Mendel Ext

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.