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Chapter 14
Mendel and the Gene Idea
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What principles of inheritance did Gregor Mendel
discover by breeding garden pea plants?
Figure 14.1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Collection of Genes in an organism
Deck of Cards VS
Paint
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Blending hypothesis
(Paint)
수 세대를 건너 재등장하는 형질은 어떻게
설명하나?
VS
Particulate inheritance model
(Deck of Cards = Gene idea)
유전자는 희석되지 않는 상태로 섞여서 세대를
통해 전달될 수 있다
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gregor Mendel (1822-1884)
•Augustinian monk
•Expert plant breeder:
majored in physics and
plant physiology
•Discovered the basic
principles of genetics in
1866 (just seven years after
Darwin’s On the Origin of
Species)
Fig. 2.2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 14.1: Mendel used the scientific
approach to identify two laws of inheritance
•
Mendel discovered the basic principles of heredity by breeding garden
peas in carefully planned experiments
Mendel’s Experimental, Quantitative Approach
•
Advantages of pea plants for genetic study
– There are many varieties with distinct heritable features, or characters
(such as flower color); character variants (such as purple or white flowers)
are called traits
– Mating can be controlled
– Each flower has sperm-producing organs (stamens:수술) and eggproducing organ (carpel:암술)
– Cross-pollination (fertilization between different plants) involves dusting
one plant with pollen from another
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fig. 14-2
TECHNIQUE
Crossing pea plants
1
2
Parental
generation
(P)
Stamens
Carpel
3
4
RESULTS
First
filial
generation
offspring
(F1)
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5
(1) Removed stamens from
purple flower
(2) Transferred spermbearing pollen from
stamens of white flower
to egg-bearing carpel of
purple flower
(3) Pollinated carpel
matured into pod
(4) Planted seeds from pod
(5) Examined offspring: all
purple flowers in first filial
generation offspring (F1)
Key Components of Mendel’s Mission(1)
•
Mendel chose to track only those characters that occurred in two
distinct alternative forms
•
He also used varieties that were true-breeding (순종교배 plants
that produce offspring of the same variety when they self-pollinate)
•
In a typical experiment, Mendel mated two contrasting, true-breeding
varieties, a process called hybridization
•
The true-breeding parents are the P generation
•
The hybrid offspring of the P generation are called the F1 generation
•
When F1 individuals self-pollinate or cross- pollinate with other F1
hybrids, the F2 generation is produced
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(1) The Law of Segregation
•
When Mendel crossed
contrasting, true-breeding
white and purple flowered
pea plants, all of the F1
hybrids were purple
•
When Mendel crossed
the F1 hybrids, many of
the F2 plants had purple
flowers, but some had
white
•
Mendel discovered a
ratio of about three to
one, purple to white
flowers, in the F2
generation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Q: When F1 hybrid pea plants are allowed to
self-pollinate, which traits appear in the F2
generation?
•
Mendel reasoned that only the
purple flower factor was affecting
flower color in the F1 hybrids
•
Mendel called the purple flower
color a dominant trait and the
white flower color a recessive trait
•
The factor for white flowers was not
diluted or destroyed because it
reappeared in the F2 generation
•
Mendel observed the same pattern of
inheritance in six other pea plant
characters, each represented by two
traits
•
What Mendel called a “heritable
factor” is what we now call a gene
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s Model
•
Mendel developed a hypothesis to explain the 3:1 inheritance pattern
he observed in F2 offspring
•
Four related concepts make up this model
• These concepts can be related to what we now know about
genes and chromosomes
•
(1) Alternative versions of genes
account for variations in inherited
characters
•
For example, the gene for flower color in
pea plants exists in two versions, one for
purple flowers and the other for white
flowers
•
These alternative versions of a gene are
now called alleles
•
Each gene resides at a specific locus on
a specific chromosome
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
The second concept is that for each character an organism inherits two
alleles, one from each parent
•
Mendel made this deduction without knowing about the role of chromosomes
•
The third concept is that if the two alleles at a locus differ, then one (the
dominant allele) determines the organism’s appearance, and the other (the
recessive allele) has no noticeable effect on appearance
•
In the flower-color example, the F1 plants had purple flowers because the
allele for that trait is dominant
•
The fourth concept (now known as the law of segregation) states that the
two alleles for a heritable character separate (segregate) during gamete
formation and end up in different gametes
•
Thus, an egg or a sperm gets only one of the two alleles that are present in the
somatic cells of an organism
•
This segregation of alleles corresponds to the distribution of homologous
chromosomes to different gametes in meiosis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
Mendel’s segregation
model accounts for the 3:1
ratio he observed in the F2
generation of his
numerous crosses
•
The possible combinations
of sperm and egg can be
shown using a Punnett
square, a diagram for
predicting the results of
a genetic cross between
individuals of known
genetic makeup
•
A capital letter represents
a dominant allele, and a
lowercase letter represents
a recessive allele
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Useful Genetic Vocabulary
•
An organism with two identical alleles for a character is said to be
homozygous for the gene controlling that character
•
An organism that has two different alleles for a gene is said to be
heterozygous for the gene controlling that character
•
Unlike homozygotes, heterozygotes are not true-breeding
표현형(phenotype) 과 유전자형 (genotype)
•
Because of the different effects of dominant
and recessive alleles, an organism’s traits do
not always reveal its genetic composition
•
Therefore, we distinguish between an organism’s
phenotype, or physical appearance, and its
genotype, or genetic makeup (= composition)
•
In the example of flower color in pea plants,
PP and Pp plants have the same phenotype
(purple) but different genotypes (see next
slide)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Testcross의 활용도
•
우성인 표현형을 보고
유전자형을 어떻게 예측할 수
있을까?
