1. Linkage & Recombination
Higher Biology
Linkage & Recombination

2. 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.

3. 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.

4. Linked Genes
Two or more genes which are carried on the same chromosome are said to be linked.
simple gene map in peas

5. 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.

6. 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.

7. Linked Genes
crossing over to separate linked genes

8. 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

9. 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 AB ab AB ab

10. Linked Genes Male gametes (AB) (ab) Female gametes We sometimes
use brackets
around the
gametes to show
they are linked
together.
Male gametes
(AB)
(ab)
Female
gametes
(AB)(AB)
(AB)(ab)
(AB)(ab)
(ab)(ab)
If the alleles
A & B were
not linked the
ratio would be
9:3:3:1
The F2 will be:
AABB
AaBb
aabb
Ratio : : 1

11. Linked Genes
Complete the “Dihybrid Cross- Linked Genes” sheet from your diagram pack.

12. Linked Genes- separating them

13. Recombination Try to answer the following questions:
What process separates linked genes?
How many different types of gametes would be formed from a heterozygous individual with linked genes?
How many different types of gametes would be formed from a heterozygous individual when the linked genes are separated?
What do we call the additional 2 types of gametes?

14. 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!

15. Summary table Unlinked genes (phenotypic ratio) Linked genes 9:3:3:1
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

16. 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.

17. Chromosome Mapping Broadly speaking:
The recombination frequency for two genes (%) is a measure of crossing over between them.
The frequency of crossing over between two
genes is proportional to the distance
between them.
Number of F2 recombinants
X 100
Total number of F2

18. 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.

19. Constructing a Gene Map
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.

20. 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

21. Constructing a Gene Map
Identify the two most distantly linked genes.
B/b & C/c- 18 units apart
C/c
B/b
18 units

22. Constructing a Gene Map
Identify the gene that is closest to the one on the left (B/b)
B/b & A/a- 5 units apart
A/a
C/c
B/b
5 units
18 units

23. Constructing a Gene Map
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.
C/c
B/b
A/a
D/d
5 units
3 units
18 units

24. 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

25. Constructing a Gene Map
L/l
S/s
B/b
T/t
4 units
12 units
25 units

26. Practice Questions Torrance TYK page 93 Q3 & 4 TYK page 96 Q2a, b, c
AYK pages Q6 & 7

27. 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.