During meiosis, chromosome pairs line up during the first prophase on the meiotic spindle. During prophase, the chromosones physically exchange material by breaking and rejoining (see adjacent diagram).
The position of these breakpoints or cross-overs occurs randomly and more than one cross-over can occur with the same random probability.
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A B C
a b c
The vertical line represents the cross-over and results in the following formation :-
A b c
a B C
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Page 1
CROSSES 2
Clearly, the further apart two genes are, the greater the chance of a cross-over occuring between them. This provides us with a convenient mechanism for determining the relative position of two genes on a chromosone - a gene map.
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Page 2
TEST CROSSES 3
To make mapping simpler, we always perform test crosses. In such a case, one parent is heterozygous, for linked genes which we wish to map, and the other is homozygous for the recessive alleles of the linked genes.
In this way, we only measure recombination in one parent as any recombination in the homzygous recessive parent does not affect the outcome.
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Page 3
F1 GENERATION 4
Thus the F1 generation is a mixture of parental type (AB and ab) and recombinant type (Ab and aB) progeny. The frequency of recombinant types in the total population (usually fairly rare) is a direct reflection of the frequency of recombination (cross-overs) in the heterozygous parent.
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RECOMBINATION
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MAPPING
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Page 4
NO RECOMBINATION 5
A heterozygous parent (a) crossed with a recessive homozygous parent (b) can produce gametes in which there is no recombination (c).
The phenotypes AB and ab in the progeny reflect the two chromosones in the heterozygous parent.
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(a) (b)
A B a b
X
a b a b
(c)
A B or a bBBBB
Page 5
RECOMBINATION 6
A heterozygous parent (a) crossed with a recessive homozygous parent (b) can also produce gametes in which recombination has occured (c) if there has been a cross-over.
The phenotypes Ab and aB reflect the two recombinant chromosones formed by the heterozygous parent.
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(a) (b)
A B a b
X
a b a b
(c)
A b or a BBBBB
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Page 6
types Ab and aB reflect the two recombinant chromosones formed by the heterozygous parent.
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(a) (b)
A B a b
X
a b a b
(c)
A b or a BBBBB
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TEST CROSSES
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COEFFICIENT OF COINCIDENCE
If all cross-over events are truly random, then the frequency of double cross-overs (DCO's) should merely be the product of the two individual single cross-over (SCO) frequencies.
In reality this never occurs due to a phenomenon called negative interference, whereby one cross-over event reduces the likelihood of another occuring nearby. The coefficient of coincidence is a measure of the theoretical frequency of DCO's and that actually observed.
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A B C If AB = 0.6 and BC = 0.4 then the DCO between ABC
| 0.6 | 0.4 | should be 0.6 x 0.4 = 0.24
Thus C of C = observed DCO frequency
expected DCO frequency
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Page 12
MAPPING DISTANCES 10
The distance between two genes is proportional to the frequency of recombination. This includes single cross-overs (SCO), such as abC + ABc, and double cross-overs (DCO), such as AbC + aBc.
Go to the next page to make an estimate of the mapping distance between two genes.
SCO = abC + ABc
DCO = AbC + aBc
ny with cross-overs
freq.rec = total progeny with cross-overs = abC + AbC = SCO + DCO
total progeny total
Page 10
RECOMBINANT TYPES
Meiotic Events
Single Cross-overs
Double Cross-overs
Parental Types
Recombinant Types
This page repeats the previous display, but shows the progeny formed when one of the gametes produced is used for fertilisation. Note, not all meiotic events generate a cross-over.
You must press 'RESET FOR SINGLE' before you first press 'SINGLE STEPS' even if the display is clear. When you have finished studying single meiotic events, press 'RUN'. followed by 'RESET' to finish.....
This page repeats the previous display, but shows the progeny formed when one of the gametes produced is used for fertilisation. Note, not all meiotic events generate a cross-over.
You must press 'RESET FOR SINGLE' before you first press 'SINGLE STEPS' even if the display is clear. When you have finished studying single meiotic events, press 'RUN'. followed by 'RESET' to finish.....
To determine gene order, look for the smallest class. This is always the double cross-overs.
Write out the three possible orders and consider the double cross-overs produced by each of these three possibilities. Only one corresponds to the actual double cross-over class therefore this one indicates the gene order.
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A B C
= aBc/AbC
a b c
B A C
= bAc/BaC
b a c
A C B
= aCb/AcB
a c b
Page 9
Thus C of C = observed DCO frequency
expected DCO frequency
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CROSSOVER EVENTS
Page 7
Meiotic Events
Single Cross-overs
Double Cross-overs
This card displays the random occurrence of cross-overs between two chromosones. The boxes show each cross-over as it occurs. Any 'hits' before 'A' or after 'C' indicate parental types where no cross-over has occurred.
Press 'RUN' to observe cross-overs, then 'RESET'. Go to next page.
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