Transcript Ch 7 (part 1) Linkage and Gene Mapping

Linkage, Recombination and
Eukaryotic Mapping
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
Thomas Hunt Morgan
1866-1945
Observed: certain traits in Drosophila
tended to inherited together.
Suggested: these genes were on the
same chromosome.
Also suggested: gene close together
rarely shuffled
Alfred Sturtevant
1891-1970
Let’s compare inheritance of 2
linked genes with the inheritance of
2 genes that assort independently
(like what we have already studied
according to Mendel’s principles.)
According to Mendel
(non-linked genes—this is review, ch. 3)
• AaBb x AaBb
• AaBb
• aAbB
• AABB
• aabb
Independent Assortment
3:1
Non-recombinants
Recombinants
Linked genes
• Genes located close together on the same chromosome
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?
Use the Test Cross Method
If Completely Linked:
M
D
m
x
d
M
m
D
d
50%
MmDd x
m d
m
or
If Independent
mmdd
d
m d
m d
50%
ALL NON-RECOMBINANTS
MmDd or mmdd
Mmdd or mmDd
1:1:1:1
50% nonrecombinants
50% recombinants
Sometimes however the linkage is “broken” by a process called
CROSSING OVER”. “Incomplete Linkage”
It is another way to
recombination
Crossing Over
(meiosis—prophase 1)
Physical exchange of DNA
Intrachromosomal
Recombinantion
How do we know if crossing over
occurs between linked genes?
• Again use a test cross and look the offspring
distribution.
• Let’s continue to use the same test cross:
M
D
m
d
x
m d
m
d
Calculation of Recombination
Frequency
8 +7
55+53+8+7
X 100 = 12%
RF allows you to prediction proportion of
expected offspring with linked genes
If you know that RF = 16%
T= warty
d= dull color
Recombination Frequencies (RF)
• Can be used to determine the order of genes on a chromosome.
• Chromosome maps determined from RF are called“Genetic 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)
•First a few simple examples of displaying a “genetic map”
• 2 point test crosses were done and the results follow
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
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
2 point test cross map:
data 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
Genetic Map
RF values from 2 point test cross
b
.
c
20 m.u.
Genetic Map
RF values from 2 point test cross
d
b
10 m.u.
c
20 m.u.
30 m.u.
3 point test cross
(to be applied to your problem)
But is the order of the genes correct?
Interference and Co-efficient of coincidence (CC)
•
CC= # of observed double crossovers/# of expected double crossovers
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