Chapter 13.
Meiosis &
Sexual Reproduction
AP Biology
Modified from: Kim Foglia,
Explore Biology
How about the rest of us?
 What if a complex multicellular organism
(like us) wants to reproduce?

joining of egg + sperm
 Do we make egg & sperm by mitosis?
46
egg
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+
46
92
sperm
zygote
Human female karyotype
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Human male karyotype
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How do we make sperm & eggs?
 reduce 46 chromosomes  23 chromosomes

half the number of chromosomes
23
46
meiosis
23
46
egg
23
46
23
sperm
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fertilization
Meiosis: production of gametes
 Alternating processes,
alternating stages

chromosome number
must be reduced
 diploid  haploid
 2n  n
 humans: 46  23
 meiosis reduces
chromosome number

fertilization restores
chromosome number
 haploid  diploid
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 n  2n
Homologous chromosomes
 Paired chromosomes

both chromosomes of a pair carry genes
 control same inherited characters
 homologous = same information
diploid
2n
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homologous
chromosomes
double stranded
homologous chromosomes
Double division
of meiosis
DNA replication
1st division of
meiosis separates
homologous pairs
2nd division of
meiosis separates
sister chromatids
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Steps of meiosis
 Meiosis 1
interphase
 prophase 1
 metaphase 1
 anaphase 1
 telophase 1

 Meiosis 2
prophase 2
 metaphase 2
 anaphase 2
 telophase 2
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
1st division of
meiosis separates
homologous pairs
(2n  1n)
2nd division of
meiosis separates
sister chromatids
(1n  1n)
* just like mitosis *
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Crossing over
 During Prophase 1
homologous pairs swap
pieces of chromosome
 sister chromatids intertwine
 crossing over

tetrad
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synapsis
Crossing over
 3 steps
What are the
advantages of
sexual reproduction?
cross over
 breakage of DNA
 re-fusing of DNA

 New combinations of traits
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Genetic variation
 Meiosis & crossing over introduce
great genetic variation to population

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drives evolution
The value of meiosis
 Meiosis introduces genetic variation
gametes of offspring do not have same
genes as gametes from parents
 genetic recombination

 random assortment in humans produces 223
(8,388,608) different combinations
Mom
APfrom
Biology
from Dad
new gametes
made by offspring
And more variation…
 Crossing over

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creates completely new
combinations of traits
in next generation
Random fertilization
 Any 2 parents will produce a zygote
with over 70 trillion (223 x 223) diploid
combinations
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Sources of genetic variability
 Genetic variability in sexual reproduction

independent assortment
 homologous chromosomes in Meiosis 1

crossing over
 between homologous chromosomes in
prophase 1

random fertilization
 random ovum fertilized by a random sperm
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metaphase1
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both
genetic similarity & differences.
Michael & Kirk
Douglas
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Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
Cell A has 4 chromosomes and Cell D has 2
chromosomes. What process created Cell D?
1. Mitosis
2. Asexual
3.
4.
Reproduction
Fusion
Meiosis
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Cel
lA
Cel
lB
Cel
lC
Cel
lD
1
0
Cell A is considered _______ while Cell D is
considered ________.
1.
2.
3.
4.
Haploid; Diploid
Tetrad; Diploid
Diploid; Haploid
Homologous; Haploid
Cel
lA
0%
1
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0%
0%
2
3
Cel
lB
Cel
lC
Cel
lD
0%
4
1
0
Cell A is considered _______ while
Cell D is considered ________.
A. somatic cell;
gamete
B. gamete; somatic
cell
C. stem cell; somatic
cell
D. stem cell; gamete
0%
et
e
m
at
st
e
m
ce
ll;
so
m
ce
ll;
m
st
e
3
0
ga
ic
ic
at
so
m
et
e;
ga
m
0%
ce
ll
ce
et
e
m
ga
l;
ce
l
at
ic
so
m
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0%
ll
0%
How can we best describe the diagram
below?
1.
2.
3.
4.
Two sister chromatids
Homologous chromosomes
One replicated chromosome
Haploid chromsomes
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0%
1
0%
2
0%
3
0%
4
1
0
What process is best demonstrated by the
diagram below?
1.
2.
3.
4.
Anaphase I
Segregation of alleles
Crossing over
Independent assortment
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0%
1
0%
2
0%
3
0%
4
1
0
Which of the following leads to
genetic variation during meiosis?
A.Crossing Over
B.DNA replication
C.Independent assortment
spindle formation
1. All D.Mitotic
of the above (A, B, C,
2.
3.
4.
5.
D)
A&B
A&D
A, B, & D
A&C
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0%
1
0%
0%
2
3
0%
0%
4
5
1
0
Mitosis vs. Meiosis
 Mitosis






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1 division
daughter cells
genetically identical
to parent cell
produces 2 cells
2n  2n
produces cells for
growth & repair
no crossing over
 Meiosis

2 divisions
daughter cells
genetically different
from parent
produces 4 cells
2n  1n
produces gametes

crossing over




Mitosis vs. Meiosis
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Changes in Chromosomes
Number
 Euploidy = correct # of chromosomes
 Aneuploidy = a change in
chromosomes number due to nondisjunction during meiosis
 Monosomy- only 1 copy of an individual
chromosome
 Trisomy- 3 copies of an individual
chromosome
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Non-disjuction
Primary nondisjuction = Meiosis I
Secondary nondisjuction = Meiosis II
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Trisomy 21 : Down Syndrome







Delayed mental and social skills
Decreased muscle tone at birth
Asymmetrical or odd-shaped skull
Small skull
Small mouth with protruding tongue
Broad short hands
Increased risk of developing
Leukemia and Alzheimer’s later in life
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Trisomy 18 : Edward’s Syndrome
 Most children die in the first year of life, some have lived 10









years
Growth deficiency
Feeding difficulties
Breathing difficulties
Developmental delays
Mental Retardation
Overlapped, flexed fingers
Webbing of the second and third toes
Clubfeet
Structural heart defects at birth
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Trisomy 13 : Patau Syndrome










Mental retardation, severe
Seizures
Small head
Scalp defects
Cleft lip and/or palate
Eyes close set (hypotelorism) –may fuse
Extra digits (polydactyl)
Hernias
Undescended testicle
Children die in the first year of life
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Karyotype
 A visual display of the chromosomes arranged by
size, shape, and banding pattern
 Used to identify aneuploid conditions
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Procedure: Amniocentesis and
Karyotyping
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Karyotyping
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Changes in Sex Chromosome #






Turners Syndrome (XO) – missing Barr Body
Kleinfelter’s Syndrome (XXY)
Swyer Syndrome (XY female)
La Chapelle Syndrome (XX male)
Poly-X Females (XXX, XXXX)
Jacob’s Syndrome (XYY males)
-SRY gene (located on short arm of Y chromosome)
-hormone= testis-determining factor
-Barr Body – Inactive X chromosome (XX)
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Chromosomal Mutations
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Deletion Syndromes
 Williams Syndrome (deletion of a piece
of chromosome 7)
 Cri du chat (cat’s cry) (deletion of a
piece of chromosome 5)
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Translocation Syndromes
 Alagille syndrome – Chromosomes 2
and 20 exchange segments
 Cancers
Chronic myelogenous leukemia (2 and
9)
 Burkitt lymphoma (8 and 14)

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What are the
DISadvantages of
sexual reproduction?
Any Questions??
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