Higher Biology

Linkage & Recombination

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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

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

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

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

X

The F1 would all be AaBb At this stage there is no indication that

genes A and B are linked (on the same chromosome)

AABB aabb

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Linked Genes

If two F1 individuals are then self-crossed:

X

If A and B are very tightly linked, and there has been no crossing over, then the gametes will be:

Draw a Punnett Square:

AaBb AaBb

abABabAB

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Linked Genes

Male gametes

(AB) (ab)

Femalegamete

s

(AB)

(ab)

We sometimesuse brackets around the gametes to showthey are linkedtogether.

(AB)(AB)

(AB)(ab) (ab)(ab)

(AB)(ab)

The F2 will be:

AABB AaBb aabb

Ratio 1 : 2 : 1

If the allelesA & B were not linked the ratio would be9:3:3:1

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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

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!

Recombinant gametes will change the expected phenotypic ratio.

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Summary table

Unlinked genes

(phenotypic ratio)

Linked genes(phenotypic

ratio)

2 true breeding individuals crossed to F2

Heterozygous in a back cross to F1

9:3:3:1 3:1

1:1:1:1 1:1

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Chromosome Mapping

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 recombination frequency for two genes (%) is a measure of crossing over between them.

The frequency of crossing over between twogenes is proportional to the distance

between them.

Number of F2 recombinantsX 100

Total number of F2

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Chromosome Mapping

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. 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)

2. 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

18 units

B/b C/c

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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

18 units

B/b C/cA/a

5 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

It is equally correct to draw the genes in the reverse order.

18 units

B/b C/cA/a

5 units

D/d

3 units

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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

25 units

B/b S/sT/t

4 units

L/l

12 units

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Practice Questions

Torrance1. TYK page 93 Q3 & 42. TYK page 96 Q2a, b, c3. TYK page 101 Q14. 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.