPI, GEN MODIFIER, PENETRASI DAN EKSPRESIVITAS

  2. Pleiotrophy • One gene has effect(s) on multiple phenotypes • Many examples • Cystic fibrosis • Marfan syndrome • Porphyria variegata • Cannot metabolize porphyrin, deep red urine • Becomes toxic to brain (also abdominal pain muscular weakness fever insomnia headaches vision problems, delerium, etc.) • King George III of England (U.S. Revolution) may have suffered from condition

  3. GEN MODIFIER • Gen modifier adalah gen yang mengubahsedikitkuantitaskenampakan gen lain (dapatdiukur) • Gen modifier dapatberperansebagaipenghambat (inhibitor genes), pendukung (enhancer genes), penekan (supressor genes). • Gen utamamengaturpenampakanfenotipedarisuatusifattetapimungkinberubahkarenapengaruhbeberapaataubanyak gen minor

  4. GEN MODIFIER • Gen Pendukung (Enhancer genes) • Gen minor dapatsangatmempengaruhi gen utama (major gen) dalammenampakkansuatusifat • Contoh : sifatwarnakepalapadatikus yang disebabkan gen resesif. Seleksiterusmenerusterhadapsifattersebutmeningkatkanpigmentasiwarnabulukarenapeningkatanjumlah gen modifier • Gen penghambat (Inhibitor genes) • Dapatmencegahpenampakkandari gen lain • Contoh : warnakulitpadabawangmerah (epistasisdominandanresesif) • Gen penekan (supressor genes) • Berperanmenekan gen mutansehinggafenotipe yang normal tidaktampakmeskipun gen mutantersebutada. • Contoh : gen resesifsu yang menekan gen Hw pada Drosophila yang menyebabkansayapberbulu. Padapersilanganduatetuatakberbuluketurunannyadapatmempunyaisayapberbulu Hw Hwsusu x hw hw Su Su tidakberbulutidakberbulu F1 Hw hw Su su berbulu Hw = sayapberbulu Su = tidakmenekan Hw

  5. penetrasi • Penetrasi = proporsiindividu yang menampakkanfenotipe yang diharapkan. • Penetrasisempurna (100%) bilasemuaindividumenampakkansifattersebut • Penetrasitidaksempurnabilasifatterlihatpadabeberapaindividutapitidakterlihatpadaindividu lain meskipun gen tersebutterdapatpadasemuaindividutersebut. P : GG x gg kuninghijau F1 : Gg (kuningsemua) F2 : 3 G_ : 1 gg Penetrasi = 100% Sorghum P: RR x rr BatangmerahBatanghijau F1 Rr (batangmerah) F2 : 3 merah : 1 hijau (dibawahsuhu normal) dalampopulasiada 40 tanaman, padasuhutinggidiperolehnisbah 20:20 bukan 30:10. Penetrasi = 66 2/3%

  6. Phenotype Is Not Always a Direct Reflection of Genotype • Penetrance – the percentage of individuals that show at least some degree of expression of the mutant genotype • Partial penetrance • Expressivity – the range of expression of the mutant phenotype • Can be the result of either or both genetic background differences or environmental effects

  7. Expressivity and Penetrance Variable (or Incomplete) Penetrance: Variable Expressivity: Variable Expressivity and Penetrance:

  8. Expressivity • “Eyeless” mutation in Drosophila • Reduces eye size from a partial reduction to complete elimination (average 0.25 to 0.50)

  9. Genetic Background Effects • Genetic suppression – mutant allele at a locus partially or completely restores the wt phenotype of another locus homozygous (or hemizygous) for a mutant allele • Position effect – the physical location of a gene influences its expression (relative position to other genetic material • Translocations or inversions • Heterochromatin effects…

  10. Position Effect • (a) female heterozygote for white eye genotype showing normal dominant phenotype • (b) chromosomal rearrangement leading to variegated effect (also female heterozygote for white eye)

  11. Environmental Effects • Temperature effects • Evening primrose produces red flowers at 23C and white flowers at 18C • Siamese cats and Himalayan rabbits have darker fur on cooler areas of body (tail, feet, ears) • Enzymes lose catalytic function at higher temperature • Temperature sensitive mutations • Mutant allele only expressed (phenotype) at [generally] lower temperature • ts phage mutants, restrictive and permissive temperatures • Heat-shock genes

  12. Environmental Effects:Temperature-sensitive Alleles vestigal alleles himalayan Rabbits

  13. Nutritional Effects • Nutritional mutations • Prevent synthesis of nutrient molecules • Auxotrophs • Phenotype expressed or not depending upon the diet • Phenylketonuria • Loss of enzyme to metabolize phenylalanine • Severe problems unless low Phe diet • Galactosemia (very bad again) and lactose intolerance (unpleasant)…

  14. Terimakasih

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