Transcript Pp - HCC Learning Web

Mendel and the Gene Idea
© 2011 Pearson Education, Inc.
Introduction
• In 1865, Gregor Mendel worked out the rules of inheritance
through a series of brilliant experiments on garden peas.
• Early in the 20th century, Walter Sutton and Theodor Boveri
formulated the chromosome theory of inheritance, which
proposes that meiosis causes the patterns of inheritance that
Mendel observed.
• Genetics is the branch of biology that focuses on
inheritance.
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Mendel’s Experimental System
• Gregor Mendel was a 19th-century monk and active member
of his city’s Agricultural Society.
• Mendel was interested in heredity. Heredity is the
transmission of traits from parents to their offspring. A trait is
any characteristic of an individual.
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What Question Was Mendel Trying to Answer?
• Mendel was addressing the basic question of why offspring
resemble their parents and how transmission of traits occurs.
• In his time, two hypotheses had been formulated to try to
answer this question:
1. Blending inheritance – parental traits blend such that
their offspring have intermediate traits.
2. Inheritance of acquired characteristics – parental
traits are modified and then passed on to their offspring.
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Garden Peas: The First Model Organism in Genetics
• Genetics, the branch of biology that focuses on the
inheritance of traits, uses model organisms because the
conclusions drawn from them can be applied to other
species.
• Mendel chose the common garden pea (Pisum sativum) as
his model organism because:
– It is easy to grow.
– Its reproductive cycle is short.
– It produces large numbers of seeds.
– Its matings are easy to control.
– Its traits are easily recognizable.
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Pea plants were particularly well suited for use
in Mendel's breeding experiments for all of the
following reasons except that
a) peas show easily observed variations in a number of characters,
such as pea shape and flower color.
b) it is possible to control matings between different pea plants.
c) it is possible to obtain large numbers of progeny from any given
cross.
d) peas have an unusually long generation time.
e) many of the observable characters that vary in pea plants are
controlled by single genes.
© 2011 Pearson Education, Inc.
Pea plants were particularly well suited for use
in Mendel's breeding experiments for all of the
following reasons except that
a) peas show easily observed variations in a number of characters,
such as pea shape and flower color.
b) it is possible to control matings between different pea plants.
c) it is possible to obtain large numbers of progeny from any given
cross.
d) peas have an unusually long generation time.
e) many of the observable characters that vary in pea plants are
controlled by single genes.
© 2011 Pearson Education, Inc.
How Did Mendel Arrange Matings?
• Peas normally pollinate themselves, a process called selffertilization.
• Mendel could prevent this by removing the male reproductive
organs containing pollen from each flower. He then used this
pollen to fertilize the female reproductive organs of flowers
on different plants, thus performing cross-pollination.
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© 2011 Pearson Education, Inc.
What Traits Did Mendel Study?
• Mendel worked with pea varieties that differed in seven easily
recognizable traits:
- Seed shape
- Seed color
- Pod shape
- Pod color
- Flower color
- Flower and pod position
- Stem length
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• An individual’s observable features comprise its
phenotype. Mendel’s pea population had two
distinct phenotypes for each of the seven traits.
• Mendel worked with pure lines that produced
identical offspring when self-pollinated. He used
these plants to create hybrids by mating two
different pure lines that differed in one or more
traits.
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Inheritance of a Single Trait
• Mendel's first experiments involved crossing pure lines that
differed in just one trait.
• The adults in the cross were the parental generation, the
offspring are the F1 generation (for "first filial").
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P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
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224 white
flowered
plants
The Monohybrid Cross
• Mendel’s first experimented with crossing plants that differed
in only one trait.
• When Mendel crossed plants with round seeds and plants
with wrinkled seeds, all of the F1 offspring had round seeds.
– This contradicted the hypothesis of blending inheritance.
– The genetic determinant for wrinkled seeds seemed to
have disappeared.
• Mendel allowed the F1 progeny to self-pollinate.
– The wrinkled seed trait reappeared in the next F2
generation.
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RESULT
PREDICTION
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Dominant and Recessive Traits
• Mendel called the genetic determinant for wrinkled seeds
recessive and the determinant for round seeds dominant.
– In modern genetics, the terms dominant and recessive
identify only which phenotype is observed in individuals
carrying two different genetic determinants.
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• Mendel repeated these experiments
with each of the other traits.
In each case, the dominant trait
was present in a 3:1 ratio over the
recessive trait in the F2 generation.
