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Chapter 13
Meiosis and Sexual
Life Cycles
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Variations on a Theme
• Living organisms are distinguished by their
ability to reproduce their own kind
• Genetics is the scientific study of heredity and
variation
• Heredity is the transmission of traits from one
generation to the next
• Variation is demonstrated by the differences in
appearance that offspring show from parents
and siblings
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Concept 13.1: Offspring acquire genes from
parents by inheriting chromosomes
• In a literal sense, children do not inherit
particular physical traits from their parents
• It is genes that are actually inherited
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Inheritance of Genes
• Genes are the units of heredity, and are made
up of segments of DNA
• Genes are passed to the next generation
through reproductive cells called gametes
(sperm and eggs)
• Each gene has a specific location called a
locus on a certain chromosome
• Most DNA is packaged into chromosomes
• One set of chromosomes is inherited from each
parent
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Comparison of Asexual and Sexual Reproduction
• In asexual reproduction, one parent produces
genetically identical offspring by mitosis
• A clone is a group of genetically identical
individuals from the same parent
• In sexual reproduction, two parents give rise
to offspring that have unique combinations of
genes inherited from the two parents
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Concept 13.2: Fertilization and meiosis alternate
in sexual life cycles
• A life cycle is the generation-to-generation
sequence of stages in the reproductive history
of an organism
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Sets of Chromosomes in Human Cells
• Human somatic cells (any cell other than a
gamete) have 23 pairs of chromosomes
• A karyotype is an ordered display of the pairs
of chromosomes from a cell
• The two chromosomes in each pair are called
homologous chromosomes, or homologs
• Chromosomes in a homologous pair are the
same length and carry genes controlling the
same inherited characters
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-3b
TECHNIQUE
5 µm
Pair of homologous
replicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
• The sex chromosomes are called X and Y
• Human females have a homologous pair of X
chromosomes (XX)
• Human males have one X and one Y
chromosome
• The 22 pairs of chromosomes that do not
determine sex are called autosomes
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• Each pair of homologous chromosomes
includes one chromosome from each parent
• The 46 chromosomes in a human somatic cell
are two sets of 23: one from the mother and
one from the father
• A diploid cell (2n) has two sets of
chromosomes
• For humans, the diploid number is 46 (2n = 46)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• A gamete (sperm or egg) contains a single set
of chromosomes, and is haploid (n)
• For humans, the haploid number is 23 (n = 23)
• Each set of 23 consists of 22 autosomes and a
single sex chromosome
• In an unfertilized egg (ovum), the sex
chromosome is X
• In a sperm cell, the sex chromosome may be
either X or Y
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Behavior of Chromosome Sets in the Human
Life Cycle
• Fertilization is the union of gametes (the
sperm and the egg)
• The fertilized egg is called a zygote and has
one set of chromosomes from each parent
• The zygote produces somatic cells by mitosis
and develops into an adult
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• At sexual maturity, the ovaries and testes
produce haploid gametes
• Gametes are the only types of human cells
produced by meiosis, rather than mitosis
• Meiosis results in one set of chromosomes in
each gamete
• Fertilization and meiosis alternate in sexual life
cycles to maintain chromosome number
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-5
Key
Haploid gametes (n = 23)
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
The Variety of Sexual Life Cycles
• The alternation of meiosis and fertilization is
common to all organisms that reproduce
sexually
• The three main types of sexual life cycles differ
in the timing of meiosis and fertilization
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• In animals, meiosis produces gametes, which
undergo no further cell division before
fertilization
• Gametes are the only haploid cells in animals
• Gametes fuse to form a diploid zygote that
divides by mitosis to develop into a multicellular
organism
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-6a
Key
Haploid (n)
Diploid (2n)
n
Gametes
n
n
MEIOSIS
2n
Diploid
multicellular
organism
(a) Animals
FERTILIZATION
Zygote
2n
Mitosis
Fig. 13-6
Key
Haploid (n)
n
Gametes
n
Mitosis
n
n
MEIOSIS
FERTILIZATION
Diploid
multicellular
organism
(a) Animals
Zygote
2n
Mitosis
Mitosis
n
Spores
Mitosis
Mitosis
n
n
n
n
MEIOSIS
2n
Haploid unicellular or
multicellular organism
Haploid multicellular organism
(gametophyte)
Diploid (2n)
n
Gametes
n