•
Such an individual must have
one dominant allele, but the
individual could be either
homozygous dominant or
heterozygous
•
The answer is to carry out a
testcross (검정교배): breeding
the mystery individual with a
homozygous recessive
individual
•
If any offspring display the
recessive phenotype, the
mystery parent must be
heterozygous
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(2) The Law of Independent Assortment
•
Mendel derived the law of segregation by following a single character
•
The F1 offspring produced in this cross were monohybrids (단성잡종),
individuals that are heterozygous for one character
•
A cross between such heterozygotes is called a monohybrid cross
•
Mendel identified his second law of inheritance by following two
characters at the same time
•
Crossing two true-breeding parents differing in two characters
produces dihybrids in the F1 generation, heterozygous for both
characters
•
A dihybrid cross, a cross between F1 dihybrids, can determine
whether two characters are transmitted to offspring as a package or
independently
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Q: How are two characters transmitted from
parents to offspring?
– As a package?
– Independently?
Do the alleles for one character assort into
gametes dependently or independently of
the alleles for a different character?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fig. 14-8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR

F1 Generation
YyRr
Hypothesis of
dependent
assortment
Predictions
yr
Hypothesis of
independent
assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
4
Sperm
1/ YR 1/
2
2 yr
1/
4
1/
2
YR
1/
4
1/
4
Yr
yR
1/
4
yr
YR
YYRR YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
Eggs
1/
2
YyRr
1/
4
Yr
Eggs
yr
YyRr
3/
4
yyrr
1/
4
yR
1/
4
Phenotypic ratio 3:1
1/
4
yr
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
32
Phenotypic ratio approximately 9:3:3:1
Proposed biological mechanism of the shuffling: The Law of
Independent Assortment
• Each pair of alleles assorts
independently of gamete
formation
• The allele for pea shape in
a Y-carrying gamete can be
either R or r with equal
chance and vice versa for a
gamete with y
Y
y
• Gametes in F1:
YR:Yr: yR:yr = 1:1:1:1
R
 Possible zygotes in F2: 16
r
 Genotypes: 9
Fig. 2.16
Ref: Genetics edited by Hartwell et al.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
 Phenotypes: 4
Dihybrid cross shows parental and recombinant types
• Mating of yellow round &
green wrinkled
• F1 dihybrid: all dominant
phenotypes of yellow round
Pollens
Eggs
















Fig. 2.15 top
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• F1 self-fertilization:
production of F2 progeny
with parental and
recombinant types
•
1=2=2=4 (9): Y–R–
1=2 (3):
Y–rr
1=2 (3):
yyR–
1 (1):
yyrr
•
Using a dihybrid cross, Mendel developed the law of independent
assortment
•
The law of independent assortment states that each pair of alleles
segregates independently of each other pair of alleles during gamete
formation
•
Strictly speaking, this law applies only to genes on different, nonhomologous
chromosomes
•
Genes located near each other on the same chromosome tend to be
inherited together
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
확률의 법칙이 지배하는 멘델법칙!!!
