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Table of Contents pages iii
Unit 1 What is Biology? Unit 2 Ecology Unit
3 The Life of a Cell Unit 4 Genetics Unit 5
Change Through Time Unit 6 Viruses, Bacteria,
Protists, and Fungi Unit 7 Plants Unit 8
Invertebrates Unit 9 Vertebrates Unit 10 The
Human Body
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Table of Contents pages vii-xiii
Unit 1 What is Biology? Chapter 1
Biology The Study of Life Unit 2 Ecology
Chapter 2 Principles of Ecology Chapter
3 Communities and Biomes Chapter 4
Population Biology Chapter 5 Biological
Diversity and Conservation Unit 3 The Life of a
Cell Chapter 6 The Chemistry of Life
Chapter 7 A View of the Cell Chapter 8
Cellular Transport and the Cell Cycle
Chapter 9 Energy in a Cell
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Table of Contents pages vii-xiii
Unit 4 Genetics Chapter 10 Mendel and
Meiosis Chapter 11 DNA and Genes
Chapter 12 Patterns of Heredity and Human
Genetics Chapter 13 Genetic
Technology Unit 5 Change Through Time
Chapter 14 The History of Life Chapter
15 The Theory of Evolution Chapter 16
Primate Evolution Chapter 17 Organizing
Lifes Diversity
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Table of Contents pages vii-xiii
Unit 6 Viruses, Bacteria, Protists, and Fungi
Chapter 18 Viruses and Bacteria
Chapter 19 Protists Chapter 20 Fungi
Unit 7 Plants Chapter 21 What Is a
Plant? Chapter 22 The Diversity of
Plants Chapter 23 Plant Structure and
Function Chapter 24 Reproduction in Plants
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Table of Contents pages vii-xiii
Unit 8 Invertebrates Chapter 25 What Is
an Animal? Chapter 26 Sponges,
Cnidarians, Flatworms, and
Roundworms Chapter 27
Mollusks and Segmented Worms Chapter 28
Arthropods Chapter 29 Echinoderms and
Invertebrate
Chordates
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Table of Contents pages vii-xiii
Unit 9 Vertebrates Chapter 30 Fishes
and Amphibians Chapter 31 Reptiles and
Birds Chapter 32 Mammals Chapter 33
Animal Behavior Unit 10 The Human Body
Chapter 34 Protection, Support, and
Locomotion Chapter 35 The Digestive and
Endocrine Systems Chapter 36 The Nervous
System Chapter 37 Respiration,
Circulation, and Excretion Chapter 38
Reproduction and Development Chapter 39
Immunity from Disease
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Unit Overview pages 250-251
Genetics
Mendel and Meiosis
DNA and Genes
Patterns of Heredity and Human Genetics
Genetic Technology
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Chapter Contents page vii
Chapter 10 Mendel and Meiosis 10.1 Mendels
Laws of Heredity 10.1 Section Check 10.2
Meiosis 10.2 Section Check Chapter 10
Summary Chapter 10 Assessment
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Chapter Intro-page 252
What Youll Learn
You will identify the basic concepts of genetics.
You will examine the process of meiosis.
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10.1 Section Objectives page 253
Section Objectives

• Relate Mendels two laws to the results he
  obtained in his experiments with garden peas.

• Predict the possible offspring of a genetic cross
  by using a Punnett square.

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Section 10.1 Summary pages 253-262
Why Mendel Succeeded

• It was not until the mid-nineteenth century that
  Gregor Mendel, an Austrian monk, carried out
  important studies of hereditythe passing on of
  characteristics from parents to offspring.

• Characteristics that are inherited are called
  traits.

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Section 10.1 Summary pages 253-262
Why Mendel Succeeded

• Mendel was the first person to succeed in
  predicting how traits are transferred from one
  generation to the next.

• A complete explanation requires the careful study
  of geneticsthe branch of biology that studies
  heredity.

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Section 10.1 Summary pages 253-262
Mendel chose his subject carefully

• Mendel chose to use the garden pea in his
  experiments for several reasons.

• Garden pea plants reproduce sexually, which means
  that they produce male and female sex cells,
  called gametes.

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Section 10.1 Summary pages 253-262
Mendel chose his subject carefully

• The male gamete forms in the pollen grain, which
  is produced in the male reproductive organ.

• The female gamete forms in the female
  reproductive organ.

• In a process called fertilization, the male
  gamete unites with the female gamete.

• The resulting fertilized cell, called a zygote
  (ZI goht), then develops into a seed.

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Section 10.1 Summary pages 253-262
Mendel chose his subject carefully

• The transfer of pollen grains from a male
  reproductive organ to a female reproductive organ
  in a plant is called pollination.

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Section 10.1 Summary pages 253-262
Mendel chose his subject carefully

• When he wanted to breed, or cross, one plant with
  another, Mendel opened the petals of a flower and
  removed the male organs.

Remove male parts
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Section 10.1 Summary pages 253-262
Mendel chose his subject carefully

• He then dusted the female organ with pollen from
  the plant he wished to cross it with.

