Download Mendel`s 2 nd Law – Independent Assortment

Mendel’s 1st Law - Segregation
Each individual has two alleles (copies) of every gene. These
alleles separate during sperm/egg cell formation
Mendel’s 2nd Law – Independent Assortment
During gamete formation, different pairs of alleles segregate
independently of each other
Potential gametes
AaBb
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB & ab
Ab & aB
P
X
aa bb
AA BB
X
F1
Aa Bb
F2
100
AB
100
100
100
Ab
aB
ab
P
X
BB CC
bb cc
X
F1
Bb Cc
F2
1339
151
154
1195
BC
bC
Bc
bc
Gametes
All parental
50% parental 50% recombinant
X
F1
F2
1339
BC
151
bC
154
1195
bC
bc
10.7 m.u.
RF = 10.7 % so
B
C
DCO lead to underestimation of genetic map distances
Solution is to use three point cross
A female Drosophila is heterozygous at three autosomal linked gene loci:
e+
h+
c+
brown body
hairless legs
straight wings
e
h
c
ebony body
hairy legs
curvy wings
In order to map these genes, this heterozygote is crossed to a homozygous male recessive fly
and the following phenotypes of progeny were obtained:
ehc
e+ h+ c+
e+ h c
e h+ c+
e h c+
e h+ c
e+ h+ c
e+ h c+
356
347
2
3
42
19
53
22
844
Why didn’t we do the cross the other way round i.e. with a heterozygous male?
Which is the middle gene?
Calculate the map distances between the genes.
Is there any evidence of interference?
(1)
(2)
(4)
(2)