• Concept 14.2: The laws of probability
govern Mendelian inheritance
•
Mendel’s laws of
segregation and
independent assortment
reflect the rules of
probability
•
When tossing a coin, the
outcome of one toss has
no impact on the outcome
of the next toss
•
In the same way, the
alleles of one gene
segregate into gametes
independently of another
gene’s alleles
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Multiplication and Addition Rules
Applied to Monohybrid Crosses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Solving Complex Genetics Problems
•
We can apply the
multiplication and
addition rules to predict
the outcome of crosses
involving multiple
characters
•
A dihybrid or other
multicharacter cross is
equivalent to two or more
independent monohybrid
crosses occurring
simultaneously
•
In calculating the
chances for various
genotypes, each
character is considered
separately, and then the
individual probabilities
are multiplied
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
유전현상은 멘델법칙의 예측보다 훨씬
복잡하다
• Concept 14.3: Inheritance patterns are
often more complex than predicted by
simple Mendelian genetics
•
The relationship between genotype and phenotype is rarely as simple
as in the pea plant characters Mendel studied
•
Many heritable characters are not determined by only one gene
with two alleles
•
However, the basic principles of segregation and independent
assortment apply even to more complex patterns of inheritance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Extending Mendelian Genetics for a Single Gene
•
Inheritance of characters by a single gene may deviate (=escape) from simple
Mendelian patterns in the following situations:
–
When alleles are not completely dominant or recessive (Degrees of
dominance)
–
When a gene has more than two alleles (Multiple alleles)
–
When a gene produces multiple phenotypes (Pleiotropy)
우성의 정도에 따른 경우 (Degrees of Dominance)
•
Complete dominance occurs when phenotypes of the heterozygote and
dominant homozygote are identical
•
In incomplete dominance, the phenotype of F1 hybrids is somewhere
between the phenotypes of the two parental varieties
•
In codominance, two dominant alleles affect the phenotype in separate,
distinguishable ways
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Spectrum of Dominance
Summary of dominance relationships
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.10 Incomplete
dominance in snapdragon
color (금어초).
P Generation
White
CWCW
Red
CRCR
금어초/용머리
모양꽃/
Confession
CR
Gametes
CW
F1 Generation
Pink
CRCW
1/
Gametes 1/2 CR
2
CW
Sperm
1/
F2 Generation
 Unlike the Mendel’s
monohybrid, the phenotypic
and genotypic ratios are
identical
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
1/
2
CR
1/
2
CW
Eggs
2
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
•
In codominance
–
•
Two dominant alleles affect the phenotype in separate,
distinguishable ways
The human blood group MN (another example: blood type AB)
–
Is an example of codominance
–
M or N homozygote have red blood cells with only M or N
molecules, respectively
–
MN heterozygote have RBC cells with both M and N
molecules (not intermediate between the M and N
phenotypes)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Relation Between Dominance and Phenotype
•
대립유전자의 Dominance는 우성대립유전자가 열성대립유전자를 정복하는
방식이 아니라 표현형에서 우성임이 확인되기 때문이다 (A dominant allele
does not subdue a recessive allele; alleles don’t interact that way)
•
Alleles are simply variations in a gene’s nucleotide sequence
•
For any character, dominance/recessiveness relationships of alleles
depend on the level at which we examine the phenotype
우성와 표현형간의 관계는 관찰대상인 수준에 따라 달라진다
•
Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation
of lipids in the brain
– At the organismal level, the allele is recessive (recessive
homozygote만 질병증상이 나타남)
– At the biochemical level, the phenotype (i.e., the enzyme activity level)
is incompletely dominant (heterozygote의 경우 효소활성이 중간
정도임) no symptom (절반의 효소활성만으로 충분한 기능 발휘)
– At the molecular level, the alleles are codominant (heterozygote의
경우 정상효소와 비정상효소분자의 개수는 동일함)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Multiple Alleles
•
Most genes exist in populations in
more than two allelic forms
•
For example, the four phenotypes
of the ABO blood group in humans
are determined by three alleles
for the enzyme (I) that attaches A
or B carbohydrates to red blood
cells: IA, IB, and i (see Next slide)
•
The enzyme encoded by the IA
allele adds the A carbohydrate,
whereas the enzyme encoded by
the IB allele adds the B
carbohydrate; the enzyme
encoded by the i allele adds
neither
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pleiotropy
•
Most genes have multiple phenotypic effects, a property called
pleiotropy
•
For example, pleiotropic alleles are responsible for the multiple
symptoms of certain hereditary diseases, such as cystic fibrosis and
sickle-cell disease
낫형 빈혈증의 다양한 증상
Multiple defects in the homozygous
SS individual : muscle cramps
(근육경련), short breath, fatigue,
resistance to Malaria
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Extending Mendelian Genetics for Two or More Genes
•
Some traits may be determined by two or more genes
Epistasis
•
In epistasis, a gene at one locus alters the phenotypic expression of a gene at
a second locus (= One gene’s alleles mask [hide,
completely interfere with] the effects of another gene’s alleles)
•
For example, in mice and many other mammals, coat color depends on two
genes
•
One gene determines the pigment color (with alleles B for black and b for
brown)
•
The other gene (with alleles C for color and c for no color) determines whether
the pigment will be deposited in the hair
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Coat color in Labrador retrievers
• Coat color depends on the allelic combinations of two coat color genes
• The dominant B allele of the 1st gene determines black
• The recessive bb homozygote is brown
• The dominant E allele of the 2nd gene has no effect on black or brown coat
color
• The recessive homozygotic alleles (ee) hide the effect of any
combinations of the 1st gene alleles to produce golden color
• Recessive epistasis : the allele causing the epistasis is recessive
Same Mom?