Pollen grains
Transfer pollen
Female part
Male parts
Cross-pollination
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Section 10.1 Summary pages 253-262
Mendel chose his subject carefully

• This process is called cross-pollination.

• By using this technique, Mendel could be sure of
  the parents in his cross.

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Section 10.1 Summary pages 253-262
Mendel was a careful researcher

• He studied only one trait at a time to control
  variables, and he analyzed his data
  mathematically.

• The tall pea plants he worked with were from
  populations of plants that had been tall for many
  generations and had always produced tall
  offspring.

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Section 10.1 Summary pages 253-262
Mendel was a careful researcher

• Such plants are said to be true breeding for
  tallness.

• Likewise, the short plants he worked with were
  true breeding for shortness.

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Section 10.1 Summary pages 253-262
Mendels Monohybrid Crosses

• A hybrid is the offspring of parents that have
  different forms of a trait, such as tall and
  short height.

• Mendels first experiments are called monohybrid
  crosses because mono means one and the two
  parent plants differed from each other by a
  single traitheight.

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Section 10.1 Summary pages 253-262
The first generation

• Mendel selected a six-foot-tall pea plant that
  came from a population of pea plants, all of
  which were over six feet tall.

• He cross-pollinated this tall pea plant with
  pollen from a short pea plant.

• All of the offspring grew to be as tall as the
  taller parent.

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Section 10.1 Summary pages 253-262
The second generation

• Mendel allowed the tall plants in this first
  generation to self-pollinate.

• After the seeds formed, he planted them and
  counted more than 1000 plants in this second
  generation.

• Three-fourths of the plants were as tall as the
  tall plants in the parent and first generations.

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Section 10.1 Summary pages 253-262
The second generation
P1

• One-fourth of the offspring were as short as the
  short plants in the parent generation.

Short pea plant
Tall pea plant
F1

• In the second generation, tall and short plants
  occurred in a ratio of about three tall plants to
  one short plant.

All tall pea plants
F2
3 tall 1 short
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Section 10.1 Summary pages 253-262
The second generation

• The original parents, the true-breeding plants,
  are known as the P1 generation.

• The offspring of the parent plants are known as
  the F1 generation.

• When you cross two F1 plants with each other,
  their offspring are the F2 generation.

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Section 10.1 Summary pages 253-262
The second generation
Seed shape
Flower color
Seed color
Flower position
Pod color
Pod shape
Plant height
Dominant trait
axial (side)
purple
round
yellow
green
tall
inflated
Recessive trait
terminal (tips)
green
white
yellow
short
wrinkled
constricted
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Section 10.1 Summary pages 253-262
The second generation

• In every case, he found that one trait of a pair
  seemed to disappear in the F1 generation, only to
  reappear unchanged in one-fourth of the F2 plants.

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Section 10.1 Summary pages 253-262
The rule of unit factors

• Mendel concluded that each organism has two
  factors that control each of its traits.

• We now know that these factors are genes and that
  they are located on chromosomes.

• Genes exist in alternative forms. We call these
  different gene forms alleles.

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Section 10.1 Summary pages 253-262
The rule of unit factors

• An organisms two alleles are located on
  different copies of a chromosomeone inherited
  from the female parent and one from the male
  parent.

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Section 10.1 Summary pages 253-262
The rule of dominance

• Mendel called the observed trait dominant and the
  trait that disappeared recessive.

• Mendel concluded that the allele for tall plants
  is dominant to the allele for short plants.

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Section 10.1 Summary pages 253-262
The rule of dominance

• When recording the results of crosses, it is
  customary to use the same letter for different
  alleles of the same gene.

Short plant
Tall plant
t
t
T
T
t
T
F1
All tall plants
t
T
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Section 10.1 Summary pages 253-262
The rule of dominance

• An uppercase letter is used for the dominant
  allele and a lowercase letter for the recessive
  allele.

Short plant
Tall plant
t
t
T
T
t
T
F1

• The dominant allele is always written first.

All tall plants
t
T
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Section 10.1 Summary pages 253-262
The law of segregation

• The law of segregation states that every
  individual has two alleles of each gene and when
  gametes are produced, each gamete receives one of
  these alleles.

• During fertilization, these gametes randomly pair
  to produce four combinations of alleles.

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Section 10.1 Summary pages 253-262
Phenotypes and Genotypes
Law of segregation
Tt Tt cross

• Two organisms can look alike but have different
  underlying allele combinations.

F1
Tall plant
Tall plant
T
t
T
t
F2
Tall
Tall
Short
Tall
t
t
t
T
t
T
T
T
3
1
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Section 10.1 Summary pages 253-262
Phenotypes and Genotypes

• The way an organism looks and behaves is called
  its phenotype.

• The allele combination an organism contains is
  known as its genotype.

• An organisms genotype cant always be known by
  its phenotype.

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Section 10.1 Summary pages 253-262
Phenotypes and Genotypes

• An organism is homozygous for a trait if its two
  alleles for the trait are the same.

• The true-breeding tall plant that had two alleles
  for tallness (TT) would be homozygous for the
  trait of height.

