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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

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Presentation Transcript
outline
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
slide3

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

slide4

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
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
Linked genes
  • Genes located close together on the same chromosome
linked genes1
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
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%

1:1:1:1

ALL NON-RECOMBINANTS

slide9

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
Crossing Over (meiosis—prophase 1)

Physical exchange of DNA

Intrachromosomal Recombinantion

how do we know if crossing over occurs between linked genes
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

rf allows you to prediction proportion of expected offspring with linked genes
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
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)
slide15

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
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
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
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
3 point test cross to be applied to your problem
3 point test cross

(to be applied to your problem)

interference and co efficient of coincidence cc
Interference and Co-efficient of coincidence (CC)
  • CC= # of observed double crossovers/# of expected double crossovers
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