n
Gametes
FERTILIZATION
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
n
2n
Mitosis
(b) Plants and some algae
Zygote
FERTILIZATION
2n
Zygote
(c) Most fungi and some protists
• Plants and some algae exhibit an alternation
of generations
• This life cycle includes both a diploid and
haploid multicellular stage
• The diploid organism, called the sporophyte,
makes haploid spores by meiosis
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• Each spore grows by mitosis into a haploid
organism called a gametophyte
• A gametophyte makes haploid gametes by
mitosis
• Fertilization of gametes results in a diploid
sporophyte
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-6b
Key
Haploid (n)
Diploid (2n)
Mitosis
n
Haploid multicellular organism
(gametophyte)
Mitosis
n
n
n
n
Spores
MEIOSIS
Gametes
FERTILIZATION
2n
Diploid
multicellular
organism
(sporophyte)
2n
Mitosis
(b) Plants and some algae
Zygote
• In most fungi and some protists, the only
diploid stage is the single-celled zygote; there
is no multicellular diploid stage
• The zygote produces haploid cells by meiosis
• Each haploid cell grows by mitosis into a
haploid multicellular organism
• The haploid adult produces gametes by mitosis
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Fig. 13-6c
Key
Haploid (n)
Haploid unicellular or
multicellular organism
Diploid (2n)
Mitosis
Mitosis
n
n
n
n
Gametes
MEIOSIS
FERTILIZATION
2n
Zygote
(c) Most fungi and some protists
n
• Depending on the type of life cycle, either
haploid or diploid cells can divide by mitosis
• However, only diploid cells can undergo
meiosis
• In all three life cycles, the halving and doubling
of chromosomes contributes to genetic
variation in offspring
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Concept 13.3: Meiosis reduces the number of
chromosome sets from diploid to haploid
• Like mitosis, meiosis is preceded by the
replication of chromosomes
• Meiosis takes place in two sets of cell divisions,
called meiosis I and meiosis II
• The two cell divisions result in four daughter
cells, rather than the two daughter cells in
mitosis
• Each daughter cell has only half as many
chromosomes as the parent cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Stages of Meiosis
• In the first cell division (meiosis I), homologous
chromosomes separate
• Meiosis I results in two haploid daughter cells
with replicated chromosomes; it is called the
reductional division
• In the second cell division (meiosis II), sister
chromatids separate
• Meiosis II results in four haploid daughter cells
with unreplicated chromosomes; it is called the
equational division
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-7-3
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unreplicated chromosomes
• At the end of meiosis, there are four daughter
cells, each with a haploid set of unreplicated
chromosomes
• Each daughter cell is genetically distinct from
the others and from the parent cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
A Comparison of Mitosis and Meiosis
• Mitosis conserves the number of chromosome
sets, producing cells that are genetically
identical to the parent cell
• Meiosis reduces the number of chromosomes
sets from two (diploid) to one (haploid),
producing cells that differ genetically from each
other and from the parent cell
• The mechanism for separating sister
chromatids is virtually identical in meiosis II and
mitosis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-9
MITOSIS
MEIOSIS
Parent cell
Chromosome
replication
Prophase
Chiasma
Chromosome
replication
Prophase I
Homologous
chromosome
pair
2n = 6
Replicated chromosome
MEIOSIS I
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
2n
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
SUMMARY
Property
Mitosis
Meiosis
DNA
replication
Occurs during interphase before
mitosis begins
Occurs during interphase before meiosis I begins
Number of
divisions
One, including prophase, metaphase,
anahase, and telophase
Two, each including prophase, metaphase, anaphase, and
telophase
Synapsis of
homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many chromosomes
as the parent cell; genetically different from the parent
cell and from each other
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes by half
and introduces genetic variability amoung the gametes
•
Three events are unique to meiosis, and all
three occur in meiosis l:
– Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect
and exchange genetic information
– At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead
of individual replicated chromosomes
– At anaphase I, it is homologous
chromosomes, instead of sister chromatids,
that separate
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Concept 13.4: Genetic variation produced in
sexual life cycles contributes to evolution
• Mutations (changes in an organism’s DNA) are
the original source of genetic diversity
• Mutations create different versions of genes
called alleles
• Reshuffling of alleles during sexual
reproduction produces genetic variation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Origins of Genetic Variation Among Offspring
• The behavior of chromosomes during meiosis