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Section 10.1 Summary pages 253-262
Phenotypes and Genotypes

• An organism is heterozygous for a trait if its
  two alleles for the trait differ from each other.

• Therefore, the tall plant that had one allele for
  tallness and one allele for shortness (Tt) is
  heterozygous for the trait of height.

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Section 10.1 Summary pages 253-262
Mendels Dihybrid Crosses

• Mendel performed another set of crosses in which
  he used peas that differed from each other in two
  traits rather than only one.

• Such a cross involving two different traits is
  called a dihybrid cross.

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Section 10.1 Summary pages 253-262
The first generation

• Mendel took true-breeding pea plants that had
  round yellow seeds (RRYY) and crossed them with
  true-breeding pea plants that had wrinkled green
  seeds (rryy).

• He already knew the round-seeded trait was
  dominant to the wrinkled-seeded trait.

• He also knew that yellow was dominant to green.

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Section 10.1 Summary pages 253-262
The first generation
Dihybrid Cross
round yellow x wrinkled green
P1
Round yellow
Wrinkled green
All round yellow
F1
F2
9
3
3
1
Round yellow
Round green
Wrinkled yellow
Wrinkled green
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Section 10.1 Summary pages 253-262
The second generation

• Mendel then let the F1 plants pollinate
  themselves.

• He found some plants that produced round yellow
  seeds and others that produced wrinkled green
  seeds.

• He also found some plants with round green seeds
  and others with wrinkled yellow seeds.

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Section 10.1 Summary pages 253-262
The second generation

• He found they appeared in a definite ratio of
  phenotypes9 round yellow 3 round green 3
  wrinkled yellow 1 wrinkled green.

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Section 10.1 Summary pages 253-262
The law of independent assortment

• Mendels second law states that genes for
  different traitsfor example, seed shape and seed
  colorare inherited independently of each other.

• This conclusion is known as the law of
  independent assortment.

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Section 10.1 Summary pages 253-262
Punnett Squares

• In 1905, Reginald Punnett, an English biologist,
  devised a shorthand way of finding the expected
  proportions of possible genotypes in the
  offspring of a cross.

• This method is called a Punnett square.

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Section 10.1 Summary pages 253-262
Punnett Squares

• If you know the genotypes of the parents, you can
  use a Punnett square to predict the possible
  genotypes of their offspring.

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Section 10.1 Summary pages 253-262
Monohybrid crosses

• A Punnett square for this cross is two boxes tall
  and two boxes wide because each parent can
  produce two kinds of gametes for this trait.

Heterozygous tall parent
T
t
T
t
T
t
T
T
TT
Tt
t
t
Tt
tt
T
t
Heterozygous tall parent
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Section 10.1 Summary pages 253-262
Monohybrid crosses

• The two kinds of gametes from one parent are
  listed on top of the square, and the two kinds of
  gametes from the other parent are listed on the
  left side.

Heterozygous tall parent
T
t
T
t
T
t
T
T
TT
Tt
t
t
Tt
tt
T
t
Heterozygous tall parent
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Section 10.1 Summary pages 253-262
Monohybrid crosses

• It doesnt matter which set of gametes is on top
  and which is on the side.

• Each box is filled in with the gametes above and
  to the left side of that box. You can see that
  each box then contains two allelesone possible
  genotype.

• After the genotypes have been determined, you can
  determine the phenotypes.

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Section 10.1 Summary pages 253-262
Punnett Square of Dihybrid Cross
Dihybrid crosses
Gametes from RrYy parent
Ry
RY
rY
ry

• A Punnett square for a dihybrid cross will need
  to be four boxes on each side for a total of 16
  boxes.

RRYY
RRYy
RrYY
RrYy
RY
RRYy
RRYy
RrYy
Rryy
Ry
Gametes from RrYy parent
RrYY
RrYy
rrYY
rrYy
rY
RrYy
Rryy
rrYy
rryy
ry
51
Section 10.1 Summary pages 253-262
Punnett Square of Dihybrid Cross
Dihybrid crosses
Gametes from RrYy parent
Ry
RY
rY
ry
RRYY
RRYy
RrYY
RrYy
RY
F1 cross RrYy RrYy
RRYy
RRYy
RrYy
Rryy
round yellow
Ry
Gametes from RrYy parent
round green
RrYY
RrYy
rrYY
rrYy
rY
wrinkled yellow
RrYy
Rryy
rrYy
rryy
ry
wrinkled green
52
Section 10.1 Summary pages 253-262
Probability

• In reality you dont get the exact ratio of
  results shown in the square.

• Thats because, in some ways, genetics is like
  flipping a coinit follows the rules of chance.

• The probability or chance that an event will
  occur can be determined by dividing the number of
  desired outcomes by the total number of possible
  outcomes.

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Section 10.1 Summary pages 253-262
Probability

• A Punnett square can be used to determine the
  probability of getting a pea plant that produces
  round seeds when two plants that are heterozygous
  (Rr) are crossed.

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Section 10.1 Summary pages 253-262
Probability
r
R

• The Punnett square shows three plants with round
  seeds out of four total plants, so the
  probability is 3/4.