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A cross between homozygous purple-flowered and
homozygous white-flowered pea plants results in
offspring with purple flowers. This demonstrates
a)
b)
c)
d)
e)
the blending model of genetics.
true-breeding.
dominance.
a dihybrid cross.
the mistakes made by Mendel.
© 2011 Pearson Education, Inc.
A cross between homozygous purple-flowered and
homozygous white-flowered pea plants results in
offspring with purple flowers. This demonstrates
a)
b)
c)
d)
e)
the blending model of genetics.
true-breeding.
dominance.
a dihybrid cross.
the mistakes made by Mendel.
© 2011 Pearson Education, Inc.
A Reciprocal Cross
• Mendel wanted to determine if gender influenced inheritance.
• He performed a reciprocal cross, in which the mother's
phenotype in the first cross is the father's phenotype in the
second cross, and the father's phenotype in the first cross is
the mother's phenotype in the second cross.
• The results of the two crosses were identical. This
established that it does not matter whether the genetic
determinants for seed shape are located in the male or
female parent.
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© 2011 Pearson Education, Inc.
Particulate Inheritance
• To explain these results, Mendel proposed a hypothesis
called particulate inheritance, which suggests that
“hereditary determinants” maintain their integrity from
generation to generation.
This directly contradicts both the blending inheritance and
inheritance of acquired characteristics hypotheses.
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Genes, Alleles, and Genotypes
• “Hereditary determinants” for a trait are now called genes.
• Mendel also proposed that each individual has two versions
of each gene. Today these different versions of a gene are
called alleles. Different alleles are responsible for the
variation in the traits that Mendel studied.
• The alleles found in an individual are called its genotype. An
individual’s genotype has a profound effect on its phenotype.
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Allele for purple flowers
Locus for flower-color gene
Pair of
homologous
chromosomes
Allele for white flowers
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The Principle of Segregation
• Mendel developed the principle of segregation: the two
members of each gene pair must segregate—that is,
separate—into different gamete cells during the formation of
eggs and sperm in the parents.
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Genetic Notational Convention
• Mendel used a letter to indicate the gene for a particular trait.
For example, R represented the gene for seed shape. He
used uppercase (R) to show a dominant allele and lowercase
(r) for a recessive allele.
• Individuals have two alleles of each gene.
– Individuals with two copies of the same allele (RR or rr)
for a gene are said to be homozygous.
– Those with different alleles (Rr) are heterozygous.
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Crossing Pure Lines
• Pure-line individuals always produce offspring with the same
phenotype because they are homozygous—no other allele is
present.
• A mating between two pure lines that differ in one trait (RR
and rr) results in offspring that all have a heterozygous
genotype (Rr) and a dominant phenotype.
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The Monohybrid Cross
• A mating of two heterozygous parents results in offspring that
are ¼ RR, ½ Rr, and ¼ rr, which produces a 3:1 ratio of
“phenotypes”.
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© 2011 Pearson Education, Inc.
Testing the Model
• Mendel's genetic model—a set of hypotheses that explains
how a particular trait is inherited—explains the results of
these crosses.
• A Punnett square is now used to predict the genotypes and
phenotypes of the offspring from a cross.
© 2011 Pearson Education, Inc.
Producing a Punnett Square
1. Write the gamete genotypes for one parent along the top of
the diagram.
2. Write the gamete genotypes for the other parent down the left
side of the diagram.
3. Draw empty boxes under the row and to the right of the
column of gametes.
4. Fill in each box with the genotypes written at the top of the
corresponding column and at the left of the corresponding
row.
5. Predict the ratios of each possible offspring genotype and
phenotype by tallying the resulting genotypes in all the boxes.
© 2011 Pearson Education, Inc.
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Sperm from F1 (Pp) plant
F2 Generation
P
Eggs from
F1 (Pp) plant
p
P
p
PP
Pp
Pp
pp
3
© 2011 Pearson Education, Inc.
:1
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
© 2011 Pearson Education, Inc.
1
Imagine crossing a pea heterozygous at the loci for flower
color (white versus purple) and seed color (yellow versus
green) with a second pea homozygous for flower color
(white) and seed color (yellow). What types of gametes
will the first pea produce?
– two gamete types: white/white and purple/purple
– two gamete types: white/yellow and purple/green
– four gamete types: white/yellow, white/green, purple/yellow,
purple/green
– four gamete types: white/purple, yellow/green,white/white, and
purple/purple
– one gamete type: white/purple/yellow/green
© 2011 Pearson Education, Inc.
