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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
Application of Mendel’s 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
Bateson and Punnett also studied peas:
Flower Color:
Pollen seed shape:
P = purple
L = long
True Breeding lines: PPLL
x
p = white
l = round
ppll
PpLl
Phenotype
Number
Purple long 284
Purple round 21
White long
21
While round 55
What is going on????
P
F1
Exp Ratio
Exp Number
9
3
3
1
215
71
71
24
Test cross F1 to double recessive:
Parents
PpLl X
ppll
Gametes
PL
pl
Pl
pL
pl
Expect 1:1:1:1 ratio of phenotypes
Bateson and Punnett observed 7:1:1:7
Some gamete types more common that others…but why???
Independent loci
Linked loci
Hair Color
Hair Color
Eye Color
Eye Color
Test cross to determine if genes are linked in coupling or repulsion
PL//pl x pl//pl
Gametes 1/2 PL all pl
1/2 pl
F1
Pl//pL X pl//pl
1/2 Pl
all pl
1/2 pL
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 1:1:1:1
Linked genes sorted on
chromosomes
Explained why linkage not
perfect
Thomas H. Morgan
P: PL//PL X
pl//pl
F1: (PL//pl)
P p P p
L L l
= centrosome
l
P P p p
L L l
l
Effects of crossing over on number of possible gamete types
With x-over
No x-over
P p P p
P P pp
L L l l
L L l l
p
pL
L
P
P
Pl
l
PL
L
p
pl
l
Alfred H. “Hot Dog” Sturtevant
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 1:1:1:1 ratio of phenotypes
Bateson and Punnett observed 7:1:1:7
2/16 = 12.5% recomb.
P
12.5 map units
L
A 5 units L
How do we know if it’s P- A - L , or P - L- A?
If x-over frequency between P and A is 7.5% then:
P
A
7.5 map units
L
5 map units
If x-over frequency between P and A is 17.5% then:
P
L
12.5 map units
A
5 map units
Fig 15.7
sa
(antennae
mutation)
9
17
b
cn
9.5
vg
be (eye
color)