RR
Rr
R
Rr
rr
r
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Section 10.1 Summary pages 253-262
Probability
r
R

• It is important to remember that the results
  predicted by probability are more likely to be
  seen when there is a large number of offspring.

RR
Rr
R
Rr
rr
r
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Section 10.1 Summary pages 253-262
Punnett Square
Click image to view movie.
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Section 1 Check
Question 1
The passing on of characteristics from parents
to offspring is __________.
A. genetics
B. heredity
C. pollination
D. allelic frequency
CA Biology/Life Sciences 2a-5e
58
Section 1 Check
The answer is B. Genetics is the branch of
biology that studies heredity.
CA Biology/Life Sciences 2a-5e
59
Section 1 Check
Question 2
What are traits?
Answer
Traits are characteristics that are inherited.
Height, hair color and eye color are examples of
traits in humans.
CA Biology/Life Sciences 4
60
Section 1 Check
Question 3
Gametes are __________.
A. male sex cells
B. female sex cells
C. both male and female sex cells
D. fertilized cells that develop into adult
organisms
CA Biology/Life Sciences 2a
61
Section 1 Check
The answer is C. Organisms that reproduce
sexually produce male and female sex cells,
called gametes.
CA Biology/Life Sciences 2a
62
Section 1 Check
Question 4
How did Mendel explain the results of his cross
between tall and short plants, depicted in the
diagram?
Short plant
Tall plant
t
t
T
T
t
T
F1
All tall plants
t
T
CA Biology/Life Sciences 3b
63
Section 1 Check
When Mendel crossed a tall pea plant with a short
pea plant, all the offspring plants were tall. In
such crosses when only one trait was observed,
Mendel called the observed trait dominant.
Tall plant
t
t
T
T
t
T
F1
All tall plants
t
T
CA Biology/Life Sciences 3b
64
Section 1 Check
Question 5
Which of the following genotypes represents a
plant that is homozygous for height?
A. Tt
B. Hh
C. tT
D. tt
CA Biology/Life Sciences 3a
65
Section 1 Check
The answer is D. An organism is homozygous for a
trait if its two alleles for the trait are the
same. It can be either homozygous dominant or
homozygous recessive.
CA Biology/Life Sciences 3a
66
10.2 Section Objectives page 263
Section Objectives

• Analyze how meiosis maintains a constant number
  of chromosomes within a species.

• Infer how meiosis leads to variation in a
  species.

• Relate Mendels laws of heredity to the events
  of meiosis.

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Section 10.2 Summary pages 263-273
Genes, Chromosomes, and Numbers

• Genes do not exist free in the nucleus of a
  cell they are lined up on chromosomes.

• Typically, a chromosome can contain a thousand
  or more genes along its length.

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Section 10.2 Summary pages 263-273
Diploid and haploid cells

• In the body cells of animals and most plants,
  chromosomes occur in pairs.

• A cell with two of each kind of chromosome is
  called a diploid cell and is said to contain a
  diploid, or 2n, number of chromosomes.

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Section 10.2 Summary pages 263-273
Diploid and haploid cells

• This pairing supports Mendels conclusion that
  organisms have two factorsallelesfor each
  trait.

• Organisms produce gametes that contain one of
  each kind of chromosome.

• A cell containing one of each kind of
  chromosome is called a haploid cell and is
  said to contain a haploid, or n, number of
  chromosomes.

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Section 10.2 Summary pages 263-273
Diploid and haploid cells

• This fact supports Mendels conclusion
  that parent organisms give one allele for
  each trait to each of their offspring.

Chromosome Numbers of Common Organisms
Organism
Body Cell (2n)
Gamete (n)
Fruit fly
8
4
Garden pea
14
7
Corn
20
10
Tomato
24
12
Leopard Frog
26
13
Apple
34
17
Human
46
23
Chimpanzee
48
24
Dog
78
39
1260
630
Adders tongue fern
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Section 10.2 Summary pages 263-273
Diploid and haploid cells
Chromosome Numbers of Common Organisms

• This table shows the diploid and haploid
  number of chromosomes of some species.

Organism
Body Cell (2n)
Gamete (n)
Fruit fly
8
4
Garden pea
14
7
Corn
20
10
Tomato
24
12
Leopard Frog
26
13
Apple
34
17
Human
46
23
Chimpanzee
48
24
Dog
78
39
1260
630
Adders tongue fern
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Section 10.2 Summary pages 263-273
Homologous chromosomes

• The two chromosomes of each pair in a diploid
  cell are called homologous chromosomes.

• Each pair of homologous chromosomes has genes
  for the same traits.

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Section 10.2 Summary pages 263-273
Homologous chromosomes

• On homologous chromosomes, these genes are
  arranged in the same order, but because there
  are different possible alleles for the same
  gene, the two chromosomes in a homologous
  pair are not always identical to each other.

Homologous Chromosome 4
a
A
Terminal
Axial
Inflated
D
d
Constricted
T
t
Short
Tall
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Section 10.2 Summary pages 263-273
Why meiosis?

• When cells divide by mitosis, the new cells
  have exactly the same number and kind of
  chromosomes as the original cells.

