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Learning objectives: Students should be able to …
• Describe how Mendel’s principles of segregation and independent
assortment are a consequence of chromosome movement in
meiosis.
• Calculate expected frequencies of genotypes and phenotypes in
monohybrid, dihybrid, and X-linked crosses.
• Analyze the results of crosses and pedigrees to determine whether
phenotypes are autosomal or X-linked, dominant or recessive,
linked or on different chromosomes.
• Explain what a dominant allele is and how this applies to
incomplete dominance, codominance, gene interactions, and
polygenic inheritance.
© 2011 Pearson Education, Inc.
Chapter 13 outline: Mendel and the gene
• Mendel
• Outline (cont)
– how inheritance works
• sex linkage
– model organism
• crossing over
– exp with a single trait
• incomplete dominance
– exp with two traits
• codominance
• Sutton & Boveri (1902)
• multiple allelism
– meiosis to explain Mendel’s findings
• pleiotropic genes
– chromosome theory of inheritance
• quantitative traits
• Thomas Hunt Morgan
– testing chromosome theory of inheritance
• Nettie Stevens
• Applying Mendel’s rules to humans
– sex chromosome discovery
• (controlled experiments not
possible)
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Key Concepts
In many species, individuals have two alleles of each gene. The
principle of segregation states that prior to the formation of eggs
and sperm, the alleles of each gene separate so that each egg or
sperm cell receives only one of them.
The principle of independent assortment states that alleles of
different genes are transmitted to egg cells and sperm cells
independently of each other.
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Key Concepts
Genes are located on chromosomes. The principle of segregation
is explained by the separation of homologous chromosomes in
anaphase of meiosis I. The principle of independent assortment
applies to genes found on different chromosomes and is explained
by chromosomes lining up randomly in metaphase of meiosis I.
There are important exceptions and extensions to the basic
patterns of inheritance that Mendel discovered.
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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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How Did Mendel Arrange Matings?
• Peas normally pollinate themselves, a process called self-fertilization.
• 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 crosspollination.
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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, and stem length.
• 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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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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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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.
• 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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© 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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Is inheritance of seed shape related to sex?
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 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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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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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.
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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.
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Single Trait Cross
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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.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Using a Testcross to Confirm Predictions
• 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.
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© 2011 Pearson Education, Inc.
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.
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© 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.
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 wildtype 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
• 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.
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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.
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© 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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Extending Mendel’s Rules
Once Mendel’s work was rediscovered, researchers began to
analyze traits and alleles whose inheritance was more
complicated.
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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.
• Note that the terms linkage and sex-linkage differ in meaning. If a
single gene is sex-linked, it means that it is found on a sex
chromosome.
• 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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© 2011 Pearson Education, Inc.
The First Studies of Linked Genes
• To determine whether linked genes behave as predicted, Morgan
performed an experiment using Drosophila, in which he mated two
flies that were heterozygous for two sex-linked traits.
• The results of this experiment included some fruit flies with novel
phenotypes. Morgan referred to these flies as recombinant because
the combination of alleles on their X chromosome was different
from the combinations of alleles present in the parental generation.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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.
© 2011 Pearson Education, Inc.
Extending Mendel’s Rules
• By studying a simple genetic system, Mendel discovered the most
fundamental rules of inheritance.
• Most genes are inherited in a more complex fashion, however, 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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An example
of
codominance
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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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Pleiotropy
• The alleles Mendel analyzed affected only a single trait. Some
genes, however, influence many traits—these genes are said to be
pleiotropic.
• An example of pleiotropy in humans is the gene responsible for
Marfan syndrome. Although research suggests that just a single
gene is involved, individuals with Marfan syndrome exhibit a wide
array of phenotypic effects, including increased height and limb
length, and potentially severe heart problems.
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The Physical Environment Also Affects Phenotype
• Mendel controlled the physical environment of his plants to be sure
that it did not affect phenotype when he studied plant height.
• Most phenotypes are strongly influenced by the physical
environment in addition to their corresponding genotypes.
• The combined effect of genes and environment is referred to as
gene-by-environment interaction.
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Gene-by-Environment Interactions
• The human genetic disease phenylketonuria (PKU) is a good
example of a gene-by-environment interaction.
– Untreated, this disease causes phenylalanine to accumulate in
the body of affected individuals and results in profound mental
retardation.
– Individuals placed on a low-phenylalanine diet, however,
develop normally.
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Interactions with Other Genes Affect Phenotypes
• The expression of many genes is dependent upon the presence or
absence of other genes.
• When these types of gene-by-gene interactions occur, the
phenotype produced by an allele depends on the action of alleles of
other genes.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
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.
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© 2011 Pearson Education, Inc.
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.
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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.
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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.
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© 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.
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Example: Autosomal dominant trait:
Huntington’s disease
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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.
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Example: X-linked trait from a recessive
allele
A child can show the trait without
the parent showing the trait
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