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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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
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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
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Two or more genes which are carried on
the same chromosome are said to be
linked.
Insert linked picture Fig 3.1 p18 Scholar
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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

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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)
We sometimes
use brackets
around the
gametes to show
they are linked
together.
The F2 will be:
AABB
Ratio
1
AaBb
:
2
aabb
:
1
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Linked Genes
1.
Complete the “Dihybrid Cross- Linked
Genes” sheet from your diagram pack.
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Practice Questions

1.
2.
3.
Torrance
TYK page 93 Q2
AYK page 89 Q3
AYK pages 101 Q2.
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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.
12