• Imagine if mitosis were the only means of cell
  division.

• Each pea plant parent, which has 14
  chromosomes, would produce gametes that
  contained a complete set of 14 chromosomes.

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Section 10.2 Summary pages 263-273
Why meiosis?

• The F1 pea plants would have cell nuclei with
  28 chromosomes, and the F2 plants would have
  cell nuclei with 56 chromosomes.

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Section 10.2 Summary pages 263-273
Why meiosis?

• There must be another form of cell division
  that allows offspring to have the same number
  of chromosomes as their parents.

• This kind of cell division, which produces
  gametes containing half the number of
  chromosomes as a parents body cell, is
  called meiosis.

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Section 10.2 Summary pages 263-273
Why meiosis?

• Meiosis consists of two separate divisions,
  known as meiosis I and meiosis II.

• Meiosis I begins with one diploid (2n) cell.

• By the end of meiosis II, there are four
  haploid (n) cells.

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Section 10.2 Summary pages 263-273
Why meiosis?

• These haploid cells are called sex cells
  gametes.

• Male gametes are called sperm.

• Female gametes are called eggs.

• When a sperm fertilizes an egg, the resulting
  zygote once again has the diploid number of
  chromosomes.

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Section 10.2 Summary pages 263-273
Why meiosis?

• This pattern of reproduction, involving the
  production and subsequent fusion of haploid
  sex cells, is called sexual reproduction.

Meiosis
Haploid gametes (n23)
Sperm Cell
Meiosis
Egg Cell
Fertilization
Multicellular diploid adults (2n46)
Diploid zygote (2n46)
Mitosis and Development
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Section 10.2 Summary pages 263-273
The Phases of Meiosis

• During meiosis, a spindle forms and the
  cytoplasm divides in the same ways they do
  during mitosis.

• However, what happens to the chromosomes in
  meiosis is very different.

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Sect ion 10.2 Summary pages 263-273
The Phases of Meiosis
Click image to view movie.
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Section 10.2 Summary pages 263-273
Interphase

• During interphase, the cell replicates its
  chromosomes.

• After replication, each chromosome consists of
  two identical sister chromatids, held together
  by a centromere.

Interphase
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Section 10.2 Summary pages 263-273
Prophase I

• The chromosomes coil up and a spindle forms.

• As the chromosomes coil, homologous
  chromosomes line up with each other gene by
  gene along their length, to form a four-part
  structure called a tetrad.

Prophase I
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Section 10.2 Summary pages 263-273
Prophase I

• The chromatids in a tetrad pair tightly.

• In fact, they pair so tightly that non-sister
  chromatids from homologous chromosomes can
  actually break and exchange genetic material in
  a process known as crossing over.

Prophase I
85
Section 10.2 Summary pages 263-273
Prophase I

• Crossing over can occur at any location on a
  chromosome, and it can occur at several
  locations at the same time.

Prophase I
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Section 10.2 Summary pages 263-273

• It is estimated that during prophase I of
  meiosis in humans, there is an average of
  two to three crossovers for each pair of
  homologous chromosomes.

Prophase I
Nonsister chromatids
Sister chromatids
Crossing over in tetrad
Tetrad
Homologous chromosomes
Gametes
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Section 10.2 Summary pages 263-273
Prophase I

• Crossing over results in new combinations of
  alleles on a chromosome.

Nonsister chromatids
Sister chromatids
Crossing over in tetrad
Tetrad
Homologous chromosomes
Gametes
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Section 10.2 Summary pages 263-273
Metaphase I

• During metaphase I, the centromere of each
  chromosome becomes attached to a spindle fiber.

• The spindle fibers pull the tetrads into the
  middle, or equator, of the spindle.

Metaphase I
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Section 10.2 Summary pages 263-273
Anaphase I

• Anaphase I begins as homologous chromosomes,
  each with its two chromatids, separate and
  move to opposite ends of the cell.

• This critical step ensures that each new cell
  will receive only one chromosome from each
  homologous pair.

Anaphase I
90
Section 10.2 Summary pages 263-273
Telophase I

• Events occur in the reverse order from the
  events of prophase I.

• The spindle is broken down, the chromosomes
  uncoil, and the cytoplasm divides to yield two
  new cells.

Telophase I
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Section 10.2 Summary pages 263-273
Telophase I

• Each cell has half the genetic information of
  the original cell because it has only one
  chromosome from each homologous pair.

Telophase I
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Section 10.2 Summary pages 263-273
The phases of meiosis II

• The second division in meiosis is simply a
  mitotic division of the products of meiosis I.

• Meiosis II consists of prophase II, metaphase
  II, anaphase II, and telophase II.

Meiosis II
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Section 10.2 Summary pages 263-273
The phases of meiosis II

• During prophase II, a spindle forms in each of
  the two new cells and the spindle fibers
  attach to the chromosomes.

Prophase II
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Section 10.2 Summary pages 263-273
The phases of meiosis II

• The chromosomes, still made up of sister
  chromatids, are pulled to the center of the
  cell and line up randomly at the equator
  during metaphase II.

