OUTLINE 15

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OUTLINE 15 E. Violation of independent
assortment 1. Bateson and Punnet 2. Linkage 3.
Crossing over 4. Chromosome mapping 5. Effects
of linkage and crossing over on genotypic and
phenotypic ratios
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Application of Mendels Rules assumes 1. One
allele completely dominates the other 2. All
genes have 2 allelic forms 3. All traits are
monogenic (affected by only one locus) 4.
All chromosomes occur in homologous pairs 5.
All genes assort independently 6. An allele is
completely expressed when either dominant or
homozygous 7. Each trait is controlled by a
different set of factors
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Bateson and Punnett also studied peas Flower
Color P purple p white Pollen seed shape L
long l round True Breeding lines PPLL
x ppll P PpLl F1
Phenotype Number Exp Ratio Exp Number Purple
long 284 9 215 Purple round 21 3 71 White
long 21 3 71 While round 55 1 24
What is going on????
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Test cross F1 to double recessive Parents PpLl
X ppll Gametes PL pl Pl pL pl
Expect 1111 ratio of phenotypes Bateson and
Punnett observed 7117
Some gamete types more common that othersbut
why???
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Independent loci
Linked loci
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Test cross to determine if genes are linked in
coupling or repulsion PL//pl x
pl//pl Pl//pL X pl//pl Gametes 1/2 PL
all pl 1/2 Pl all pl 1/2 pl 1/2
pL F1 1/2 PL//pl (purple, long) 1/2 (purple,
round) 1/2 pl//pl (white, round) 1/2 (white,
long) With independent assortment PpLl x ppll
expect 1111
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Linked genes sorted on chromosomes
Explained why linkage not perfect
Thomas H. Morgan
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P PL//PL X pl//pl
F1 (PL//pl)
p
P
p
P
L
L
l
l
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Effects of crossing over on number of possible
gamete types
With x-over
No x-over
p
P
p
P
p
p
P
P
L
L
l
l
L
L
l
l
P
p
P
p
l
L
l
L
pL
Pl
PL
pl
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Alfred H. Hot Dog Sturtevant
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Test cross F1 to double recessive Parents PpLl
X ppll PL//pl X pl//pl Gametes PL
pl PL parental (7) Pl pl parental (7) pL
pL recombinant (1) pl Pl recombinant
(1) Expect 1111 ratio of phenotypes Bateson
and Punnett observed 7117 2/16
12.5 recomb.
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L
P
12.5 map units
5 units
A
L
How do we know if its P- A - L , or P - L-
A? If x-over frequency between P and A is 7.5
then
P
A
L
7.5 map units
5 map units
If x-over frequency between P and A is 17.5 then
P
L
A
12.5 map units
5 map units
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Fig 15.7