Download Mendelian Genetics

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
The Monastery Garden with the greenhouse which
Gregor J. Mendel, O.S.A., had built in 1870. Its appearance
before 1902.Courtesy of Villanova University Archives.
Gregor J. Mendel, O.S.A., experimental garden (35x7
meters) in the grounds of the Augustinian Monastery in
Old Brno.Its appearance before 1922. 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
F1
F2
親代
第一子代
第二子代
F1 Results of One Monohybrid
Cross
F2 Results of
Monohybrid Cross
Mendel’s
Monohybrid
Cross Results
F2 plants showed
dominant-torecessive ratio that
averaged 3:1
5,474 round
1,850 wrinkled
6,022 yellow
2,001 green
882 inflated
299 wrinkled
428 green
152 yellow
705 purple
224 white
651 long stem
207 at tip
787 tall
277 dwarf
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
purple
flowers,
tall
TRUEBREEDING
PARENTS:
AABB
GAMETES:
AB
x
AB
white
flowers,
dwarf
aabb
ab
ab
AaBb
F1 HYBRID
OFFSPRING:
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
AaBb
X
AaBb
Four Phenotypes:
– Tall, purple-flowered
(9/16)
– Tall, white-flowered (3/16)
– Dwarf, purple-flowered
(3/16)
– Dwarf, white-flowered
(1/16)
Explanation of Mendel’s
Dihybrid Results
If the two traits
are coded for by
genes
on separate
chromosomes,
sixteen gamete
combinations are
possible
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
16 Allele
Combinations in F2
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/16
1/16
1/16
1/16
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
1/16
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
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
(line of bell-shaped curve indicates
continuous variation in population)
Range of values for the trait
Number of individuals with
some value of the trait
Number of individuals with
some value of the trait
族群中連續變異
(Continuous variation)
Range of values for the trait
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