AUTOSOMAL
LINKAGE
Genes that do not obey Mendel’s
Second Law
© 2007 Paul Billiet ODWS
Reminder
In monohybrid inheritance the F2 generation
gives a 3:1 ratio because of Mendel 1
 A back cross with a heterozygote (Aa) gives
a 1:1 ratio
 In dihybrid inheritance the F2 generation
gives a 9:3:3:1 ratio because of Mendel 2
 A back cross with a heterozygote for both
pairs of alleles (AaBb) gives a ratio of 1:1:1:1

© 2007 Paul Billiet ODWS
Genes which do not obey
Mendel's Second Law
Sweet Characters
Peas
Flower colour
Pollen grain
shape
Traits Alleles
Purple
F
Red
f
Long
L
Round
l
© 2007 Paul Billiet ODWS
http://www.thegardenhelper.com/pixpg/sweetpeapix.html
Looked at from a simple
dihybrid cross perspective
P
F1
Phenotypes
Purple
Long
Genotypes
FFLL
X
ffll
Phenotypes
All purple
long
Genotypes
FfLl
© 2007 Paul Billiet ODWS
Red (Pure
Round
Bred)
(Selfed)
Sweet peas, the second
generation
P
F1
F2
Phenotypes
Purple
Long
Genotypes
FFLL
X
(Pure
Bred)
ffll
Phenotypes
All purple
long
Genotypes
FfLl
Phenotypes
Purple
Long
Purple
Round
Expected
Ratios
9/16
56,25%
3/16
18,75%
© 2007 Paul Billiet ODWS
Red
Round
(Selfed)
Red Long Red Round
3/16
18,75%
1/16
6,25%
What was expected?
F2
Phenotypes
Expected
Ratios
Purple Long
Purple
Round
Red Long
Red Round
9/16
56,25%
3/16
18,75%
3/16
18,75%
1/16
6,25%
Parental
combination
© 2007 Paul Billiet ODWS
Parental
combination
Not quite what was
expected!
F2
Phenotypes
Purple Long
Purple
Round
Red
Long
Expected
Ratios
9/16
56,25%
Observed
numbers
296
19
27
85
Observed
Ratios
69,32%
4,45%
6,32%
19,91%
Parental
combination
© 2007 Paul Billiet ODWS
3/16
3/16
18,75% 18,75%
Red Round
1/16
6,25%
Parental
combination
Total
= 427
Linked genes
The parental combinations of alleles
(purple long and red round) seem to be
inherited as almost a 3:1 ratio
 As though they were behaving as a
single character
 These genes are called LINKED GENES
 That is the LOCI for these genes are
linked on the same chromosome

© 2007 Paul Billiet ODWS
Crossing over and
recombinants
BUT if these genes were perfectly linked
together they would stay in their parental
combinations (purple & long or red & round)
 There would be no Purple Round or Red
Long
 These combinations have come about
because of CROSSING OVER between the
linked alleles on their chromosomes during
Meiosis 1
 These are called RECOMBINANTS

© 2007 Paul Billiet ODWS
Meiosis & Crossing over
Early prophase 1
Anaphase 1
© 2007 Paul Billiet ODWS
Telophase 2
Are there any other
recombinants?
Yes, hidden amongst the Purple Long plants
 The genotype FfLl does not tell us enough about linked
genes.
FL
Genotypes for linked genes can be shown as:

