Download Chapter 3. Mendelian Genetics

Chapter 3.
Mendelian Genetics
Introduction
• What genetic principles account for the passing of traits
from parents to offspring?
• The “blending” hypothesis is the idea that genetic
material from the two parents blends together (like blue
and yellow paint blend to make green).
• The “particulate” hypothesis is the idea that parents
pass on discrete heritable units (genes).
• Mendel documented a particulate mechanism through
his experiments with garden peas.
Advent of Experimental Genetics
§ Gregor Mendel discovered principles of genetics
in experiments with the garden pea.
• Mendel showed that parents pass heritable factors
to offspring (heritable factors are now called
genes).
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 of plants can be controlled.
– Each pea plant has sperm-producing organs (stamens) and
egg-producing organs (carpels).
– Cross-pollination (fertilization between different plants) can
be achieved by dusting one plant with pollen from
another.
White
1
Removed
stamens from
purple flower
Stamens
Carpel
Parents
(P)
2
Purple
3
Transferred pollen from stamens of white
flower to carpel of purple flower
Pollinated carpel
matured into pod
4
Offspring
(F1)
Planted seeds
from pod
Traits Used for Mendel’s Experiment
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• Mendel chose to track only those characters that varied in an
either-or manner.
• He also used varieties that were true-breeding (plants that
produce offspring of the same variety when they selfpollinate).
• In a typical experiment, Mendel mated two contrasting, truebreeding 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, the F2 generation is
produced.
Mendel’s Genetic Experiment
Mendel’s Postulates
• Mendel developed a hypothesis to explain the 3:1
inheritance pattern he observed in F2 offspring of the
monohybrid crosses.
• Three related concepts make up this model.
• These concepts can be related to what we now know
about genes and chromosomes.
Unit Factors in Pairs
• Genetic characters are controlled by unit factors that exist in
pairs in individual organisms.
• The unit factors 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.
• Each character an organism inherits two alleles, one from
each parent.
• Mendel made this deduction without knowing about the role
of chromosomes.
• The two alleles at a locus on a chromosome may be identical,
as in the true-breeding plants of Mendel’s P generation.
• Alternatively, the two alleles at a locus may differ, as in the F1
hybrids.
Dominance/Recessiveness
• When two unlike unit factors responsible for a single
character are present in a single individual, one unit factor is
dominant to the other, which is said to be recessive.
• 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.
Segregation
• During the formation of gametes, the paired unit factors
separate or segregate randomly so that each gamete receives
one or the other with equal likelihood.
• 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.
• It, 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.
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 (3:1),
purple to white flowers, in the F2 generation.
Genetic makeup (alleles)
P plants
Gametes
PP
pp
All P
All p
F1 plants
(hybrids)
All Pp
Gametes
1
–
2
1
–
2
P
Sperm
P
F2 plants
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
P
p
PP
Pp
Pp
pp
Eggs
p
p
The Law of Segregation
• 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.
• 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.
Homologous Chromosomes Bear the
Alleles for Each Character
§ For a pair of homologous chromosomes, alleles
of a gene reside at the same locus
• Homozygous individuals have the same allele on
both homologues
• Heterozygous individuals have a different allele on
each homologue
Gene loci
Genotype:
Dominant
allele
P
a
B
P
a
b
PP
aa
Bb
Recessive
allele
Homozygous
Heterozygous
Homozygous
for the
for the
dominant allele recessive allele
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 truebreeding.
• 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.
• In the example of flower color in pea plants, PP and Pp
plants have the same phenotype (purple) but different
genotypes.
3
Phenotype
Genotype
Purple
PP
Purple
(homozygous)
1
Pp
(heterozygous)
2
Purple
1
White
Ratio 3:1
Pp
(heterozygous)
pp
(homozygous)
Ratio 1:2:1
1
• 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.
P Generation
Purple flowers White flowers
Appearance:
Genetic makeup:
PP
pp
Gametes:
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
2
1/
2
P
p
Sperm
F2 Generation
P
p
PP
Pp
Pp
pp
P
Eggs
p
3
1
• Mendel derived the law of segregation by following a
single character.
• The F1 offspring produced in this cross were
monohybrids, individuals that are heterozygous for one
character.
• A cross between such heterozygotes is called a
monohybrid cross.
Question
• P55:
• A recessive mutant allele, black, causes a very dark body
in a fruit fly (Drosophila) when homozygous. The wildtype (normal) color is gray. What F1 phenotypic ratio is
predicted when a black female is crossed with a gray
male whose father was black?
The Law of Independent Assortment
• 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.
• 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.
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P
generation
rryy
RRYY
ry
Gametes RY
rryy
RRYY
RrYy
RrYy
F1
generation
Sperm
Sperm
1
–
2
F2
generation
ry
Gametes RY
1
–
2
RY
1
–
2
1
–
4
ry
RY
Eggs
1
–
2
ry
1
–
4
RY
1
–
4
rY
1
–
4
Ry
Eggs
Hypothesized
(not actually seen)
1
–
4
RY
1
–
4
rY
1
–
4
Ry
1
–
4
ry
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
9
––
16
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
(support hypothesis)
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
Question
• p55:
• Mendel found that full pods are dominant over
constricted pods, while round seeds are dominant over
wrinkled seeds. One of his crosses was between full,
round plants and constricted, wrinkled plants. From his
cross, he obtained an F1 generation that was all full and
round. In the F2 generation, Mendel obtained his classic
9:3:3:1 ratio. Using this information, determine the
expected F1 and F2 results of a cross between
homozygous constricted, round plants and full, wrinkled
plants.
Progeny Test
• How can we tell the genotype of an individual with the
dominant phenotype?