Metaphase II
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Section 10.2 Summary pages 263-273
The phases of meiosis II

• Anaphase II begins as the centromere of each
  chromosome splits, allowing the sister
  chromatids to separate and move to opposite
  poles.

Anaphase II
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Section 10.2 Summary pages 263-273
The phases of meiosis II

• Finally nuclei, reform, the spindles break
  down, and the cytoplasm divides during
  telophase II.

Telophase II
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Section 10.2 Summary pages 263-273
The phases of meiosis II

• At the end of meiosis II, four haploid cells
  have been formed from one diploid cell.

• These haploid cells will become gametes,
  transmitting the genes they contain to
  offspring.

98
Section 10.2 Summary pages 263-273
Meiosis Provides for Genetic Variation

• Cells that are formed by mitosis are identical
  to each other and to the parent cell.

• Crossing over during meiosis, however,
  provides a way to rearrange allele
  combinations.

• Thus, variability is increased.

99
Section 10.2 Summary pages 263-273
Genetic recombination

• Reassortment of chromosomes and the genetic
  information they carry, either by crossing over
  or by independent segregation of homologous
  chromosomes, is called genetic recombination.

100
Section 10.2 Summary pages 263-273
Genetic recombination

• It is a major source of variation among
  organisms.

MEIOSIS I
MEIOSIS II
Possible gametes
Possible gametes
Chromosome A
Chromosome B
Chromosome a
Chromosome b
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Section 10.2 Summary pages 263-273
Meiosis explains Mendels results

• The segregation of chromosomes in anaphase I
  of meiosis explains Mendels observation that
  each parent gives one allele for each trait at
  random to each offspring, regardless of whether
  the allele is expressed.

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Section 10.2 Summary pages 263-273
Meiosis explains Mendels results

• The segregation of chromosomes at random
  during anaphase I also explains how factors, or
  genes, for different traits are inherited
  independently of each other.

103
Section 10.2 Summary pages 263-273
Nondisjunction

• The failure of homologous chromosomes to
  separate properly during meiosis is called
  nondisjunction.

104
Section 10.2 Summary pages 263-273
Nondisjunction

• Recall that during meiosis I, one chromosome
  from each homologous pair moves to each pole of
  the cell.

• In nondisjunction, both chromosomes of a
  homologous pair move to the same pole of the
  cell.

105
Section 10.2 Summary pages 263-273
Nondisjunction

• The effects of nondisjunction are often seen
  after gametes fuse.

• When a gamete with an extra chromosome is
  fertilized by a normal gamete, the zygote will
  have an extra chromosome.

• This condition is called trisomy.

106
Section 10.2 Summary pages 263-273
Nondisjunction

• Although organisms with extra chromosomes often
  survive, organisms lacking one or more
  chromosomes usually do not.

• When a gamete with a missing chromosome fuses
  with a normal gamete during fertilization, the
  resulting zygote lacks a chromosome.

• This condition is called monosomy.

107
Section 10.2 Summary pages 263-273
Nondisjunction

• An example of monosomy that is not lethal is
  Turner syndrome, in which human females have
  only a single X chromosome instead of two.

108
Section 10.2 Summary pages 263-273
Nondisjunction
Male parent (2n)
Meiosis
Nondisjunction
Abnormal gamete (2n)
Zygote (4n)
Female parent (2n)
Meiosis
Nondisjunction
Abnormal gamete (2n)
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Section 10.2 Summary pages 263-273
Nondisjunction

• When a gamete with an extra set of
  c chromosomes is fertilized by a normal haploid
  gamete, the offspring has three sets of
  chromosomes and is triploid.

• The fusion of two gametes, each with an extra
  set of chromosomes, produces offspring with
  four sets of chromosomesa tetraploid.

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Section 10.2 Summary pages 263-273
Chromosome Mapping

• Crossing over produces new allele
  combinations. Geneticists use the frequency
  of crossing over to map the relative positions
  of genes on a chromosome.

50
B
A
10
5
A
D
B
C
or
or
10
5
A
D
B
C
D
35
C
or
35
C
D
111
Section 10.2 Summary pages 263-273
Chromosome Mapping

• Genes that are farther apart on a chromosome
  are more likely to have crossing over occur
  between them than are genes that are closer
  together.

50
B
A
10
5
A
D
B
C
or
or
10
5
A
D
B
C
D
35
C
or
35
C
D
112
Section 10.2 Summary pages 263-273
Chromosome Mapping

• Suppose there are four genesA, B, C, and Don
  a chromosome.

35
10
5
C
B
A
D
50
113
Section 10.2 Summary pages 263-273
Chromosome Mapping

• Geneticists determine that the frequencies of
  recombination among them are as follows
  between A and B50 between A and D 10
  between B and C5 between C and D35.

• The recombination frequencies can be converted
  to map units A-B 50 A-D 10 B-C 5 C-D
  35.

114
Section 10.2 Summary pages 263-273
Chromosome Mapping

• These map units are not actual distances on the
  chromosome, but they give relative distances
  between genes. Geneticists line up the genes as
  shown.