fl
This is an example of a parental combination
This genotype would give the same phenotype as:
BUT this is an example of a recombinant
© 2007 Paul Billiet ODWS
Fl
fL
Recombinants
Note: recombinants are any
combination of alleles that are not the
same as the parental combinations
This is not exclusive to the crossing
over of linked genes
© 2007 Paul Billiet ODWS
The Genetic Diagram for Linked Genes
P
Phenotypes
Genotypes
Gametes
F1
Purple Long
X
F L
F L
f l
f l
F L
f l
Phenotypes
All
Purple
Long
Genotypes
F L
f l
© 2007 Paul Billiet ODWS
Red Round (Pure Bred)
The Genetic Diagram for Linked Genes
F1 Phenotypes
All Purple Long
(Selfed)
F L
f l
Genotypes
crossing over in
meiosis I
Gametes
F L
f l
Parental Combinations
© 2007 Paul Billiet ODWS
F l
f L
Recombinants
The Genetic Diagram for Linked
Genes
F1
Gametes
F L
f l
Parental Combinations
F2 Genotypes
F L
f l
F l
f L
© 2007 Paul Billiet ODWS
F L
F L
F L
F L
f l
F L
F l
F L
f L
f l
F
f
f
f
F
f
L
l
l
l
l
l
f l
f L
F l
f L
Recombinants
F l
F L
F l
f L
F L
f L
F l
f l
f l
f L
F l
F l
F l
f L
F l
f L
f L
f L
The ratios
Phenotypes
Expected Ratio if
the genes were
only linked
© 2007 Paul Billiet ODWS
Purple
Long
Red
Round
Purple
Round
Red
Long
3/4
75%
1/4
25%
0
0
The ratios
Phenotypes
Expected Ratio if the
genes were only
linked
Observed Ratio
© 2007 Paul Billiet ODWS
Purple
Long
3/4
75%
69,32%
Red Purple Red
Round Round Long
1/4
25%
0
0
19,91% 4,45% 6,32%
How do we know which of the Purple
Long plants are Parental
Combinations and which are
Recombinants?
 How often does crossing over
occur?
Answer: Test Cross the F1 with a
double recessive for both f l
characters

f l
© 2007 Paul Billiet ODWS
CALCULATING THE CROSS OVER VALUE
Drosophila
Characters
Traits
Alleles
Wing shape
Normal
B
Bent
b
Normal
E
Ebony
e
Body colour
© 2007 Paul Billiet ODWS
CALCULATING THE CROSS OVER VALUE
Phenotypes
Genotypes
Gametes
Heterozygous
wild type
BbEe
BE, Be, bE, be
If these genes are linked
Be and bE could only be
produced by crossing over
© 2007 Paul Billiet ODWS
X
Ebony Bent
bbee
be
CALCULATING THE CROSS OVER VALUE
Phenotypes
Wild type
Normal
Ebony
Bent
Normal
Bent Ebony
Genotypes
BbEe
Bbee
bbEe
bbee
83
82
76
71
25%
25%
25%
25%
Numbers
Approx. Ratio
Parental
combination
© 2007 Paul Billiet ODWS
Recombinants = 50% of
the offspring
Parental
combination
CALCULATING THE CROSS OVER
VALUE


These results are typical of non-linked genes
The recombinants are in the same frequency
as the parental combinations
Note: In this example bent wing flies are a bit
crippled so their offspring are not so viable. This
accounts for their low numbers
© 2007 Paul Billiet ODWS
CALCULATING THE CROSS OVER VALUE
Drosophila
Characters
Traits
Eye colour
Red
P
Pink
p
Normal
E
Ebony
e
Body colour
© 2007 Paul Billiet ODWS
Alleles
Phenotypes
Heterozygous
wild type
Genotypes
PpEe
ppee
PE, Pe, pE, pe
pe
Gametes
Phenotypes
Wild type
Genotypes
PpEe
© 2007 Paul Billiet ODWS
X
Red
Ebony
Ppee
Pink Ebony
Pink Normal
Pink Ebony
ppEe
ppee
Phenotypes
Wild type
Red
Ebony
Pink
Normal
Pink Ebony
Genotypes
PpEe
Ppee
ppEe
ppee
601
3
4
584
Numbers
Parental
combinations
Recombinants < 50%
Parental
combinations
The frequency of the recombinants is less than 50%
This is an example of linkage
© 2007 Paul Billiet ODWS
THE CROSS OVER VALUE




The % recombination in a test cross is
called the CROSS OVER VALUE (cov)
The cross over value between ebony and
pink =
7
 100  0, 59%
1192
This value is important as it tells us how far
apart the loci of the genes are
Cross over values from several pairs of
genes permit a geneticist to plot a gene
map of the chromosome
© 2007 Paul Billiet ODWS