Meiosis AHL
Outcomes
10.1
10.1.1 Describe the behaviour of the chromosomes in the
phases of meiosis.
10.1.2 Outline the formation of chiasmata in the process of
crossing over.
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.
10.1.4 State Mendel’s law of independent assortment.
10.1.5 Explain the relationship between Mendel’s law of
independent assortment and meiosis.
A video - overview
http://www.youtube.com/watch?v=kVMb4Js99tA&
feature=player_embedded
2n n
Homologous Chromosomes
Sister Chromatids and duplicated chromosomes
Replication
Prophase I and Synapsis
Prophase I and Crossing Over
Anaphase I
Infinite Genetic Possibilities!
The sex cells produced during meiosis have the potential to produce
offspring with variety.
This is due to the
processes of
crossing over
during prophase l
and the random
orientation during
metaphase l.
The sister chromatids formed
during replication are no longer
identical following crossing
over.
There are now new
combinations of genes from
maternal and paternal genes
that had no existed before.
When the sister chromatids are
separated during anaphase ll the
gametes that result will not
have identical alleles.
The random orientation that
occurs when the bivalents line up
along the equator of the cell
during metaphase l also greatly
increases variety.
There is no set pattern as to
which chromosome will end up at
either pole.
One cell could end up with most
of the paternal or maternal
chromosomes or there could be
an even mix of both.
Calculate the Possibilities?
To precisely calculate the possibilities when sperm
and eggs are being produced is very difficult.
Considering only the number of chromosomes in a
haploid cell (n) the calculation would be 2n because
there are two possible chromosomes in each pair
(maternal and paternal).
Humans
For humans the number would be
223 so the probability that a
woman could produce the same
egg twice is 1 in 8 388 608.
If you consider that we have not
considered crossing over here the
calculation is oversimplified and
only considers one gamete.
The chance that two parents can
produce two offspring that are
identical apart from twins is
infinitesimal.
Past Paper Question
10.1.4 - State Mendel’s law of independent
assortment.
Animation on random assortment
An Animation
http://www.sumanasinc.com/webcontent/animati
ons/content/independentassortment.html
http://highered.mcgraw-hill.com/olc/dl/120074/bio18.swf