Imagine crossing a pea heterozygous at the loci for flower
color (white versus purple) and seed color (yellow versus
green) with a second pea homozygous for flower color
(white) and seed color (yellow). What types of gametes
will the first pea produce?
– two gamete types: white/white and purple/purple
– two gamete types: white/yellow and purple/green
– four gamete types: white/yellow, white/green, purple/yellow,
purple/green
– four gamete types: white/purple, yellow/green,white/white, and
purple/purple
– one gamete type: white/purple/yellow/green
© 2011 Pearson Education, Inc.
Mendel’s Experiments with Two Traits
• Mendel used dihybrid crosses—matings between parents
that are both heterozygous for two traits—to determine
whether the principle of segregation holds true if parents
differ in more than one trait.
• Mendel’s experiments tested two contrasting hypotheses:
1. Independent assortment, in which alleles of different
genes are transmitted independently of each other.
2. Dependent assortment, wherein the transmission of one
allele depends upon the transmission of another.
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The Principle of Independent Assortment
• Mendel’s results supported the principle of independent
assortment.
• The Punnett square that results from a dihybrid cross
predicts:
• There should be 9 different offspring genotypes and 4
phenotypes.
• The four possible phenotypes should be present in a ratio
of 9:3:3:1.
• Based on these data, Mendel accepted the hypothesis that
alleles of different genes are transmitted independently of
one another.
• This result became known as the principle of independent
assortment.
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© 2011 Pearson Education, Inc.
Using a Testcross to Confirm Predictions (Is it RR
or Rr?? OR RRYY or RrYy??)
• In a testcross, a parent that is homozygous recessive for a
particular trait is mated with a parent that has the dominant
phenotype but an unknown genotype. Because the genetic
contribution of the homozygous recessive parent is known,
the genotype of the other parent can be inferred from the
results.
• Mendel used the testcross to further confirm the principle of
independent assortment.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
TECHNIQUE
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
If purple-flowered
parent is PP
Sperm
p
p
Recessive phenotype,
known genotype:
pp
or
If purple-flowered
parent is Pp
Sperm
p
p
P
Pp
Eggs
P
Pp
Eggs
P
p
Pp
Pp
Pp
Pp
pp
pp
RESULTS
or
All offspring purple
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1/
2
offspring purple and
1/ offspring white
2
The Chromosome Theory of Inheritance
• The chromosome theory of inheritance arose out of Sutton
and Boveri’s careful observations of meiosis. It states that
chromosomes are composed of Mendel’s hereditary
determinants, or what we now call genes.
• The physical separation of alleles during anaphase of
meiosis I is responsible for Mendel’s principle of
segregation.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
The Chromosome Theory of Inheritance
• The genes for different traits assort independently of one
another at meiosis I because they are located on different
nonhomologous chromosomes, which themselves assort
independently.
• This phenomenon explains Mendel’s principle of independent
assortment.
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© 2011 Pearson Education, Inc.
W
w
Recessive allele
For seed color
Dominant allele
For seed color
W Ww w
W
W
w
W
W
w
w
w
Principle of DEPENDENT ASSORTMENT
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Testing the Chromosome Theory
• Early in the 20th century, Thomas Hunt Morgan adopted fruit
flies (Drosophila melanogaster) as a model organism for
genetic research.
• Morgan’s first goal was to identify different phenotypes.
• He called the most common phenotype for each trait
wild-type.
• He then inferred that phenotypes that differed from the
wild-type resulted from a mutation, or a change in a
gene.
• Individuals with traits attributable to mutation are
known as mutants.
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Thomas Hunt Morgan’s Experiments
((Relationship between the sex of the progeny and the inheritance))
• Morgan identified red eyes as the wild-type for eye color, and
white eyes as a mutation.
• When he mated a wild-type female fly with a mutant male fly,
all of the F1 progeny had red eyes.
• However, when Morgan did the reciprocal cross, the F1
females had red eyes but the F1 males had white eyes.
• These experiments suggest a relationship between the sex of
the progeny and the inheritance of eye color in Drosophila.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
The Discovery of Sex Chromosomes
• Nettie Stevens analyzed beetle karyotypes and found that
females’ diploid cells contain 20 large chromosomes; but
males’ diploid cells have 19 large and 1 small (Y)
chromosomes.
