The document provides information about several key concepts in meiosis and genetics: 1) It describes the behavior of chromosomes during the phases of meiosis and explains how crossing over results in genetic variation through the formation of chiasmata during prophase I. 2) It states Mendel's Law of Independent Assortment, which is that each pair of alleles segregates independently into gametes during meiosis. 3) It explains the relationship between Mendel's law and random orientation of homologous chromosomes during metaphase I of meiosis, which results in independent assortment.

1. IB Biology
10 Genetics HL
10.1 Meiosis
All syllabus statements ©IBO 2007
All images CC or public domain or link to original material.
Jason de Nys
http://www.flickr.com/photos/carolinabio/6241450806/

2. 10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis

3. The best way to
see what happens
in this dynamic
process is to view
some animations
67% of the animations on this page are voiced
by men with deep voices. Therefore, they must
be authoritative.

4. Make your own summary
notes of what goes on
during meiosis, refer to the
animations as necessary

5. 10.1.2 Outline the formation of chiasmata in the process of crossing over
Hmmmm… chiasmata,
chiasmata,
chiasmata…

6. 10.1.2 Outline the formation of chiasmata in the process of crossing over
Hmmmm… chiasmata,
chiasmata,
chiasmata…
Singular: chiasma….
where have I heard that before?

7. Ah yes!
The place where some of the optic nerves cross
over in the brain (Option E: E2.5 maybe you haven’t done it yet,
or you might do a different option)
Greek khīasma, cross-piece,
from khīazein, to mark with
an X, from khei, khī, chi
(from the letter's shape).
http://www.wordnik.com/words/chiasma
http://en.wikipedia.org/wiki/File:Gray773.png

8. Crossing over involves the swapping of segments of
DNA between chromatids on homologous pairs
The points at which the
chromatids cross are the
chiasmata

9. Chiasmata: evidence of
exchange between
chromatids
This micrograph shows a
pair of homologous
chromosomes, each with
two chromatids, during
prophase I of meiosis in a
salamander.
Two chiasmata are visible.
© 2008 Sinauer Associates Sadava, D. et al. Life: The Science of Biology, 8th ed.
(Sunderland, MA: Sinauer Associates and W. H. Freeman & Company), 198. Used
with permission. All rights reserved

10. 10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through
crossing over in prophase I and random orientation in metaphase I
• Crossing over can occur on any part of a
chromosome.
• The size of the section swapped between
chromosomes can be almost any size.
• The number of chiasmata on each chromatid can
vary
These three points alone lead to innumerable
possibilities
View the animation to answer
the question on the next slide

11. One of the strange results
you get when you Google
“random orientation”

13. 10.1.4 State Mendel’s Law of independent assortment.
Each pair of alleles segregates into
gametes independently
Also… mmm…
http://www.flickr.com/photos/staipale/2580650017/
http://commons.wikimedia.org/wiki/File:Gregor_Mendel.png

14. An example of the independent
assortment of the gene for
colour (green [y] or yellow [Y])
and the gene for pod type
(smooth [R] or wrinkled [r])
In Italian for
your pleasure!
http://commons.wikimedia.org/wiki/File:Independent_assortment_%26_segregation-it.svg

15. 10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis
Mendel
examining his My
pea flowers babies…
http://www.flickr.com/photos/kaptainkobold/356759039/

16. Mendel new nothing of modern genetics, genes had not been
discovered. He was working from the traits he observed, the
phenotypes. He named the heritable factors alleles.
So his Law:
Each pair of alleles
segregates into gametes
independently.
…relates to the random
orientation of
homologous chromosomes
during meiosis in
metaphase I
http://commons.wikimedia.org/wiki/File:MetaphaseIF.jpg

17. Further information:
Doo do do do do ,do doo do
do do do, do do do
Three of the best sites for
IB-specific Biology
information. The top link
takes you to the PPT by
Stephen Taylor