Testcross
• How can we tell the genotype of an individual with the
dominant phenotype?
• 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: breeding the
mystery individual with a homozygous recessive individual.
• If any offspring display the recessive phenotype, the
mystery parent must be heterozygous.
TECHNIQUE
´
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
Predictions
If PP
Sperm
p
P
Eggs
Pp
If Pp
Sperm
or
p
P
Pp
Eggs
P
p
p
Pp
Pp
pp
pp
p
Pp
Pp
RESULTS
or
All offspring purple
1/2
offspring purple and
offspring white
1/2
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The Multiplication and Addition Rules
Applied to Monohybrid Crosses
• The multiplication rule states that the probability that
two or more independent events will occur together is
the product of their individual probabilities.
• Probability in an F1 monohybrid cross can be determined
using the multiplication rule.
• Segregation in a heterozygous plant is like flipping a
coin: Each gamete has a 12 chance of carrying the
dominant allele and a 12 chance of carrying the recessive
allele.
Rr
Rr
´
Segregation of
alleles into sperm
Segregation of
alleles into eggs
Sperm
1/
R
2
R
1/
2
4
r
r
r
R
R
Eggs
2
r
2
R
1/
1/
1/
1/
4
r
r
R
1/
4
1/
4
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
• The rule of addition states that the probability that any
one of two or more exclusive events will occur is
calculated by adding together their individual
probabilities.
• The rule of addition can be used to figure out the
probability that an F2 plant from a monohybrid cross will
be heterozygous rather than homozygous.
Solving Complex Genetics Problems with
the Rules of Probability
• 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 together.
Question
• A plant of genotype PpYyRr is crossed with an Ppyyrr
plant.
• What is the probability of offsprings having at least two
recessive traits?
Question
• A plant of genotype AABbCC is crossed with an AaBbCc
plant.
• What is the probability of an offspring having the
genotype AAbbCC?
Question
• p56:
• Determine the probability that a plant of genotype
CcWw will be produced from parental plants with the
genotypes CcWw and Ccww.
Question
• p56:
• In another cross involving parent plants of unknown
genotype and phenotype, the following offspring were
obtained.
F1:
3/8
3/8
1/8
1/8
full, round
full, wrinkled
constricted, round
constricted, wrinkled
Determine the genotypes and phenotypes of the parents.
Rediscovery of Mendel’s Work
• Mendel’s work was unappreciated and remained
dormant for 34 years.
• Even Darwin’s theories were viewed with skepticism in
the late 1800’s because he could not explain the mode
of inheritance of variation.
• In 1900, 16 years after Mendel died, three scientists
rediscovered and acknowledged Mendel’s work, giving
birth to the science of genetics.
• In 1900, Carl Correns, Hugo de Vries, and Erich von
Tschermak Rediscover and Confirm Mendel’s Laws
Mendelian Inheritance Has Its Physical
Basis in the Behavior of Chromosomes
• Mitosis and meiosis were first described in the late 1800s.
• The chromosome theory of inheritance states:
– Mendelian genes have specific loci (positions) on
chromosomes.
– Chromosomes undergo segregation and independent
assortment.
• The behavior of chromosomes during meiosis was said to
account for Mendel’s laws of segregation and independent
assortment.
Copyright © 2010 Pearson Education, Inc.
Green-wrinkled
seeds ( yyrr)
Yellow-round
seeds (YYRR)
P Generation
Y
Y
R R
r
´
y
y
r
Meiosis
Fertilization
Gametes
R Y
y
r
All F1 plants produce
yellow-round seeds (YyRr)
All F1 plants produce
yellow-round seeds (YyRr)
0.5 mm
F1 Generation
R
y
r
R
Y
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
y
r
Y
LAW OF INDEPENDENT
ASSORTMENT
Meiosis
r
R
Y
y
r
R
Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation.
Metaphase I
Y
y
1
1
r
R
r
R
Y
y
Anaphase I
Y
y
r
R
Metaphase II
R
r
2
2
Gametes
y
Y
Y
R
R
1
4
YR
r
1
3
4
yr
Y
Y
y
r
y
Y
y
Y
r
r
14
Yr
y
y
R
R
14
yR
3
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.
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.
Male
Affected
male
Female
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
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An Example of Pedigree
George
Sandra
Tom
Arlene
Sam
Wilma
Ann
Michael
Carla
Daniel
Alan
Tina
Christopher
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
ww
ww
Ww ww ww Ww
Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
Is a widow’s peak a dominant or recessive trait?
1st generation
(grandparents)
Ff
2nd generation
(parents, aunts,
and uncles)
FF or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
Attached earlobe
Free earlobe
Is an attached earlobe a dominant or recessive trait?
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 (i.e.,
pigmented).
• Albinism is a recessive condition characterized by a lack
of pigmentation in skin and hair.
Parents
Normal
Aa
Normal
´
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
• If a recessive allele that causes a disease is rare, then the
chance of two carriers meeting and mating is low.
• Consanguineous matings (i.e., matings between close
relatives) increase the chance of mating between two
carriers of the same rare allele.
• Most societies and cultures have laws or taboos against
marriages between close relatives.
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 (연골무형성증) is a form of dwarfism
caused by a rare dominant allele.
Parents
Dwarf
Dd
´
Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Normal
Eggs
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Copyright © 2010 Pearson Education, Inc.
Question
p58: Problem #26
Chromosomal View of Alleles
Homologous
chromosomes
Fertilization
Alleles, residing
at the same locus
Meiosis
Paired alleles,
alternate forms
of a gene
Gamete
from other
parent
Haploid gametes
(allele pairs separate)
Diploid zygote
(containing
paired alleles)
Molecular View of Alleles