7 PLEIOTROPI GEN MODIFIER PENETRASI DAN EKSPRESIVITAS Pleiotrophy

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 F 1 Hw hw Su su berbulu Hw = sayap 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 kuning F 1 : F 2 : 3 G_ : 1 gg Penetrasi = 100% Sorghum P: x gg hijau Gg (kuning semua) RR x rr Batang merah Batang hijau F 1 Rr (batang merah) F 2 : 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 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 23 C and white flowers at 18 C – 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)…

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