– Y chromosomes pair with the large X chromosome during
meiosis I.
• X and Y chromosomes are now called sex chromosomes—
they determine the sex of the offspring.
– In beetles, females have two X chromosomes while
males have an X and Y.
– Other species have other systems.
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Sex Linkage and the Chromosome Theory
• Sex chromosomes pair during meiosis I and then segregate
during meiosis II.
– This results in gametes with either an X or a Y
chromosome.
• Females produce all X gametes.
• Males produce half X gametes and half Y gametes.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
X-Linked Inheritance
• Morgan put together his experimental results with Stevens’
observations on sex chromosomes, and proposed that the
gene for white eye color in fruit flies is located on the X
chromosome and that the Y chromosome does not carry an
allele of this gene.
• Morgan's hypothesis is called X-linked inheritance (or Xlinkage). Females (XX) would then have two copies of the
gene and males (XY) would have only one.
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© 2011 Pearson Education, Inc.
X-Linked Inheritance and the Chromosome Theory
• The various inheritance patterns that can occur when genes
are carried on the sex chromosomes, such that females and
males have different numbers of alleles of that gene, is
termed sex-linked inheritance or sex-linkage.
• Non-sex chromosomes are called autosomes. Genes on
autosomes are said to show autosomal inheritance.
• The discovery of X-linked inheritance convinced most
biologists that the chromosome theory of inheritance was
correct.
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Genes Can Be Located on the Same Chromosome
• The physical association of two or more genes found on the
same chromosome is called linkage.
• Linked genes are predicted to always be transmitted together
during gamete formation and thus should violate the principle
of independent assortment.
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• Independent assortment does not apply to
linked genes.
• Linked genes segregate together EXCEPT
when crossing over and genetic recombination
happens.
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The First Studies of Linked Genes
• Morgan proposed that gametes with new, recombinant
genotypes were generated when crossing over occurred
during prophase of meiosis I in the females.
• Linked genes are inherited together unless crossing over
occurs. When crossing over takes place, genetic
recombination occurs.
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© 2011 Pearson Education, Inc.
Linkage Mapping
• Data on the percentage of recombinant offspring can be used
to estimate the location of genes, relative to one another, on
the same chromosome.
• Data on the frequency of crossing over can be used to create
a genetic map—a diagram showing the relative positions of
genes along a particular chromosome.
• Morgan proposed that genes are more likely to cross over
when they are far apart from each other than when they are
close together.
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© 2011 Pearson Education, Inc.
Extending Mendel’s Rules
• By studying a simple genetic system, Mendel discovered the
most fundamental rules of inheritance.
• However most genes are inherited in a more complex fashion
than were the traits Mendel studied in garden peas.
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Incomplete Dominance and Codominance
• Alleles of a gene are not always clearly dominant or
recessive. In some cases, incomplete dominance occurs,
and the heterozygotes have an intermediate phenotype.
• A heterozygous organism that displays the phenotype of both
alleles of a single gene is said to display codominance. In
this situation, neither allele is dominant or recessive to the
other.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
(a) The three alleles for the ABO blood groups and their
carbohydrates
IA
Allele
Carbohydrate
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
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Multiple Alleles and Polymorphic Traits
• Many genes have more than two alleles, a situation known
as multiple allelism.
• When more than two distinct phenotypes are present in a
population due to multiple allelism, the trait is called
polymorphic.
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Quantitative Traits
• Mendel worked with discrete traits, or characteristics that
are qualitatively different. In garden peas, seed color is either
yellow or green—no intermediate phenotypes exist.
– Traits that are not discrete but instead fall into a
continuum are called quantitative traits.
• Nilsson-Ehle proposed that when many genes each
contribute a small amount to the value of a quantitative trait,
then the population usually exhibits a bell-shaped curve, or
normal distribution, for the trait.
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© 2011 Pearson Education, Inc.
Quantitative Traits
• Nilsson-Ehle used wheat to propose why the distribution of
kernel color exhibited a normal distribution.
• Transmission of quantitative traits results from polygenic
inheritance, in which each gene adds a small amount to the
value of the phenotype.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
AaBbCc
AaBbCc
Sperm
1/
1/
8
8
1/
1/
Eggs
8
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
1/
8
1/
1/
8
1/
8
1/
8
1/
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
© 2011 Pearson Education, Inc.
