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Modifications to Mendelian Ratio Key Points
• Incomplete dominance and codominance• Lethal genes• Gene interaction• Epistasis• Polygenic inheritance• Linkage• Pleiotropy• Incomplete expressivity & incomplete
penetrance
Monohybrid
Aa x Aa
Gamet: A a
F2 (Punnett Square)
A a
A AA Aa
a Aa aa
F2 Genotipic Ratio
Genotype Frequency
AA 1/4
Aa 1/2
aa 1/4
F2 Phenotipic Ratio
Genotype Frequency
A_ 3/4
aa 1/4
Modifications to Mendelian Ratio
F2
Genotype Frequency
AABB 1
AABb 2
AAbb 1
AaBB 2
AaBb 4
Aabb 2
aaBB 1
aaBb 2
aabb 1
Phenotype Frequency
A_B_ 9
A_b_ 3
aaB_ 3
aabb 1
Mendelian dihybrid crossAaBb x AaBb
Modifications in Monohybrid Cross
• Incomplete Dominance
• Codominance
• Lethal genes
Rr x Rr
RR Rr rr
1 : 2 : 1
Incomplete Dominance / Codominance Snapdragon flower colour (1:2:1 ratio)
Y, the yellow-lethal mutation in mice: a dominant visible that is also a recessive lethal.
Lethal Genes
(1:2 ratio)
Modifications in Dihybrid Cross
• Incomplete Dominance/Codominance– in both pairs of alleles (1 example)– on one pair of alleles (1 example)
• Gene Interaction (2 examples)
• Epistasis (5 examples)
• Polygenic Inheritance (1 example)
F2
Genotype Frequency
AABB 1
AABb 2
AAbb 1
AaBB 2
AaBb 4
Aabb 2
aaBB 1
aaBb 2
aabb 1
Phenotype Frequency
A_B_ 9
A_bb 3
aaB_ 3
aabb 1
Mendelian dihybrid crossAaBb x AaBb
Codominance in both pairs IAIBLMLN x IAIBLMLM
Genotype Phenotype Ratio
IAIALMLM AAMM 1
IAIALMLN AAMN 2
IAIALNLN AANN 1
IAIBLMLM ABMM 2
IAIBLMLN ABMN 4
IAIBLNLN ABNN 2
IBIBLMLM BBMM 1
IBIBLMLN BBMN2
IBIBLNLN BBNN 1
Genotype Phenotype
IAIA A
IBIB B
IAIB AB
LMLM M
LNLNN
LMLN MN
Incomplete Dominance in one pair Ddh1h2 x Ddh1h2
Genotype Phenotype
DD/DdTinggi
dd Dwarf
h1h1 Smooth
h1h2 Sparse
h2h2 Hairy
Genotype Phenotype Ratio
D_ h1h1 Tall, Smooth 3
D_ h1h2 Tall, Sparse 6
D_ h2h2 Tall, Hairy 3
dd h1h1 Dwarf, Smooth 1
dd h1h2 Dwarf, Sparse 2
dd h2h2 Dwarf, Hairy 1
Comb shapes in chickens of different breeds.