Baby adopted?
Like or Unlike?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.12
An example
of Epistasis
BbEe
Eggs
1/
4 BE
1/
4 bE
1/
4
Be
1/
4
be
Sperm
1/ BE
4
1/
BbEe
4 bE
1/
4
Be
1/
4
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
9
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
: 3
: 4
be
Polygenic Inheritance (다원 유전자 유전)
멘델 연구의 특징은 either-or basis 임. 그러나 많은 수의 형질 (인간의
키나 피부색 등)은 집단내에서 점진적인 변이를 보이므로 either-or
특징으로 분류할 수 없는 정성적인 캐릭터임.
•
Quantitative characters
are those that vary in the
population along a
continuum (in gradations)
•
Quantitative variation
usually indicates
polygenic inheritance, an
additive effect of two or
more genes on a single
phenotype
•
Skin color in humans is an
example of polygenic
inheritance (see Next slide)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Nature and Nurture (천성과 교육): The
Environmental Impact on Phenotype
•
Another departure from
Mendelian genetics arises
when the phenotype for a
character depends on
environment as well as
genotype
•
The norm of reaction (표현형
표준범위) is the phenotypic
range of a genotype influenced
by the environment
•
For example, hydrangea
flowers (수국) of the same
genotype range from blueviolet to pink, depending on
soil acidity (see next slide)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
•
Norms of reaction are generally
broadest for polygenic characters
•
Such characters are called
multifactorial because genetic
and environmental factors
collectively influence phenotype
• Concept 14.4: Many human traits follow
Mendelian patterns of inheritance
•
Humans are not good subjects for genetic research
– Generation time is too long
– Parents produce relatively few offspring
– Breeding experiments are unacceptable
•
However, basic Mendelian genetics endures as the foundation of
human genetics
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pedigree Analysis
•
A pedigree is a family
tree that describes the
interrelationships of
parents and children
across generations
•
Inheritance patterns
of particular traits
can be traced and
described using
pedigrees
•
Pedigrees can also be used to make predictions about future offspring
•
We can use the multiplication and addition rules to predict the probability of
specific phenotypes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Recessively Inherited Disorders
•
Many genetic disorders are inherited in a recessive manner
•
These range from relatively mild to life-threatening
The Behavior of Recessive Alleles
•
Recessively inherited disorders
show up only in individuals
homozygous for the allele
•
Carriers are heterozygous
individuals who carry the
recessive allele but are
phenotypically normal; most
individuals with recessive
disorders are born to carrier
parents
•
Albinism is a recessive condition
characterized by a lack of
pigmentation in skin and hair
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cystic Fibrosis (낭포성 섬유증)
•
Cystic fibrosis is the most common lethal genetic disease in the United States,striking
one out of every 2,500 people of European descent
•
The cystic fibrosis allele results in defective or absent chloride transport channels
in plasma membranes
•
Symptoms include mucus buildup in some internal organs and abnormal absorption of
nutrients in the small intestine
Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications
•
Sickle-cell disease affects one out of 400 African-Americans
•
The disease is caused by the substitution of a single amino acid in the hemoglobin protein in
red blood cells
•
In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
•
Symptoms include physical weakness, pain, organ damage, and even paralysis
•
Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some
symptoms
•
About one out of ten African Americans has sickle cell trait, an unusually high frequency of
an allele with detrimental effects in homozygotes
•
Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to
being heterozygous
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dominantly Inherited Disorders
•
Some human disorders are caused by dominant alleles
•
Dominant alleles that cause a lethal disease are rare and arise by mutation
•
Achondroplasia is a form of dwarfism caused by a rare dominant allele
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Huntington’s Disease: A Late-Onset Lethal Disease
•
The timing of onset of a disease significantly affects its inheritance
•
Huntington’s disease is a degenerative disease of the nervous
system
•
The disease has no obvious phenotypic effects until the individual is
about 35 to 40 years of age
•
Once the deterioration of the nervous system begins the condition is
irreversible and fatal
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Multifactorial Disorders
• Many diseases, such as heart disease and
cancer, have both genetic and environmental
components
• Little is understood about the genetic
contribution to most multifactorial diseases
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Genetic Testing and Counseling
•
Genetic counselors
–
•
Can provide information to prospective parents concerned about a family
history for a specific disease
Using family histories
–
Genetic counselors help couples determine the odds that their children
will have genetic disorders
Fetal Testing
•
In amniocentesis, the liquid that
bathes the fetus is removed and
tested
•
In chorionic villus sampling
(CVS), a sample of the placenta is
removed and tested
•
Other techniques, such as
ultrasound and fetoscopy, allow
fetal health to be assessed visually
in utero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings