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 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
F1 :
F2 : 3 G_ : 1 gg
Penetrasi = 100%
Sorghum P:
x
gg
hijau
Gg (kuning semua)
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)…