35
10
5
C
B
A
D
50
115
Section 10.2 Summary pages 263-273
Chromosome Mapping

• The genes can be arranged in the sequence that
  reflects the recombination data.

• This sequence is a chromosome map.

35
10
5
C
B
A
D
50
116
Section 10.2 Summary pages 263-273
Polyploidy

• Organisms with more than the usual number of
  chromosome sets are called polyploids.

• Polyploidy is rare in animals and almost
  always causes death of the zygote.

117
Section 10.2 Summary pages 263-273
Polyploidy

• However, polyploidy frequently occurs in
  plants.

• Many polyploid plants are of great commercial
  value.

118
Section 10.2 Summary pages 263-273
Gene Linkage and Maps

• If genes are close together on the same
  chromosome, they usually are inherited
  together.

• These genes are said to be linked.

119
Section 10.2 Summary pages 263-273
Gene Linkage and Maps

• Linked genes may become separated on different
  homologous chromosomes as a result of crossing
  over.

• When crossing over produces new gene
  combinations, geneticists can use the
  frequencies of these new gene combinations to
  make a chromosome map showing the relative
  locations of the genes.

120
Section 2 Check
Question 1
A cell with two of each kind of chromosome
is __________.
A. diploid
B. haploid
C. biploid
D. polyploid
CA Biology/Life Sciences 3
121
Section 2 Check
Homologous Chromosome 4
The answer is A. The two chromosomes of each pair
in a diploid cell are called homologous
chromosomes. Each has genes for the same traits.
a
A
Terminal
Axial
Inflated
D
d
Constricted
T
t
Short
Tall
CA Biology/Life Sciences 3
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Section 2 Check
Question 2
Meiosis
Haploid gametes (n23)
What is the importance of meiosis in sexual
reproduction?
Sperm Cell
Meiosis
Egg Cell
Fertilization
Diploid zygote (2n46)
Multicellular diploid adults (2n46)
Mitosis and Development
CA Biology/Life Sciences 2a
123
Section 2 Check
Meiosis is cell division that produces haploid
gametes. If meiosis did not occur, each
generation would have twice as many chromosomes
as the preceding generation.
Meiosis
Haploid gametes (n23)
Sperm Cell
Meiosis
Egg Cell
Fertilization
Diploid zygote (2n46)
Multicellular diploid adults (2n46)
Mitosis and Development
CA Biology/Life Sciences 2a
124
Section 2 Check
Question 3
How does metaphase I of meiosis differ from
metaphase of mitosis?
CA Biology/Life Sciences 2a
125
Section 2 Check
During metaphase of mitosis, sister chromatids
line up on the spindle's equator independent of
each other. During metaphase I of meiosis,
homologous chromosomes are lined up side by side
as tetrads.
Centromere
Metaphase I
Sister chromatids
CA Biology/Life Sciences 2a
126
Chapter Summary 10.1
Mendels Laws of Heredity

• Genes are located on chromosomes and exist in
  alternative forms called alleles. A dominant
  allele can mask the expression of a recessive
  allele.

• When Mendel crossed pea plants differing in one
  trait, one form of the trait disappeared until
  the second generation of offspring. To explain
  his results, Mendel formulated the law of
  segregation.

127
Chapter Summary 10.1
Mendels Laws of Heredity

• Mendel formulated the law of independent
  assortment to explain that two traits are
  inherited independently.

• Events in genetics are governed by the laws of
  probability.

128
Chapter Summary 10.2
Meiosis

• In meiosis, one diploid (2n) cell produces four
  haploid (n) cells, providing a way for offspring
  to have the same number of chromosomes as their
  parents.

• In prophase I of meiosis, homologous chromosomes
  come together and pair tightly. Exchange of
  genetic material, called crossing over, takes
  place.

129
Chapter Summary 10.2
Meiosis

• Mendels results can be explained by the
  distribution of chromosomes during meiosis.

• Random assortment and crossing over during
  meiosis provide for genetic variation among the
  members of a species.

130
Chapter Summary 10.2
Meiosis

• The outcome of meiosis may vary due to
  nondisjunction, the failure of chromosomes to
  separate properly during cell division.

• All the genes on a chromosome are linked and are
  inherited together. It is the chromosome rather
  than the individual genes that are assorted
  independently.