and fertilization is responsible for most of the
variation that arises in each generation
• Three mechanisms contribute to genetic
variation:
– Independent assortment of chromosomes
– Crossing over
– Random fertilization
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Independent Assortment of Chromosomes
• Homologous pairs of chromosomes orient
randomly at metaphase I of meiosis
• In independent assortment, each pair of
chromosomes sorts maternal and paternal
homologues into daughter cells independently
of the other pairs
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• The number of combinations possible when
chromosomes assort independently into
gametes is 2n, where n is the haploid number
• For humans (n = 23), there are more than
8 million (223) possible combinations of
chromosomes
• We will do an independent assortment activity
in class with crayons and a circle paper to
demonstrate this, but n will only equal 2 or 3.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-11-3
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1 Combination 2
Combination 3 Combination 4
Crossing Over
• Crossing over produces recombinant
chromosomes, which combine genes
inherited from each parent
• Crossing over begins very early in prophase I,
as homologous chromosomes pair up gene by
gene
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• In crossing over, homologous portions of two
nonsister chromatids trade places
• Crossing over contributes to genetic variation
by combining DNA from two parents into a
single chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 13-12-5
Prophase I
of meiosis
Pair of
homologs
Nonsister
chromatids
held together
during synapsis
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
Random Fertilization
• In some species mates are chosen at random, or
there may be more than one mate at a time so
who’s the daddy could be a guess.
• Even when mates are known, which sperm gets to
the egg first is completely random. There are
millions to choose from and each is genetically
different.
• Each time fertilization occurs there will a different
combination of genes, thus producing endless
variation within every species.
Chapter 14
Mendel and the Gene Idea
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson 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
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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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-2
TECHNIQUE
1
2
Parental
generation
(P)
Stamens
Carpel
3
4
RESULTS
First
filial
generation
offspring
(F1)
5
• 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 self-pollinate)
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• 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, the F2
generation is produced
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The Law of Segregation
• When Mendel crossed contrasting, truebreeding 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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-3-3
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)

Purple
flowers
White
flowers
All plants had
purple flowers
F2 Generation
705 purple-flowered 224 white-flowered
plants
plants
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The first concept is that 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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-4
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
• 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 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
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• 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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Useful Genetic Vocabulary (on your vocab sheet)
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-6
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
1
The 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-7
TECHNIQUE

Dominant phenotype, Recessive phenotype,
unknown genotype:
known genotype:
PP or Pp?
pp
Predictions
If PP
Sperm
p
p
P
Pp
Eggs
If Pp
Sperm
p
p
or
P
Pp
Eggs
P
Pp
Pp
pp
pp
p
Pp
Pp
RESULTS
or
All offspring purple
1/2
offspring purple and
1/2 offspring white
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, individuals that are
heterozygous for one character
• A cross between such heterozygotes is called a
monohybrid cross
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• 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 © 2008 Pearson Education Inc., publishing as Pearson 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
32
Phenotypic ratio approximately 9:3:3:1
• 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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
1
carrying the dominant allele and a 2 chance of
carrying the recessive allele
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-9

Rr
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
R
1/
2
r
R
R
Eggs
4
r
2
r
2
R
1/
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-UN1
For the cross PpYyRr x Ppyyrr what would be the
chance of offspring having at least two recessive traits?
