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Higher Biology
Linkage & Recombination
Linkage & Recombination
By the end of this lesson you should be
able to:
Know what is meant by linked genes.
Know the consequences of genes being linked.
Understand the effect crossing over has on linked
genes.
Know the effect that crossing over has on distantly
located genes.
Know what cross-over values (COV) are.
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Introduction


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In the early 1900s it was noticed that the
9:3:3:1 ratio from a dihybrid cross did not
always occur.
In some F2 generations the parental
phenotypes appeared more frequently than
expected.
Sometimes recombinant phenotypes did
not appear at all.
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Linked Genes

Two or more genes which are carried on
the same chromosome are said to be
linked.
simple gene map in peas
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Linked Genes

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If there is gene linkage there is a lower
probability that the alleles will be
separated during meiosis.
This means that more of the parental
genotypes appear in the F2 generation.
Different linked genes are inherited as a
group rather than as separate units.
The closer 2 genes are on a chromosome,
the more likely it is that they will be
inherited together.
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Linked Genes


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The opposite is also true:
The further apart 2 genes are on a
chromosome, the less likely it is that they
will be inherited together as crossing over
is more likely to separate them.
REMEMBER THAT:
When the homologous chromosomes pair up
during the first meiotic division, non-sister
chromatids are able to exchange pieces of DNA.
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Linked Genes
crossing over to separate linked genes
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Linked Genes


Consider two genes A and B that are linked
on the same chromosome.
A cross is carried out
AABB


X
aabb
The F1 would all be AaBb
At this stage there is no indication that
genes A and B are linked (on the same
chromosome)
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Linked Genes

If two F1 individuals are then self-crossed:
AaBb


X
AaBb
If A and B are very tightly linked, and
there has been no crossing over, then the
gametes will be:
AB
ab
AB
Draw a Punnett Square:
ab
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Linked Genes
Male gametes
(AB)
Female
gametes
(ab)
(AB)
(AB)(AB) (AB)(ab)
(ab)
(AB)(ab) (ab)(ab)
The F2 will be:
AABB
Ratio
1
AaBb
:
2
aabb
:
1
We sometimes
use brackets
around the
gametes to show
they are linked
together.
If the alleles
A & B were
not linked the
ratio would be
9:3:3:1
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Linked Genes
1.
Complete the “Dihybrid Cross- Linked
Genes” sheet from your diagram pack.
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Linked Genes- separating them
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Recombination
Try to answer the following questions:
1. What process separates linked genes?
2. How many different types of gametes
would be formed from a heterozygous
individual with linked genes?
3. How many different types of gametes
would be formed from a heterozygous
individual when the linked genes are
separated?
4. What do we call the additional 2 types of
gametes?
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Recombination
Recombinant gametes will change
the expected phenotypic ratio.


A backcross will have different results
depending on whether the heterozygous
individual has linked or unlinked genes
Insert and complete the “Unlinked
Backcross and Linked Backcross” sheet
from your diagram pack. You can fill in the
middle boxes now!
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Summary table
Unlinked genes
(phenotypic
ratio)
2 true breeding
individuals
crossed to F2
Heterozygous in a
back cross to F1
Linked genes
(phenotypic
ratio)
9:3:3:1
3:1
1:1:1:1
1:1
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Chromosome Mapping


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A genetic map is a diagram that shows the
positions of genes on the chromosomes of
an organism.
How do we get the information to do this?
Remember- the frequency of crossing over
between two linked genes will increase if
the genes are further apart.
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Chromosome Mapping

Broadly speaking:
The frequency of crossing over between two
genes is proportional to the distance
between them.

The recombination frequency for two
genes (%) is a measure of crossing over
between them.
Number of F2 recombinants
Total number of F2
X 100
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Chromosome Mapping
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A recombinant frequency can also be called
a crossover value (COV)
A recombinant frequency (COV) of 1%
represents one unit of measure on the
genetic map.
If genes A/a and B/b show a recombination
frequency of 9%, then they will be:
9 units apart on the genetic map.
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Constructing a Gene Map
1.
2.
To construct a genetic map for a particular
chromosome, a line is drawn and one gene is
then placed on the map. (this is the
reference point)
The other genes are then placed on the map
by positioning them at the correct number
of units from the initial gene & from each
other.
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Constructing a Gene Map
Example
Genes
Recombinant Frequency (%)
A/a x B/b
5
A/a x C/c
13
A/a x D/d
3
B/b x C/c
18
B/b x D/d
8
C/c x D/d
10
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Constructing a Gene Map
1.

Identify the two most distantly linked
genes.
B/b & C/c- 18 units apart
C/c
B/b
18 units
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Constructing a Gene Map
2.

Identify the gene that is closest to the
one on the left (B/b)
B/b & A/a- 5 units apart
B/b
C/c
A/a
5 units
18 units
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Constructing a Gene Map
3.


Now place the remaining gene (D/d) on the
map.
B/b & D/d- 8 units apart
D/d & C/c -10 units apart
B/b
A/a
5 units

C/c
D/d
3 units
18 units
It is equally correct to draw the genes in the
reverse order.
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Constructing a Gene Map
Now try this yourself.
Genes
COV (%)
T/t x L/l
12
T/t x B/b
4
T/t x S/s
21
L/l x B/b
16
L/l x S/s
9
B/b x S/s
25
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Constructing a Gene Map
B/b
L/l
T/t
4 units
12 units
S/s
25 units
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Practice Questions

1.
2.
3.
4.
Torrance
TYK page 93 Q3 & 4
TYK page 96 Q2a, b, c
TYK page 101 Q1
AYK pages 102-3 Q6 & 7
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Linkage & Recombination
Can you do it?
Know what is meant by linked genes.
Know the consequences of genes being linked.
Understand the effect crossing over has on linked
genes.
Know the effect that crossing over has on distantly
located genes.
Know what cross-over values (COV) are.
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