Do Now #1
• A cell containing both sets of
homologous chromosomes is said to
be ______ , meaning “two sets”.
• Cells that contain a single set of
chromosomes is called _____.
• Where can animal cells that contain
only one set of chromosomes be
found?
Do Now # 2
Calculating Haploid & Diploid Numbers
Organisms
Haploid Number
Amoeba
N=25
Chimpanzee
N=24
Earthworm
N=18
Fern
Diploid Number
2N=1010
Hamster
N=22
Honeybee
N=28
Human
N=23
Onion
N=8
Do Now # 3
HINT: The colored
arrows represent
the ploidy (sets of
chromosomes).
Do Now # 4
After Meiosis I, are the cells haploid or diploid?
Spindle
fibers
Nucleus
Nuclear
envelope
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
Why do you share some, not all
traits of each parent?
Goal of Meiosis: reduce genetic material by half
Why?
from mom
from dad
child
too
much!
meiosis reduces
genetic content
How are haploid gamete cells
produced from diploid cells?
Overview of Meiosis
• Two Cell Divisions (Meiosis I & II)
• Begin with 1 diploid cell
• End with 4 haploid cells
• Provides variation
• Interphase
• Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis
• Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
The Key Difference Between Mitosis and Meiosis is
the Way Chromosomes Uniquely Pair and Align in
Meiosis
Mitosis
The 1st
division of
meiosis
Meiosis I
Spindle
fibers
Nucleus
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Nuclear
envelope
Anaphase I
(diploid)
Telophase I
(haploid)
What do
you see?
Early prophase
Prophase I
Late prophase
Meiosis Makes a Variety of Different Sex Cells
– Crossing-Over
Crossing-over multiplies the already huge number of
different gamete types produced by independent
assortment.
Crossing-over
• Occurs in prophase
of meiosis I
A
A
B
B
C
• Generates diversity
b
C
D D
E
F
E
F
• Creates chromosomes with new
combinations of alleles for genes A to F.
a
a
c
b
c
d
d
e
f
e
f
Crossing-over
• Occurs in prophase
of meiosis I
A
a
B
C
• Generates diversity
Letters denote genes
Case denotes alleles
C
c
D D
E
F
d
E
F
e
f
b
c
d
e
f
•Creates chromosomes with new combinations
of alleles for genes A to F.
Crossing-over
• Occurs in prophase
of meiosis I
a
A
B
C
C
D
D
• Generates diversity
Letters denote genes
Case denotes alleles
E
F
b
E
F
A
a
B
c
b
c
d
d
e
f
Creates chromosomes with new
combinations of alleles for genes A to F.
e
f
What do
you see?
Metaphase I
Remember what
happened during Mitosis
metaphase 
What do
you see?
Anaphase I
Remember what
happened during Mitosis
anaphase 
What do
you see?
Telophase I
Meiosis II
Prophase II
(haploid)
Metaphase II
(haploid)
Anaphase II
(haploid)
Telophase II
(haploid)
Four
nonidentical
haploid
daughter cells
What do
you see?
Prophase II
What do
you see?
Metaphase II
What do
you see?
Anaphase II
What do
you see?
Telophase II
What do
you see?
Results of meiosis
Cells alive!
• Gametes
• Four haploid cells
• One copy of each
chromosome, instead
of a pair
• DIVERSITY
Mitosis
# Cell Divisions
Where
Role
Sets of Genetic
Material
# of Daughter
Cells Produced
Genetically
Identical
Similarities
Meiosis
Oogenesis
First polar body
may divide
(haploid)
a
Mitosis
Oogonium
(diploid)
A
X
Polar
bodies
die
X
Meiosis I
Meiosis II
X
Primary
oocyte
(diploid)
X
a
X
a
a
A
X
A
X
Secondary
oocyte
(haploid)
Ovum (egg)
A
X
Second
polar body
(haploid)
Mature
egg
Spermatogenesis: sperm
formation
“Putting It All Together” - Fertilization
Meiosis is All About…
allowing the creation
of unique individuals
through sexual
reproduction.
Why are we so unique?
Independent assortment
The homolog of one chromosome can be inherited
with either homolog of a second chromosome.
Random assortment
The combination of maternal and parental
chromosomes received by a gamete is random.
And we have 23 pairs of chromosomes
The possible combinations in an egg or a sperm are:
223
=
# of combinations in an offspring
223 x 223 =
This doesn’t included crossing-over!
Review Meiosis
•
•
•
•
Meiosis Animation with Quiz
Step by Step Meiosis at Cells Alive!
Step by Step Animations with Text
Shockwave Video
Can mistakes
happen ?
Is this a female
or male
karyotype?
Turner’s
Syndrome:
monosomy
• Female XO
• 1/2,500 – 1/3,000
• Underdeveloped female characteristics
– Short stature
– Broad chest
– Horseshoe kidney
- absence of period (amenorrhea)
- infertile
• 98% are spontaneously aborted -miscarriage
Uniparental disomy
• Two copies of the
chromosome are
from one parent.
• Rare recessive
disorders can arise;
otherwise no physical
anomalies.
– Prader-Willie Syndrome
– Angelman Syndrome
• 1/800-1000 births
• Extra copy of
chromosome 21
• 47 chromosomes
total
• Amniocentesis
Down
Syndrome
Klinefelter's syndrome
• XXY
• Most common sex
chromosome
disorder
• 1 out of 750 males
• 47 chromosomes
• Sterile Male with
some female
characteristics
Triple X
Syndrome
• XXX
• 1 in 1,000
females
• Normal sexual
development
• Typically fertile
Fertilization
What is
fertilization?
– The fusion of
egg & sperm
– Restores the
species # of
chromosomes
– Creates a
ZYGOTE
What are the
advantages &
disadvantages
for both?
External
Fertilization
vs.
Internal
Fertilization