You will get these results if you do Punnett squares for each trait separately:
PP = ¼, Pp = ¼, pp = ¼
YY = 0, Yy = ½, yy = ½
RR = 0, Rr = ½, rr = ½
These are the genotypes that will show at least two recessive traits:
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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Extending Mendelian Genetics for a Single Gene
• Inheritance of characters by a single gene may
deviate from simple Mendelian patterns in the
following situations:
– When alleles are not completely dominant or
recessive
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-10-3
P Generation
Red
CRCR
Incomplete
dominance
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
The Relation Between Dominance and
Phenotype
• A dominant allele does not subdue a recessive
allele; alleles don’t interact
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more
common in populations than recessive alleles
• For example, one baby out of 400 in the United
States is born with extra fingers or toes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The allele for this unusual trait is dominant to
the allele for the more common trait of five
digits per appendage
• In this example, the recessive allele is far more
prevalent than the population’s dominant allele
Copyright © 2008 Pearson Education Inc., publishing as Pearson 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.
• 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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-11
Allele
IA
IB
Carbohydrate
A
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
Polygenic Inheritance
• Quantitative characters are those that vary in
the population along a continuum
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-13

AaBbCc
AaBbCc
Sperm
1/
Eggs
1/
8
1/
8
1/
8
1/
8
1/
1/
8
1/
1/
8
8
1/
8
1/
64
15/
8
1/
1/
8
8
8
1/
8
1/
8
1/
8
8
1/
Phenotypes:
64
Number of
dark-skin alleles: 0
6/
64
1
15/
64
2
20/
3
64
4
6/
64
5
1/
64
6
Nature and Nurture: 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 is the phenotypic range
of a genotype influenced by the environment
• For example, hydrangea flowers of the same
genotype range from blue-violet to pink,
depending on soil acidity
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-14
• Norms of reaction are generally broadest for
polygenic characters
• Such characters are called multifactorial
because genetic and environmental factors
collectively influence phenotype
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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 © 2008 Pearson Education Inc., publishing as Pearson 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-15a
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
Fig. 14-15b
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
(a) Is a widow’s peak a dominant or recessive trait?
• 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
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Recessively Inherited Disorders
• Many genetic disorders are inherited in a
recessive manner
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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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-16
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Cystic Fibrosis
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Chapter 15
The Chromosomal Basis of
Inheritance
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Locating Genes Along Chromosomes
• Mendel’s “hereditary factors” were genes,
though this wasn’t known at the time
• Today we can show that genes are located on
chromosomes
• The location of a particular gene can be seen
by tagging isolated chromosomes with a
fluorescent dye that highlights the gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-1
Concept 15.1: 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 © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-2
P Generation
Yellow-round
seeds (YYRR)
Y
Y
R
r

R
Green-wrinkled
seeds ( yyrr)
y
y
r
Meiosis
Fertilization
y
R Y
Gametes
r
All F1 plants produce
yellow-round seeds (YyRr)
F1 Generation
R
R
y
r
Y
Y
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
y
r
LAW OF INDEPENDENT
ASSORTMENT Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation.
