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 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

GEN MODIFIER Gen modifier adalah gen yang mengubah sedikit kuantitas kenampakan gen lain (dapat diukur) Gen modifier dapat berperan sebagai penghambat (inhibitor genes), pendukung (enhancer genes), penekan (supressor genes). Gen utama mengatur penampakan fenotipe dari suatu sifat tetapi mungkin berubah karena pengaruh beberapa atau banyak gen minor

GEN MODIFIER Gen Pendukung (Enhancer genes) Gen minor dapat sangat mempengaruhi gen utama (major gen) dalam menampakkan suatu sifat Contoh : sifat warna kepala pada tikus yang disebabkan gen resesif. Seleksi terus menerus terhadap sifat tersebut meningkatkan pigmentasi warna bulu karena peningkatan jumlah gen modifier Gen penghambat (Inhibitor genes) Dapat mencegah penampakkan dari gen lain Contoh : warna kulit pada bawang merah (epistasis dominan dan resesif) Gen penekan (supressor genes) Berperan menekan gen mutan sehingga fenotipe yang normal tidak tampak meskipun gen mutan tersebut ada. Contoh : gen resesif su yang menekan gen Hw pada Drosophila yang menyebabkan sayap berbulu. Pada persilangan dua tetua tak berbulu keturunannya dapat mempunyai sayap berbulu Hw Hw su su x hw hw Su Su tidak berbulu tidak berbulu F1 Hw = sayap berbulu Hw hw Su su berbulu Su = tidak menekan Hw

penetrasi Penetrasi = proporsi individu yang menampakkan fenotipe yang diharapkan. Penetrasi sempurna (100%) bila semua individu menampakkan sifat tersebut Penetrasi tidak sempurna bila sifat terlihat pada beberapa individu tapi tidak terlihat pada individu lain meskipun gen tersebut terdapat pada semua individu tersebut. P : GG x gg kuning hijau F1 : Gg (kuning semua) F2 : 3 G_ : 1 gg Penetrasi = 100% Sorghum P: RR x rr Batang merah Batang hijau F1 Rr (batang merah) F2 : 3 merah : 1 hijau (dibawah suhu normal) dalam populasi ada 40 tanaman, pada suhu tinggi diperoleh nisbah 20:20 bukan 30:10. Penetrasi = 66 2/3%

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

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

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

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

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)

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

Environmental Effects: Temperature-sensitive Alleles vestigal alleles himalayan Rabbits

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)