20/
64
3
15/
64
4
6/
64
5
1/
64
6
Applying Mendel’s Rules to Humans
• Because experimental crosses cannot be done in humans,
pedigrees—family trees—are used to analyze the human
crosses that already exist.
• Pedigrees record the genetic relationships among the
individuals in a family, along with each person’s sex and
phenotype for the trait being studied.
© 2011 Pearson Education, Inc.
Key
Male
1st
generation
Affected
female
Affected
male
Female
Mating
1st
generation
Ww
ww
ww
Ww
2nd
generation
Ww
ww
3rd
generation
WW
or
Ww
Widow’s
peak
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd
generation
ww
No widow’s
peak
(a) Is a widow’s peak a dominant or
recessive trait?
ff
Ff
2nd
generation
FF or Ff
Ww ww ww Ww
Ff
Offspring
Attached
earlobe
Free
earlobe
b) Is an attached earlobe a dominant
or recessive trait?
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Identifying Human Alleles as Recessive or Dominant
• If a given trait is due to a single gene, the pedigree may
reveal whether the trait is due to a dominant or recessive
allele and whether the gene responsible is located on a sex
chromosome or an autosome.
• To analyze the inheritance of a trait that shows discrete
variation, biologists begin by assuming the simplest case:
that a single autosomal gene is involved and that the alleles
present in the population have a simple dominant-recessive
relationship.
© 2011 Pearson Education, Inc.
Patterns of Inheritance: Autosomal Recessive Traits
• When a phenotype is due to an autosomal recessive allele:
– Individuals with the trait must be homozygous.
– Unaffected parents of an affected individual are likely
to be heterozygous carriers for the trait.
– Carriers have the allele and transmit it without exhibiting
the phenotype.
• In general, a recessive phenotype should show up in
offspring only when both parents have that recessive allele
and pass it on to their offspring.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
• 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
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Albinism in humans occurs when both alleles at a locus
produce defective enzymes in the biochemical pathway
leading to melanin. Given that heterozygotes are normally
pigmented, which of the following statements is/are
correct?
–
–
–
–
–
One normal allele produces as much melanin as two normal alleles.
Each defective allele produces a little bit of melanin.
Two normal alleles are needed for normal melanin production.
The two alleles are codominant.
The amount of sunlight will not affect skin color of heterozygotes.
© 2011 Pearson Education, Inc.
Albinism in humans occurs when both alleles at a locus
produce defective enzymes in the biochemical pathway
leading to melanin. Given that heterozygotes are normally
pigmented, which of the following statements is/are
correct?
–
–
–
–
–
One normal allele produces as much melanin as two normal alleles.
Each defective allele produces a little bit of melanin.
Two normal alleles are needed for normal melanin production.
The two alleles are codominant.
The amount of sunlight will not affect skin color of heterozygotes.
© 2011 Pearson Education, Inc.
Patterns of Inheritance: Autosomal Dominant Traits
• Autosomal dominant traits are expressed in any individual
with at least one dominant allele.
– In other words, individuals who are homozygous or
heterozygous for the trait will display the dominant
phenotype.
• This is the case with Huntington's disease.
© 2011 Pearson Education, Inc.
• Achondroplasia 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
© 2011 Pearson Education, Inc.
Is the Trait Autosomal or Sex-Linked?
• If a trait appears equally often in males and females, it is
likely to be autosomal. If males are much more likely to have
the trait, it is usually X-linked.
• Hemophilia is an example of an X-linked trait resulting from
a recessive allele.
– These traits usually skip generations in a pedigree.
• X-linked dominant traits rarely skip generations.
– These traits are indicated in a pedigree wherein an
affected male has all affected daughters but no affected
sons.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Relationship among
alleles of a single gene
Complete dominance
of one allele
Description
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance Heterozygous phenotype
intermediate between
of either allele
the two homozygous
phenotypes
Codominance
Both phenotypes
expressed in
heterozygotes
Example
PP
Pp
CRCR CRCW CWCW
IAIB
Multiple alleles
In the whole population, ABO blood group alleles
some genes have more
IA, IB, i
than two alleles
Pleiotropy
One gene is able to affect Sickle-cell disease
multiple phenotypic
characters
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Relationship among
two or more genes
Epistasis
Description
The phenotypic
expression of one
gene affects that
of another
Example
BbEe
BE
BbEe
bE
Be
be
BE
bE
Be
be
9
Polygenic inheritance
A single phenotypic
character is affected
by two or more genes
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AaBbCc
:3
:4
AaBbCc