Meiosis
R
r
Y
y
r
R
Y
y
Metaphase I
1
1
R
r
Y
y
r
R
Y
y
Anaphase I
R
r
Y
y
Metaphase II
r
R
Y
y
2
2
Y
Y
Gametes
R
R
1/
F2 Generation
4 YR
y
r
r
r
1/
4
Y
Y
y
r
1/
yr
4 Yr
y
y
R
R
1/
4 yR
An F1  F1 cross-fertilization
3
3
9
:3
:3
:1
Morgan’s Experimental Evidence: Scientific
Inquiry
• The first solid evidence associating a specific
gene with a specific chromosome came from
Thomas Hunt Morgan, an embryologist
• Morgan’s experiments with fruit flies provided
convincing evidence that chromosomes are the
location of Mendel’s heritable factors
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Morgan’s Choice of Experimental Organism
• Several characteristics make fruit flies a
convenient organism for genetic studies:
– They breed at a high rate
– A generation can be bred every two weeks
– They have only four pairs of chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Morgan noted wild type, or normal,
phenotypes that were common in the fly
populations
• Traits alternative to the wild type are called
mutant phenotypes
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Fig. 15-3
Correlating Behavior of a Gene’s Alleles with
Behavior of a Chromosome Pair
• In one experiment, Morgan mated male flies
with white eyes (mutant) with female flies with
red eyes (wild type)
– The F1 generation all had red eyes
– The F2 generation showed the 3:1 red:white
eye ratio, but only males had white eyes
• Morgan determined that the white-eyed mutant
allele must be located on the X chromosome
• Morgan’s finding supported the chromosome
theory of inheritance
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-4c
CONCLUSION
P
Generation
w+
X
X

w+
X
Y
w
Eggs
F1
Generation
Sperm
w+
w+
w+
w
w+
Eggs
F2
Generation
w
w+
Sperm
w+
w+
w
w
w
w+
Concept 15.2: Sex-linked genes exhibit unique
patterns of inheritance
• In humans and some other animals, there is a
chromosomal basis of sex determination
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Chromosomal Basis of Sex
• In humans and other mammals, there are two
varieties of sex chromosomes: a larger X
chromosome and a smaller Y chromosome
• Only the ends of the Y chromosome have
regions that are homologous with the X
chromosome
• The SRY gene on the Y chromosome codes for
the development of testes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-5
X
Y
• Females are XX, and males are XY
• Each ovum contains an X chromosome, while
a sperm may contain either an X or a Y
chromosome
• Other animals have different methods of sex
determination
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Inheritance of Sex-Linked Genes
• The sex chromosomes have genes for many
characters unrelated to sex
• A gene located on either sex chromosome is
called a sex-linked gene
• In humans, sex-linked usually refers to a gene
on the larger X chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Sex-linked genes follow specific patterns of
inheritance
• For a recessive sex-linked trait to be expressed
– A female needs two copies of the allele
– A male needs only one copy of the allele
• Sex-linked recessive disorders are much more
common in males than in females
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Some disorders caused by recessive alleles on
the X chromosome in humans:
– Color blindness
– Duchenne muscular dystrophy
– Hemophilia
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Concept 15.3: Linked genes tend to be inherited
together because they are located near each other
on the same chromosome
• Linked genes do not follow Mendel’s law of
independent assortment
How Linkage Affects Inheritance
• Morgan did other experiments with fruit flies to
see how linkage affects inheritance of two
characters
• Morgan crossed flies that differed in traits of
body color and wing size
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-UN1
b vg
b+ vg+
Parents
in testcross
Most
offspring

b vg
b vg
b+ vg+
b vg
or
b vg
b vg
Fig. 15-9-4
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
Double mutant
(black body,
vestigial wings)

b b vg vg
b+ b+ vg+ vg+
F1 dihybrid
(wild type)
Double mutant
TESTCROSS

b+ b vg+ vg
Testcross
offspring
b b vg vg
b vg
b+ vg
b vg+
Wild type
(gray-normal)
Blackvestigial
Grayvestigial
Blacknormal
b+ b vg+ vg
b b vg vg b+ b vg vg b b vg+ vg
Eggs
b+ vg+
b vg
Sperm
PREDICTED RATIOS
If genes are located on different chromosomes:
1
:
1
:
1
:
1
If genes are located on the same chromosome and
parental alleles are always inherited together:
1
:
1
:
0
:
0
965
:
944
:
206
:
185
RESULTS
• Morgan found that body color and wing size
are usually inherited together in specific
combinations (parental phenotypes)
• He noted that these genes do not assort
independently, and reasoned that they were on
