Chapter 13

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Chapter 13 An Introduction to Heredity Meiosis
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Heredity The transmission of traits from one
generation to the next
Genes are DNA segments A locus is a genes
specific location along the length of a
chromosome Offspring acquire genes from parents
by inheriting chromosomes
3
Characteristics of Asexual Reproduction
Asexual reproduction form of cell division A
system of cloning and creating exact genetic
replicas so there exists no genetic variation
Single-celled organisms can reproduce to create
new organisms
Budding is a form of asexual reproduction in
multi-cellular organisms
4
Characteristics of Sexual Reproduction
2 parents must give rise to offspring each
parent giving a haploid set of chromosomes
Two haploid gametes (reproductive cells) must
combine to create a new diploid organism.
Each cell of a diploid organism must have 2 sets
of chromosomes. One set donated from the mom, the
other set donated from dad.
Offspring has greater genetic variation
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Homologous Chromosomes a pair of chromosomes
containing the same linear gene sequences, each
derived from one parent.
Human cells have a diploid number of 46 2N
46 This means each cell has 23 homologous pairs
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There is one exception The sex chromosomes (pair
23), is homologous in females (XX) but is only
hemizygous in males (XY)
Karyotypes (such as these) are pictures of an
individuals chromosomes. Autosomes are non-sex
chromosomes (pairs 1- 22 in humans)
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Lets review mitosis a form of nuclear
division, whereby the chromosome number and
genetic content in the organism remains the same
in both parent and daughter cells.
Meiosis A form of nuclear division that results
in the reduction of chromosome number by half
(from diploid cell to a haploid cell)
How does mitosis and meiosis compare?
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How does meiosis increase genetic diversity?
Crossing over during prophase I Independent
Assortment or random alignment of homologous
chromosomes along metaphase plate during
metaphase I Segregation (separation) of
homologous chromosomes in Anaphase I and
segregation of sister chromatids in Anaphase
II Random Fertilization
Genetic diversity in a species increases the
species chances of survival and prevents
extinction. In a diverse gene pool, there is a
higher probability of a gene that increases
fitness during times of instability or stress.
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Varieties in Sexual Life Cycles
Type 1 Dominant Stage (stage it spends most of
its life in) Diploid Multicellular (Ex. Humans)
2N
N
Mitosis/Cytokinesis
Meiosis/cytokinesis
2N
N
Fertilization
Diploid Multicellular
Haploid unicellular
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Varieties in Sexual Life Cycles
Type 2 Dominant Stage Haploid Multicellular
(Ex. Fungi and Algae)
N
Diploid Single Cell
Mitosis/cytokinesis
Haploid multicellular
2N
N
fertilization
Meiosis/Cytokinesis
One cell from multicellular haploid cluster is
designated a haploid gamete
N
N
N
N
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Sordaria (fungi) lab 3 Example of Type 2 Life
Cycle

• Gene that codes for ascospore color
• Black () wildtype black spore ?
• Tan (tn) mutant brown spore ?
• Sordaria is a haploid organism, so fungis
  phenotype for spore color is dependent on the
  single gene that it inherits

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Example of Type 2 Life Cycle
Areas where both haploid strands can fuse, become
diploid, and undergo immediate meiosis/cross over.
Black spores
Tan spores
Both haploid strands grown on an agar plate
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Example of Type 2 Life Cycle
Two haploid genes fuses to become diploid
Cross over might occur
Ascus containing 8 ascospores
Completion of meiosis
mitosis
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Ascopore pattern w/o crossover
Gene expression
tn
tn
tn
tn
tn
Meiosis II
tn
Mitosis
tn
tn
tn
tn
tn
tn
tn
tn
DNA replication
Meiosis I








Meiosis II

Mitosis





Ascospore with 4X4 pattern
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Ascopore pattern 1 w/ crossover
tn
tn
tn
tn
Meiosis II
tn
Mitosis
tn
tn




tn

DNA replication
Meiosis I

tn

tn
tn
tn



tn

Meiosis II

Mitosis

Cross-over


Ascospore with 2X2x2x2 pattern

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Ascopore pattern 2 w/ crossover



Meiosis II
tn
Mitosis
tn
tn
tn

tn
tn

tn
tn
DNA replication
Meiosis I

tn
tn

tn
tn




tn
Meiosis II
Mitosis



Cross-over

Ascospore with 2x4x2 pattern

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Varieties in Sexual Life Cycles
Type 3 Alternation of Generation Dominant
haploid AND diploid multicellular stages
One cell from multicellular haploid cluster is
designated a haploid gamete
2N
Meiosis/cyt
Mitosis/cyt
N
N
Diploid multicellular stage Sporophyte
4 Haploid spores
Haploid multicellular stage Gametophyte
fertilization
Mitosis/cyt
N
Meiosis/cyt
2N
N
Mitosis/Cytokinesis
2N
2N
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What can go wrong in meiosis?

• No

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Non-disjunction Disorders
Definition When members of homologous
chromosomes fail to separate during Meiosis
I or when sister chromatids fail to
separate during Meiosis II. Examples Down
Syndrome, Turners syndrome, Klinefelters
syndrome
Meiosis I - Failure to separate
Meiosis II Failure to separate
Abnormal Gametes
Normal Gametes
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Polyploidy

• Polyploidy is a term that describes the
  inheritance of more that two sets of chromosomes
  due to non-disjunction during meiosis

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Interpret these karyotypes
Klinefelters syndrome
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Interpret these karyotypes
Down Syndrome
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Try these on-line activities

• http//www.biology.arizona.edu/human_bio/activitie
  s/karyotyping/karyotyping2.html

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X-inactivation (in mammals)

• In female mammals, one of the X-chromosomes turns
  off and condenses into a compact barr body.
• The barr body is reactivated in the ovary cells
  only during meiosis for reproduction.

Two X-chromosomes (autosomes not shown)
Barr body due to X-inactivation
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Other non-disjuntion disorders

• XO individual (missing 1-X chromosome)
  physically female (same phenotype as normal
  female who has barr body), but sterile since
  there is no barr body to reactivate during
  puberty
• XXX female sterile
• XYY male abnormally tall
• XXY male one X becomes a barr body, so
  phenotypically male, but is sterile when barr
  body is reativated. He has abnormally small
  testes

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What is a mutation?

• Mutations are changes in the DNA. During meiosis,
  there are 4 different types of chromosomal
  mutations that can occur.

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• Deletion A segment of the chromosome is removed
  (not just
• one nuclotide)

A
B
A
B
C
C
D
E
E
F
F
G
H
G
H
2. Duplication A segment of the chromosome is
repeated
E
D
E
F
A
B
A
B
C
C
D
B
C
F
G
H
G
H
3. Inversion A segment within a chromosome is
reversed
4. Translocation A segment from one chromosome
moves to another, non-homologous one
A
B
A
B
C
C
D
D
E
E
F
F
G
H
G
H
M
N
O
A
B
C
D
E
F
G
H
C
D
E
F
G
H
A
B
M
N
O
P
Q
R
P
Q
R