extra credit are the b and vg genes on the same chromosome? if so, how far apart are they?
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Extra Credit Are the b and vg genes on the same chromosome? If so, how far apart are they?. A BC — • ————————— — • ————————— a b c. A b C. a B c. - PowerPoint PPT PresentationTRANSCRIPT
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Extra Credit
Are the b and vg genes on the
same chromosome?
If so, how far apart are they?
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A B C—•—————————
—•————————— a b c
Multiple crossovers can be useful for mapping three
genes at a time!
A B Ca b c
A B C—•—————————
—•————————— a b c
A b Ca B c
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3 recessive phenotypes in maize (corn), coded by three
linked genes
l l lazy or prostrate growthg g glossy leavess s sugary endosperm
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To map the genes, mate a triple heterozygote to triple
recessive homozygote
Ll Gg Ss x ll gg ss
Gene order is not known,so the order shown here is arbitrary.
Linkage phase is not know
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How many different kinds of gametes do you get from triple
heterozygote?
L or l G or g S or s 2 * 2 * 2
Ll Gg Ss
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L G S / l g s x l g s / l g sRecomb.Wildtpe for all lazy, gloss, sugary
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Where to begin?
Parental types will constitute ≥ 50% of all progeny, so…
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Rule 1
Two most-frequent gametes types are the parental types
Tells us the linkage phase of heterozygous parent:
L G S or L g S or l g S or L g s
l g s l G s L G s l G S
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L G S // l g s x l g s // l g s
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Linkage phase in heterozygous parent?
L G S or L g S or l g S or L g sl g s l G s L G s l G
S
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Rule 2
The double-recombinant gametes will be the two least frequent types
A B C
a b c
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L G S / l g s x l g s / l g s
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Rule 3
Effect of double crossovers is to interchange the members of the middle pair of alleles between the chromosomes
A B C
a b c
A b C
a B c
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Double-crossover types:L G s and l g S
Which gene is in the middle?
L s G
l S g
Parental types:L G S and l g s
L S G
l s g
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Now you know linkage phase of heterozygous parent and
gene order
L S Gl s g
How far apart are the genes?
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Count the crossovers between adjacent genes
• In parents, L allele on same homolog as S and l on same homolog as s. So if these get broken up ---> crossover between L and S loci
• In parents, S on same homolog as G and s on same homolog as g. If these get broken up --> recombination between S and G loci
L S Gl s g
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ProgenyPhenotype
ProgenyGenotype #
Crossover or Non-Crossover?
wildtype L G S / l g s 286 Parental (NCO)
lazy l G S / l g s 33 si ngle CO be twee n L and S
gloss y L g S / l g s 59 si ngle CO be twee n S and G
s uga ry L G s / l g s 4 double CO
lazy,gl ossy l g S / l g s 2 double CO
lazy,su gary l G s / l g s 44 si ngle CO be twee n S and G
gloss y,s uga ry L g s / l g s 40 si ngle CO be twee n L and S
lazy,gl ossy ,su gary l g s / l g s 272 Parental (NCO)
Total 740
L S Gl s g
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Rule 4: Reciprocal products expected to occur in approximately equal numbers
LGS ≈ lgs (286 ≈ 272) LgS ≈ lGs (59 ≈ 44) Lgs ≈ lGS (40 ≈ 33)LGs ≈ lgS (4 ≈ 2)
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Rule 5
Don't forget to include the double recombinants when calculating
recombination frequency!
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l G S 33L g s 40
L G s 4l g S 2
79
Rec Freq L-S Rec Freq S-GL g S 59l G s 44L G s 4l g S 2
109
ProgenyPhenotype
ProgenyGenotype #
Crossover or Non-Crossover?
wildtype L G S / l g s 286 Parental (NCO)
lazy l G S / l g s 33 si ngle CO be twee n L and S
gloss y L g S / l g s 59 si ngle CO be twee n S and G
s uga ry L G s / l g s 4 double CO
lazy,gl ossy l g S / l g s 2 double CO
lazy,su gary l G s / l g s 44 si ngle CO be twee n S and G
gloss y,s uga ry L g s / l g s 40 si ngle CO be twee n L and S
lazy,gl ossy ,su gary l g s / l g s 272 Parental (NCO)
Total 740
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79/740 or 10.7% of gametes recombinant between L & S. So,
map distance between L & S = 10.7 map units
109/740 or 14.8 % of gametes recombinant between S & G. So, map distance between S & G=14.8 map units
l G S 33L g s 40
L G s 4l g S 2
79
Rec Freq L-S
Rec Freq S-GL g S 59l G s 44L G s 4l g S 2
109
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Genetic Map
10.7 mu 14.8 mu_____________________________
L S G
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Interference
Assuming independence, expected probability of double crossovers is the probability of recombination in one region times the probability of recombination in other (product rule).
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Maize exampleProbability of recombination between L and S is 10.7% Probability of recombination between S and G is 14.8%
If crossovers independent, probability of double crossover should then be
0.107 * 0.148 = 0.0158
In 740 events, the double crossover class should occur
0.0158 * 740 = 12 times
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Expected DCO = 12Observed DCO = 6
Typical Result: O < E
Conclusion: Crossing over in one region reduces probability of crossing over in adjacent regions
This is Interference
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Why?
Physical constraints that prevent two chiasmata in close proximity during meiosis?
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Quantifying Interference
Coefficient of coincidence = Obs DCOExp DCO
cc = 6/12
Interference = (1 - cc) = 1 - 0.5 = 0.5
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Recombination is not independent at small distancesIf distance between genes is small (<10
map units in Drosophila) no double crossovers occur (interference is complete, I=1)
At large distances (> 45 map units, Interference disappears, Obs = Exp and I=0
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In Drosophila, the allele b gives black body (wild type is tan); at a separate gene, the allele wx gives waxy wings (nonwaxy is wild type); and at a third gene, the allele cn gives cinnabar eyes (red is wild type). A female that is heterozygous for these three genes is testcrossed, and 980 progeny are classified as follows for body color, wing phenotype, and eye color:
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Extra Credit Question for Next Time
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PhenotypeHaploid
Genotype Numberbrown, nonwaxy, red + + + 6black, waxy, cinnabar b wx cn 6brown, waxy, cinnabar + wx cn 79black, nonwaxy, red b + + 77brown, nonwaxy, cinnabar + + cn 369black, waxy, red b wx + 369brown, waxy, red + wx + 38black, nonwaxy, cinnabar b + cn 36
a) What is the linkage phase of the heterozygous female parent?
b) What is the order of the three genes?
c) Construct a linkage map with the genes in their correct order and indicate the map distances between the genes.
d) Calculate the Interference.
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Where to begin?Recombinant genotypes of all sorts will
constitute <= 50% of all progeny
0% Rec
50% Rec 100% Rec
50% Rec
50% Rec
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Three-locus
mapping
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To map the genes, mate a triple heterozygote to triple
recessive homozygoteL G S x l g s
l g s l g s
Gene order is not known, so the order shown here is arbitrary
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Multiple crossovers can lead to inaccuracy
A C—•—————————
—•————————— a c
B
b