the same chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• However, nonparental phenotypes were also
produced
• Understanding this result involves exploring
genetic recombination, the production of
offspring with combinations of traits differing
from either parent
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Genetic Recombination and Linkage
• The genetic findings of Mendel and Morgan
relate to the chromosomal basis of
recombination
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Recombination of Unlinked Genes: Independent
Assortment of Chromosomes
• Mendel observed that combinations of traits in
some offspring differ from either parent
• Offspring with a phenotype matching one of the
parental phenotypes are called parental types
• Offspring with nonparental phenotypes (new
combinations of traits) are called recombinant
types, or recombinants
• A 50% frequency of recombination is observed
for any two genes on different chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-UN2
Gametes from yellow-round
heterozygous parent (YyRr)
Gametes from greenwrinkled homozygous
recessive parent ( yyrr)
YR
yr
Yr
yR
YyRr
yyrr
Yyrr
yyRr
yr
Parentaltype
offspring
Recombinant
offspring
Recombination of Linked Genes: Crossing Over
• Morgan discovered that genes can be linked,
but the linkage was incomplete, as evident
from recombinant phenotypes
• Morgan proposed that some process must
sometimes break the physical connection
between genes on the same chromosome
• That mechanism was the crossing over of
homologous chromosomes
Animation: Crossing Over
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-10a
Testcross
parents
Black body, vestigial wings
(double mutant)
Gray body, normal wings
(F1 dihybrid)
Replication
of chromosomes
Meiosis I
b+ vg+
b vg
b vg
b vg
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
b+ vg+
b+
Meiosis I and II
vg
b vg+
b vg
Meiosis II
Recombinant
chromosomes
b+ vg+
b vg
Eggs
b+ vg
b vg+
b vg
Sperm
Replication
of chromosomes
Fig. 15-10b
Recombinant
chromosomes
Eggs
Testcross
offspring
b+ vg+
b vg
b+ vg
b vg+
944
Wild type
Black(gray-normal) vestigial
206
Grayvestigial
185
Blacknormal
965
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Parental-type offspring
Recombinant offspring
391 recombinants
Recombination
 100 = 17%
=
frequency
2,300 total offspring
b vg
Sperm
Mapping the Distance Between Genes Using
Recombination Data: Scientific Inquiry
• Alfred Sturtevant, one of Morgan’s students,
constructed a genetic map, an ordered list of
the genetic loci along a particular chromosome
• Sturtevant predicted that the farther apart two
genes are, the higher the probability that a
crossover will occur between them and
therefore the higher the recombination
frequency
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• A linkage map is a genetic map of a
chromosome based on recombination
frequencies
• Distances between genes can be expressed as
map units; one map unit, or centimorgan,
represents a 1% recombination frequency
• Map units indicate relative distance and order,
not precise locations of genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Genes that are far apart on the same
chromosome can have a recombination
frequency near 50%
• Such genes are physically linked, but
genetically unlinked, and behave as if found on
different chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Concept 15.4: Alterations of chromosome number
or structure cause some genetic disorders
• Large-scale chromosomal alterations often
lead to spontaneous abortions (miscarriages)
or cause a variety of developmental disorders
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Abnormal Chromosome Number
• In nondisjunction, pairs of homologous
chromosomes do not separate normally during
meiosis
• As a result, one gamete receives two of the
same type of chromosome, and another
gamete receives no copy
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n–1
n–1
n+1
n–1
n
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
n
• Aneuploidy results from the fertilization of
gametes in which nondisjunction occurred
• Offspring with this condition have an abnormal
number of a particular chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• A monosomic zygote has only one copy of a
particular chromosome
• A trisomic zygote has three copies of a
particular chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Polyploidy is a condition in which an organism
has more than two complete sets of
chromosomes
– Triploidy (3n) is three sets of chromosomes
– Tetraploidy (4n) is four sets of chromosomes
• Polyploidy is common in plants, but not animals
• Polyploids are more normal in appearance than
aneuploids
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-14
This mouse is tetraploid. He’s a bit larger than normal.