Gene Interaction
Chicken Comb Shapes (9:3:3:1 ratio)
a. rose, Wyandottes
b. pea, Brahmas
c. walnut, hybrid
d. single, Leghorns
RRpp (rose)
RrPp (walnut)
rrPP (pea)X
9 R_P_ walnut
3 R_pp rose
3 rrP_ pea
1 rrpp single
Gene Interaction Pig Coat Colour (9:6:1 ratio)
AABB x aabb
red white
F1 AaBb
red
F2
9 A_B_ red (interaction between A & B)
3 A_bb sand yellow (because of A)
3 aaB_ sand yellow (because of B)
1 aabb white (no A or B)
Modifications to Mendelian Ratio Key Points
• Incomplete dominance and codominance• Lethal genes• Gene interaction• Epistasis• Polygenic inheritance• Linkage• Pleiotropy• Incomplete expressivity & incomplete
penetrance
Epistasis (recessive)
Mice Coat Colour (9:3:4 ratio) CCaa x ccAA
black albino
F1 CcAa
agouti
F2
9 C_A_ agouti (A determines agouti)
3 C_aa black (a determines black)
3 ccA_ albino (cc epistasis towards A_)
1 ccaa albino (cc epistasis towards aa)
Epistasis (Dominant) Cucurbita pepo fruit colour (12:3:1 ratio)
AABB x aabb
white green
F1 AaBb
white
F2
9 A_B_
3 A_bb
3 aaB_ yellow (B determines yellow)
1 aabb green (b determines green)
12 white (A epistasis towards B_ and bb)
Epistasis Fowl plumage colour (13:3 ratio)
IICC x iicc
white white
F1 IiCc
white
F2
9 I_C_ white (because of inhibitor I)
3 I_cc white (because of I and cc)
3 iiC_ coloured (C determines colour)
1 iicc white (cc has no colour)
Leghorn Wyandotte
Epistasis Corn kernel colour (9:7 ratio)
AAcc x aaCC
yellow yellow
F1 AaCc
purple
F2
9 A_C_ purple (A and C present)
3 A_cc yellow (C absent)
3 aaC_ yellow (A absent)
1 aacc yellow (A and C absent)
SUBSTRATE X SUBSTRATE Y ANTOCYANINE
A B
Gene
Enzyme
A B
Epistasis Seed capsules of the shepherd’s purse (15:1 ratio)
AABB x aabb
triangular ovoid
F1 AaBb
triangular
F2
9 A_B_ triangular (A and B present)
3 A_bb triangular (A present)
3 aaB_ triangular (B present)
1 aabb ovoid (A and B absent)
Only when both pathways are blocked by homozygous recessive alleles is the triangular phenotype suppressed and an ovoid capsule
produced.© 2003 John Wiley and Sons Publishers
A & B completely dominant 9 3 3 1 Mendel’s cross
aa epistasis towards B and b 9 3 4 Mice coat colour
A epistasis towards B and b 12 3 1 Squash fruit colour
A epistasis towards B & b; bb epistasis towards A & a
13* 3 Fowl plumage colour
aa epistasis towards B and b; bb epistasis towards A & a
9 7 Corn kernel colour
A epistasis towards B & b; B epistasis towards A and a
15 1 Fowl leg plumage
Polygenic Inheritance Colour of wheat kernels (1:4:6:4:1 ratio)
Parents: AABB x aabb
dark red white
F1 AaBb medium red
F2 Genotype
1 AABB
4 AABb, AaBB
6 AAbb, aaBB, AaBb
7 aaBb, Aabb
1 aabb
Phenotype
DARK RED
MEDIUM DARK RED
MEDIUM RED
LIGHT RED
WHITE
Pleiotropy: A condition where changes in one gene can affect more than one phenotype. Example: Phenylketonuria – a disease caused by the deficiency of phenylalanine hydroxylase leading to accumulation of phenylalanine in the plasma. Clinical manifestation includes fair skin, blonde hair,mental retardation, musty odour.
Incomplete Expressivity is seen in cases where the same genotype may, for unknown reasons, have variability in their phenotypes. Example: Genetic diseases such as diabetes.
Incomplete Penetrance is seen when an individual with a particular genotype, for unknown reasons, does not express the phenotype.
Modifications to Mendelian Ratio Key Points
• Incomplete dominance and codominance• Lethal genes• Gene interaction• Epistasis• Polygenic inheritance• Linkage• Pleiotropy• Incomplete expressivity & incomplete
penetrance
Berikan hukum-hukum Mendel. Terangkan keadaan-keadaan yang akan memberikan keputusan lain daripada keputusan Mendel.
State Mendel’s laws. Explain the conditions whereby Mendel’s laws were modified.
Times New Roman, font size 12, single spacingMaximum 3 pages (3 x 49 lines) excluding pictures
Last date of submission – 2nd August 2006
Population Genetics
1. Hardy-Weinberg’s law.
2. Allele frequency can be obtained in 2 ways: from the population and from Hardy-Weinberg’s formula.
3. At equilibrium, genotype frequencies can be determined by Hardy-Weinberg’s formula.
4. Factors influencing allele and genotype frequency.