Linkage, Recombination and Eukaryotic Mapping

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Linkage, Recombination and Eukaryotic Mapping
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Outline

• Introduction
• Complete Linkage compared to independent
  assortment
• Crossing over with linked genes
• coupling and repulsion gene arrangements
• Predicting outcomes with linked genes
• Gene mapping with recombination frequencies
• 2-point test cross

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Observed certain traits in Drosophila tended to
inherited together.
Thomas Hunt Morgan
1866-1945
Suggested these genes were on the same
chromosome.
Also suggested gene close together rarely
shuffled
Alfred Sturtevant
1891-1970
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Lets compare inheritance of 2 linked genes with
the inheritance of 2 genes that assort
independently (like what we have already studied
according to Mendels principles.)
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According to Mendel(non-linked genesthis is
review, ch. 3)

• AaBb x AaBb
• AaBb
• aAbB
• AABB
• aabb

Independent Assortment
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Non-recombinants
Recombinants
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Linked genes

• Genes located close together on the same
  chromosome

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Linked Genes
On the same chromosome genes travel together
and arrive a the same destination.
But how can you tell when a gene is linked or if
it is independent?
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Use the Test Cross Method
If Independent
If Completely Linked
MmDd x mmdd
m d
M D
x

m d
m d
m d m d
M D m d
MmDd or mmdd Mmdd or mmDd
or
50 non-recombinants
50 -recombinants
50
50
1111
ALL NON-RECOMBINANTS
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Sometimes however the linkage is broken by a
process called CROSSING OVER. Incomplete
Linkage
It is another way to
recombination
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Crossing Over (meiosisprophase 1)
Physical exchange of DNA
Intrachromosomal Recombinantion
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How do we know if crossing over occurs between
linked genes?

• Again use a test cross and look the offspring
  distribution.
• Lets continue to use the same test cross

m d
M D
x
m d
m d
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Calculation of Recombination Frequency
8 7 555387
X 100 12
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RF allows you to prediction proportion of
expected offspring with linked genes
If you know that RF 16
T warty d dull color
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Recombination Frequencies (RF)

• Can be used to determine the order of genes on a
  chromosome.
• Chromosome maps determined from RF are
  calledGenetic Maps.
• Distances are given in map units (m.u.) or
  centimorgans (cM)
• 1 m.u. 1 recombination
• 50 RF is expected with independent assortment
  (or distant genes)

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• First a few simple examples of displaying a
  genetic map

• 2 point test crosses were done and the results
  follow

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1. Genetic Map using RF

• A to B 5 m.u.
• B to C 10 m.u.
• A to C 15 m.u.
• Then a simple genetic map using RF is

A 5 m.u. B
10 m.u. C
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2. Genetic Map using RF

• A to D 8 m.u.
• B to D 13 m.u.
• C to D 23 m.u.

Another set of 2 pt test cross
Now include D to the previous map, what would
the map look like now?

A 5 m.u. B
10 m.u. C
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2 point test cross mapdata given from a series
of test crosses

• 
  RF
• a and b 50
• a and c 50
• b and c 20
• b and d 10
• c and d 28

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Genetic MapRF values from 2 point test cross
b
c
. 20 m.u.
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Genetic MapRF values from 2 point test cross
d b
c
10 m.u. 20 m.u.
30 m.u.
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• 3 point test cross
• (to be applied to your problem)

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But is the order of the genes correct?
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Interference and Co-efficient of coincidence (CC)

• CC of observed double crossovers/ of expected
  double crossovers

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