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.
Insert linked picture Fig 3.1 p18 Scholar

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
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

8. 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

9. 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)
The F2 will be:
AABB
AaBb
aabb
Ratio : : 1

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

11. Practice Questions Torrance TYK page 93 Q2 AYK page 89 Q3
AYK pages 101 Q2.

12. 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.