Alterations of Chromosome Structure
• Breakage of a chromosome can lead to four
types of changes in chromosome structure:
– Deletion removes a chromosomal segment
– Duplication repeats a segment
– Inversion reverses a segment within a
chromosome
– Translocation moves a segment from one
chromosome to another
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-15
(a)
(b)
(c)
(d)
A B C D E
F G H
A B C D E
F G H
A B C D E
F G H
A B C D E
F G H
Deletion
Duplication
A B C E
F G H
A B C B C D E
Inversion
A D C B E
R
F G H
M N O C D E
Reciprocal
translocation
M N O P Q
F G H
A B P Q
R
F G H
Human Disorders Due to Chromosomal
Alterations
• Alterations of chromosome number and
structure are associated with some serious
disorders
• Some types of aneuploidy appear to upset the
genetic balance less than others, resulting in
individuals surviving to birth and beyond
• These surviving individuals have a set of
symptoms, or syndrome, characteristic of the
type of aneuploidy
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Down Syndrome (Trisomy 21)
• Down syndrome is an aneuploid condition that
results from three copies of chromosome 21
• It affects about one out of every 700 children
born in the United States
• The frequency of Down syndrome increases
with the age of the mother, a correlation that
has not been explained
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-16
Down syndrome is caused
by having three copies of
chromosome 21.
Aneuploidy of Sex Chromosomes
• Nondisjunction of sex chromosomes produces
a variety of aneuploid conditions
• Klinefelter syndrome is the result of an extra
chromosome in a male, producing XXY
individuals
• Monosomy X, called Turner syndrome,
produces X0 females, who are sterile; it is the
only known viable monosomy in humans
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Disorders Caused by Structurally Altered
Chromosomes
• The syndrome cri du chat (“cry of the cat”),
results from a specific deletion in chromosome
5
• A child born with this syndrome is mentally
retarded and has a catlike cry; individuals
usually die in infancy or early childhood
• Certain cancers, including chronic
myelogenous leukemia (CML), are caused by
translocations of chromosomes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-17
Normal chromosome 9
Normal chromosome 22
Reciprocal
translocation
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Concept 15.5: Some inheritance patterns are
exceptions to the standard chromosome theory
• There are two normal exceptions to Mendelian
genetics
• One exception involves genes located in the
nucleus, and the other exception involves
genes located outside the nucleus
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Inheritance of Organelle Genes
• Extranuclear genes (or cytoplasmic genes) are
genes found in organelles in the cytoplasm
• Mitochondria, chloroplasts, and other plant
plastids carry small circular DNA molecules
• Extranuclear genes are inherited maternally
because the zygote’s cytoplasm comes from
the egg
• The first evidence of extranuclear genes came
from studies on the inheritance of yellow or
white patches on leaves of an otherwise green
plant
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-19
• Some defects in mitochondrial genes prevent
cells from making enough ATP and result in
diseases that affect the muscular and nervous
systems
– For example, mitochondrial myopathy and
Leber’s hereditary optic neuropathy
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-UN4
Sperm
P generation D
gametes
C
B
A
Egg
E
+
c
b
a
d
F
f
The alleles of unlinked
genes are either on
separate chromosomes
(such as d and e) or so
far apart on the same
chromosome (c and f)
that they assort
independently.
This F1 cell has 2n = 6
chromosomes and is
heterozygous for all six
genes shown (AaBbCcDdEeFf).
Red = maternal; blue = paternal.
D
Each chromosome
has hundreds or
thousands of genes.
Four (A, B, C, F) are
shown on this one.
e
C
B
A
F
e
d
E
cb
a
f
Genes on the same chromosome whose alleles are so
close together that they do
not assort independently
(such as a, b, and c) are said
to be linked.