第三章 Mendelian Genetics: How are Traits Inherited?

前言

孟德爾對遺傳模式的洞察力

孟德爾 (Gregor Mendel): 理性與感性的結合 數學與生物

22. 7. 1822 in Hyncice - 6. 1. 1884 in Brno

Brno, Cze

GregorJ. Mendel, O.S.A., experimental garden (35x7 meters) in the grounds of the Augustinian Monastery in Old Brno.Itsappearance before 1922. Courtesy of Villanova University Archives.

The Monastery Garden with the greenhouse whichGregorJ. Mendel, O.S.A., had built in 1870. Its appearance

before 1902.Courtesy of Villanova University Archives.

Mendelianum

Lost of Mendel

1. The ignorance of comtemporary researchers: W.O. Focke and Karl Nageli

2. Lack of knowledge of modern biology.

3. Astray by apomictic propagation

4. The rise of evolution theory emphazised on variations.

5. Personality of humility.

Rediscovery of Mendel

Hugo de Vries: Holland botanist

Carl Correns: German botanist

Erich Von Tschermak-Seysenegg: Autira botanist

Wiiliam Bateson: English biologist, Second father of Genetics

純種系 (True-breeding lineage)雜種子代 (Hybrid off-springs)

P 親代F1 第一子代F2 第二子代

F1 Results of One Monohybrid Cross

F2 Results of Monohybrid Cross

Mendel’s Monohybrid Cross Results

787 tall 277 dwarf

651 long stem

207 at tip

705 purple 224 white

152 yellow428 green

299 wrinkled882 inflated

6,022 yellow 2,001 green

5,474 round 1,850 wrinkled

F2 plants showed dominant-to-recessive ratio that averaged 3:1

單一雜交 (Mono-hybrid crosses)

預測單一雜交的結果

棋盤格法 (Punnett-square method)

雙基因雜交 (Di-hybrid crosses)Experimental cross between individuals

that are homozygous for different versions

of two traits

A Dihybrid Cross - F1 Results

AABB aabbx

AaBb

AB AB ab ab

TRUE-BREEDING PARENTS:

GAMETES:

F1 HYBRID OFFSPRING:

purple flowers, tall

white flowers,dwarf

All purple-flowered, tall

F1 Results of Mendel’s Dihybrid Crosses

• All plants displayed the dominant form of

both traits

• We now know:

– All plants inherited one allele for each trait

from each parent

– All plants were heterozygous (AaBb)

Phenotypic Ratios in F2

Four Phenotypes:– Tall, purple-flowered (9/16)

– Tall, white-flowered (3/16)

– Dwarf, purple-flowered (3/16)

– Dwarf, white-flowered (1/16)

AaBb X AaBb

Explanation of Mendel’s Dihybrid Results

If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible

aB

AB

AB

abAb

Ab

aB

ab

1/4

1/4

1/4

1/4

1/4 1/4 1/4 1/4

AaBb aabbAabb aaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBb aaBB aaBbAaBB

1/161/161/161/16

1/161/161/161/16

1/161/161/161/16

1/16 1/16 1/16 1/16

16 Allele Combinations in F2

aB

AB

AB

abAb

Ab

aB

ab

1/4

1/4

1/4

1/4

1/4 1/4 1/4 1/4

AaBb aabbAabb aaBb

AABB AABb AaBB AaBb

AABb AAbb AaBb Aabb

AaBb aaBB aaBbAaBB

1/161/161/161/16

1/161/161/161/16

1/161/161/161/16

1/16 1/16 1/16 1/16

Tremendous Variation

Number of genotypes possible in offspring

as a result of independent assortment and

hybrid crossing is

3n

(n is the number of gene loci

at which the parents differ)

Impact of Mendel’s Work

• Mendel presented his results in 1865

• Paper received little notice

• Mendel discontinued his experiments in 1871

• Paper rediscovered in 1900 and finally appreciated

Do Mendel’s Laws Always Apply?

Lethality affects the ratio of distribution

顯性關係

不完全顯性 (Incomplete dominance)

共顯性 (Codominance): 如 ,ABO 血型是多重對偶基因系統 (Multiple allele system)

ABO 血型是多重對偶基因系統

單一基因的多重影響

既基因多效性 (Pleiotropy)

如鐮刀型貧血症 (Sickle-cell anemia)

鐮刀型貧血症 (Sickle-cell anemia)

鐮刀型貧血症 (Sickle-cell anemia)

基因對間的交互作用 如 , 上位現象 (Epistasis)

例 : 哺乳類的毛色

•Two genes involved

- One gene influences melanin production

•Two alleles - B (black) is dominant over b (brown)

- Other gene influences melanin deposition

•Two alleles - E promotes pigment deposition and is dominant over e

Albinism• Phenotype results when pathway for

melanin production is completely blocked

• Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway

基因對間的交互作用

例 : 家禽的頭冠形狀

如何解釋較不易預期的變異 有關意外表現型

族群中連續變異

族群中連續變異 (Continuous variation)

Range of values for the trait

Nu

mb

er o

f in

div

idu

als

wit

hso

me

valu

e o

f th

e t

rait

(line of bell-shaped curve indicates continuous variation in population)

Range of values for the traitN

um

ber

of

ind

ivid

ual

s w

ith

som

e va

lue

of

the

tra

it

環境影響表現型的例子Temperature Effects

on Phenotype

• Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway

• Melanin is produced in cooler areas of body

Environmental Effects on Plant Phenotype

• Hydrangea macrophylla

• Action of gene responsible for floral color is influenced by soil acidity

• Flower color ranges from pink to blue