Reduction-Division
Genetic Recombination
1
2
3
n=23
human
sex cell
sperm
n=23
n=23
2n=46
diploid (2n)
n=23
n=23
haploid (n)
n=23
Meiosis I
Meiosis II
4
Haploid
(1n)
n=23
human
sex cell
egg
n=23
2n=46
diploid (2n)
Meiosis I
Polar
Bodies
(die)
n=23
Meiosis II
5
6
chromatin
Nucleus
cell
membrane
nucleolus
7
Prophase I
8
9
10
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
11
LOCI
12
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
13
14
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
15
16
XX chromosome - female
XY chromosome - male
17
Meiosis I
Homologs
separate
18
spindle fiber
centrioles
aster
fibers
TETRAD
19
20
OR
21
 Formula:
 Example:2n
then
2n
= 4
1n = 2
thus 22 = 4
combinations
22
In
terms of Independent
Assortment -how many
different combinations of
sperm could a human male
produce?
23
Formula:
2n
Human chromosomes: 2n =
46
n = 23
223 = ~8 million combinations
24
25
26
27
cytokinesis
28
Meiosis II
Sister
Chromatids
Separate
29
No Interphase II or very short
 No DNA Replication
 Remember: Meiosis II is similar
to mitosis

30
Same
as Prophase in mitosis
Nucleus & nucleolus disappear
 Chromosomes condense
 Spindle forms

31
Same
as Metaphase in mitosis
Chromosomes (not homologs) line up at equator
32
Same
as Anaphase in mitosis
SISTER CHROMATIDS
separate
33
 Same
as Telophase in mitosis.
 Nuclei and Nucleoli reform, spindle
disappears
 CYTOKINESIS occurs.
 Remember: FOUR HAPLOID
DAUGHTER cells are produced.
 Called GAMETES (eggs and sperm)
1n Sperm cell
fertilizes 1n egg
to form 2n zygote
34
35
36
 Also
known as GENETIC
RECOMBINATION
 Important to population as the raw
material for NATURAL SELECTION.
 All organisms are NOT alike
 Strongest
“most fit” survive to
reproduce & pass on traits
37
What
are the 3 sources of
genetic recombination or
variation?
38
1.
CROSSING OVER (prophase I)
2.
INDEPENDENT ASSORTMENT
(metaphase I)
3.
RANDOM FERTILIZATION
39
A
cell containing 20 chromosomes
(diploid) at the beginning of
meiosis would, at its completion,
produce cells containing how many
chromosomes?
40
10
1n)
chromosomes (haploid or
41
 An
organized
picture of the
chromosomes of a
human arranged in
pairs by size from
largest to smallest.
 Pairs 1-22 called
AUTOSOMES
 Last pair are SEX
CHROMOSOMES
Male - XY
42
Female - XX
43
Down Syndrome – Trisomy 21
Female - XX
44
 The
fusion of a sperm and egg to
form a zygote.
 A zygote is a FERTILIZED EGG
n=23
egg
sperm
n=23
2n=46
zygote
45
A
cell containing 40 chromatids at
the beginning of meiosis would, at
its completion, produce cells
containing how many chromosomes?
46
10
chromosomes
47