131
Chapter Assessment
Question 1
Heterozygous tall parent
Predict the possible genotypes of the offspring
of parents who are both heterozygous for height.
t
T
T
t
T
t
T
t
Heterozygous tall parent
CA Biology/Life Sciences 3a, 2g
132
Chapter Assessment
There are three different possible genotypes TT,
Tt, and tt.
Heterozygous tall parent
t
T
T
t
T
t
T
t
Heterozygous tall parent
CA Biology/Life Sciences 3a, 2g
133
Chapter Assessment
Question 2
The law of __________ states that every
individual has two alleles of each gene and
gametes that are produced each receive one of
these alleles.
C. independent assortment
A. dominance
D. segregation
B. recessive traits
CA Biology/Life Sciences 3b
134
Chapter Assessment
Law of segregation
Tt x Tt cross
The answer is D. Mendel's law of segregation
explained why two tall plants in the F1
generation could produce a short plant.
F1
Tall plant
Tall plant
T
t
T
t
F2
Short
Tall
Tall
Tall
t
t
t
T
t
T
T
T
1
3
1
CA Biology/Life Sciences 3b
135
Chapter Assessment
Question 3
The allele combination an organism contains is
known as its __________.
A. phenotype
B. genotype
C. homozygous trait
D. heterozygous trait
CA Biology/Life Sciences 3
136
Chapter Assessment
The answer is B. The genotype gives the allele
combination for an organism. The genotype of a
tall plant that has two alleles for tallness is
TT.
CA Biology/Life Sciences 3
137
Chapter Assessment
Question 4
What is the phenotype of a plant with the
following genotype TtrrYy
A. tall plant producing round yellow seeds
B. short plant producing round yellow seeds
C. tall plant producing wrinkled yellow seeds
D. short plant producing round green seeds
CA Biology/Life Sciences 3a, 2g
138
Chapter Assessment
The answer is C. This plant is heterozygous
dominant for tallness and seed color, and
homozygous recessive for seed shape. If the
genotype of an organism is known, its phenotype
can be determined.
CA Biology/Life Sciences 3a, 2g
139
Chapter Assessment
Punnett Square of Dihybrid Cross
Gametes from RrYy parent
Ry
RY
rY
ry
RRYY
RRYy
RrYY
RrYy
RY
F1 cross RrYy RrYy
RRYy
RRYy
RrYy
Rryy
round yellow
Ry
Gametes from RrYy parent
round green
RrYY
RrYy
rrYY
rrYy
rY
wrinkled yellow
RrYy
Rryy
rrYy
rryy
ry
wrinkled green
CA Biology/Life Sciences 3a, 2g
140
Chapter Assessment
Question 5
During which phase of meiosis do the tetrads
separate?
A. anaphase I
B. anaphase II
C. telophase I
D. telophase II
CA Biology/Life Sciences 2a
141
Chapter Assessment
The answer is A. The tetrads separate during
anaphase I. The sister chromatids separate during
anaphase II.
CA Biology/Life Sciences 2a
142
Chapter Assessment
Question 6
Look at the diagram and determine which of the
following has the TT genotype.
T
t
A. 1
T
1
2
B. 2
C. 3
t
3
4
D. 4
CA Biology/Life Sciences 3a, 2g
143
Chapter Assessment
The answer is A. Only 1 has the genotype TT. Both
2 and 3 have the genotype Tt, and only 4 has
genotype tt.
T
t
1
2
T
TT
Tt
4
3
t
Tt
tt
CA Biology/Life Sciences 3a, 2g
144
Chapter Assessment
Question 7
The failure of homologous chromosomes to separate
properly during meiosis is _________.
A. crossing over
B. nondisjunction
C. trisomy
D. genetic recombination
CA Biology/Life Sciences 2a
145
Chapter Assessment
The answer is B. Nondisjunction can result in
several types of gametes, including one with an
extra chromosome and one missing a chromosome, as
well as gametes inheriting a diploid set of
chromosomes.
CA Biology/Life Sciences 2a
146
Chapter Assessment
Male parent (2n)
Meiosis
Nondisjunction
Abnormal gamete (2n)
Zygote (4n)
Female parent (2n)
Meiosis
Nondisjunction
Abnormal gamete (2n)
CA Biology/Life Sciences 2a
147
Chapter Assessment
Question 8
Which of the following statements is true?
A. Individual genes follow Mendel's law of
independent assortment.
B. Genes that are close together on the same
chromosome are usually inherited together.
CA Biology/Life Sciences 2a
148
Chapter Assessment
Question 8
Which of the following statements is true?
C. Genes that are farther apart on a
chromosome are less likely to have
crossing over occur between them than
genes that are closer together.
D. Crossing over occurs in only one
location on a chromosome at a time.
CA Biology/Life Sciences 2a
149
Chapter Assessment
The answer is B. Genes that are close together
on the same chromosome are usually inherited
together, and are said to be linked. Linked genes
may become separated as a result of crossing over.
CA Biology/Life Sciences 2a
150
Chapter Assessment
Question 9
Organisms with more than the usual number of
chromosome sets are called __________.
A. diploids
B. haploid
C. triploids
D. polyploids
CA Biology/Life Sciences 2
151
Chapter Assessment
The answer is D. Polyploidy is rare in animals
but occurs frequently in plants. Because the
flowers and fruits of polyploid plants are often
larger than normal, these plants have great
commercial value.
CA Biology/Life Sciences 2
152
Chapter Assessment
Question 10
How many different kinds of eggs or sperm can a
person produce?
A. 23
B. 46
C. 529
D. over 8 million
CA Biology/Life Sciences 2a
153
Chapter Assessment
The answer is D. The number of chromosomes in a
human is 23. Because each chromosome can line up
at the cell's equator in two different ways, the
number of possible type of egg or sperm is 223.
CA Biology/Life Sciences 2a
154
Photo Credits
Photo Credits

• PhotoDisc
• Alton Biggs

155
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