1. Human Biology Sylvia S. Mader Michael Windelspecht
Chapter 18
Patterns of
Chromosome Inheritance
Lecture Outline
Part 3
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2. 18.4 Meiosis
What is crossing-over?
Crossing-over is the exchange of genetic information between ______________ sister chromatids during synapsis.
This occurs during ___________ of meiosis and increases genetic variation.

3. c. resulting chromatids
18.4 Meiosis
What is crossing-over?
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sister chromatids
of a chromosome
sister chromatids
of its homologue A a A A a a B b B b B b
crossing-over
between
chromatids
1 and 3 C c C c C c D d D D d d 1 2 3 4 1 2 3 4 1 2 3 4
a. synapsis
b. crossing-over
c. resulting chromatids
Figure Synapsis and crossing-over increase variability. 3

4. What is independent alignment?
18.4 Meiosis
What is independent alignment?
In metaphase I, homologous pairs align independently at the equator.
Maternal or paternal member may be oriented toward either pole.
It gives rise to various possible combinations of maternal and paternal chromosomes in gametes descended from this cell.

5. Independent alignment
18.4 Meiosis
Independent alignment
Figure Independent alignment at metaphase I increases variability.

6. Meiosis II Prophase II Chromosomes _________ again. Metaphase II
Chromosomes align at the equator.
Anaphase II
Sister chromatids separate to opposite poles.
Telophase II
4 daughter cells result, each with 23 _________________ chromosomes.

7. Cells have one chromosome from each homologous pair.
18.4 Meiosis
Meiosis II
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Prophase II
Cells have one chromosome
from each homologous pair.
Figure The phases of meiosis.

8. Cells have one chromosome from each homologous pair.
18.4 Meiosis
Meiosis II
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haploid with
paired sister
chromatids
haploid with
paired sister
chromatids
Prophase II
Cells have one chromosome
from each homologous pair.
Metaphase II
Chromosomes align
at the equator.
Figure The phases of meiosis. 8

9. Meiosis II 18.4 Meiosis Figure 18.10 The phases of meiosis. 9
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haploid with
paired sister
chromatids
haploid with
paired sister
chromatids
Prophase II
Cells have one chromosome
from each homologous pair.
Metaphase II
Chromosomes align
at the equator.
Anaphase II
Sister chromatids separate and
become daughter chromosomes.
Figure The phases of meiosis. 9

10. Meiosis II 18.4 Meiosis Figure 18.10 The phases of meiosis. 10
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haploid with
paired sister
chromatids
haploid with
paired sister
chromatids
Telophase II
Spindle disappears, nuclei form,
and cytokinesis takes place.
Prophase II
Cells have one chromosome
from each homologous pair.
Metaphase II
Chromosomes align
at the equator.
Anaphase II
Sister chromatids separate and
become daughter chromosomes.
Figure The phases of meiosis. 10

11. Meiosis II 18.4 Meiosis Figure 18.10 The phases of meiosis. 11
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haploid
haploid with
paired sister
chromatids
haploid with
paired sister
chromatids
haploid
Telophase II
Spindle disappears, nuclei form,
and cytokinesis takes place.
Daughter cells
Meiosis results in four
haploid daughter cells.
Prophase II
Cells have one chromosome
from each homologous pair.
Metaphase II
Chromosomes align
at the equator.
Anaphase II
Sister chromatids separate and
become daughter chromosomes.
Figure The phases of meiosis. 11

12. The production of sperm and eggs
18.4 Meiosis
The production of sperm and eggs
___________________
The process of making sperm in males.
A continual process after puberty.
About 400 million sperm are produced per day.

13. The production of sperm and eggs
18.4 Meiosis
The production of sperm and eggs
______________
The process of making eggs in females.
During meiosis, 1 egg and 3 ___________ are formed.
Polar bodies act to hold discarded chromosomes and thus disintegrate.
Normally, 1 egg per month is produced, and ____ are produced during the entire reproductive cycle. 13

14. The production of sperm and eggs
18.4 Meiosis
The production of sperm and eggs
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SPERMATOGENESIS
primary
spermatocyte 2 n
Meiosis I
secondary
spermatocytes n
Meiosis II
spermatids n
Metamorphosis
and maturation
sperm n
OOGENESIS
primary
oocyte 2n
Meiosis I
first
polar body n
secondary
oocyte n
Meiosis II
Meiosis II is completed
after entry of sperm.
second
polar body n
Fertilization
cont'd
egg n
sperm nucleus n
Figure A comparison of spermatogenesis and oogenesis in mammals.
fusion of sperm
nucleus and
egg nucleus
zygote 2n