Linkage and crossing over , discovery of linked genes,types of crossing over,significance and difference between linkage and crossing over, complete presentation with suitable examples and figures

2. LINKAGE AND CROSSING OVER
• SUBMITTED BY : MUHAMMAD BILAL
• SUBMITTED TO:DR WAQAS AHMAD
• (UNIVERSITY OF SARGODHA)

3. Genetic linkage
•Genetic linkage is the tendency of alleles that are
located close together on a chromosome to be
inherited together during meiosis.
•Genes whose loci are nearer to each other are less
likely to be separated on to different chromatids
during chromosomal crossover, and are therefore
said to be genetically linked.

4. Linked Genes
•Independent assortment states that during
gamete formation, the two alleles for one
gene segregate or assort independently of
the alleles for other genes.

5. •But if two genes are found on the same
chromosome, they will not assort
independently, and do not follow
Mendelian inheritance patterns.
•Genes that are inherited together are said to
be “linked.”

7. •Genes located on different chromosomes are not linked
This allows independent assortment – in a di-hybrid cross
the traits show the classic 9:3:3:1 inheritance pattern.
• Genes that are located very close together on the same
chromosome may show complete linkage.
•They may be so close to each other that they cannot be
separated by recombination during meiosis.

8. (c) Genes located far apart on the same
chromosome typically show incomplete
(partial) linkage because they are easily
separated by recombination.

9. The discovery of genetic linkage
•William Bateson and Reginald Punnett
completed a study in 1905 that determined the
movement of alleles found on the same
chromosome.
•The study used sweet peas, particularly flower
colour and pollen shape, they followed the
inheritance pattern.

10.  William
Bateson
 Reginald
Punnett

11. • For flower colour, purple is dominant over
red, and for pollen shape long is dominant
over round.
• A cross was performed using a true
breeding purple/long and red/round.
• The F1 generation was 100% purple/long.

12. •Crossing two individuals from the F1 generation resulted
in a F2 generation with four different phenotypes.
(purple/long, purple/round, red/long and red/round).
•The alleles that created these combinations did not
follow the 9:3:3:1 pattern, but supported the idea that
these alleles did not assort independently and therefore
must be linked.

14. Why Linkage?
•Linkage refers to packaging genes onto
chromosomes.
•Chromosomes (and therefore linkage) are for
organizing genes for their safe coordinated
transmission from cell to cell( parent to offspring).

15. Types of Linkage
•Depending upon the presence or absence of
new combinations or non-parental
combinations, linkage can be of two types:
1.Complete linkage
2.Incomplete linkage

16. 1.Complete linkage
•If two or more characters are inherited together and
consistently appear in two or more generations in
their original or parental combinations, it is called
complete linkage.
•Genes showing complete linkage are closely
located in the same chromosome.

17. 2.Incomplete linkage
•Incomplete linkage is exhibited by those
genes which produce some percentage of
non-parental combinations. Such genes are
located distantly on the chromosome

18. Complete vs. Incomplete Linkage

19. Significance of linkage
1.Linkage does not permit the breeders to bring
the desirable characters in one variety.
2. Linkage reduces the chance of recombination
of genes and thus helps to hold parental
characteristics together.

20. Crossing Over

21. Crossing Over
•It is a basic concept of genetics and cell biology, often
called recombination.
• The term crossing over was coined by Morgan.
•It is the exchange of genes between two chromosomes,
resulting in non-identical chromatids that comprise the
genetic material of gametes.

22. Occurrence
• This process occurs during Prophase I of Meiosis, just
prior to chromosome alignment and splitting of the
cell.
• As an exception , it can also occur in MITOSIS which
may result in heterozygosity. But the frequency of
crossing over is much higher in meiosis.
• Cross over events happen approximately 57 times in
males and around 75 times in females.

23. Mechanism of crossing over
• Crossing over, leading to recombination of linked
genes, is due to interchange of sections of
homologous chromosomes.
• At meiosis, the homologous chromosomes come
together and pair, or synapse, during prophase.
• The pairing is remarkably precise and is evidently
brought about by mutual attraction of the parts of the
chromosomes that are similar or homologous because
they contain allellic genes.

25. The enzymes involved in crossing over
• 1) Recombinase is the major enzyme regulating
recombination event
• 2) Endonuclease
• 3) Ligase enzyme
• Endonuclease is responsible for breakage of 2 non-
sister chromatids at corresponding sites. This is
followed by the exchange of segments and finally the
exchanged segments are joined or the gap is filled by
ligase enzyme.

26. • The final step is terminalisation. After crossing
over the non-sister chromatids starts to repel
each other and desynapsis takes place.
• Chiasma moves in a zipper fashion towards the
end of tetrad. This movement of chiasma is
known as terminalisation.
• As a result of terminalisation, homologous
chromosomes are separated completely.

27. Recombination Frequency
• Recombination frequency (θ) is the frequency with which a
single chromosomal crossover will take place between two
genes during meiosis.
• The recombination frequency will be 50% when two genes
are located on different chromosomes or when they
are widely separated on the same chromosome.
• This is a consequence of independent assortment. When two
genes are close together on the same chromosome, they do
not assort independently and are said to be linked.

29. FACTORS AFFECTING CROSS OVER
1. Sex
2. Mutation
3. Age
4. X-Ray effect
5. Temperature
6. Distance between genes
7. Decreased Hydration
8. Chemicals

30. 1. Sex:
There is a tendency of reduction of crossing over in male
mammals.
2. Mutation:
Mutation reduces crossing over.
3. Age:
Older age increases the rate of crossing over
4. Distance between the genes:
This is perhaps the most important factor influencing the frequency
of cross over between two gene loci. As has been repeatedly
pointed out

31. 5. X-ray Effect:
X-ray radiations increase crossing over near centromere.
6.Temperature:
High and low temperature variations increase the percentage of
crossing over in certain parts of the chromosome.
7.Decreased Hydration:
Lowering of water content in the surroundings is known to reduce the
frequency of chiasma. It may even result in complete suppression of
cross over in extreme dehydrated conditions (Sybenga 1972).
8. Chemicals:
Certain chemicals which induce chromosomal aberrations are also
known to induce or suppress crossing over. Ethyl methane sulphonate
is known to induce somatic cross over. Colchicine also prevents cross
over by preventing pairing between chromosomes.

32. Significance of Crossing-over
1.Produces new combinations of traits.
2.Through crossing over segments of
homologous chromosomes are
interchanged.
3.Crossing over plays a very important role in
the field of breeding to improve the varieties
of plants and animals.

33. Types of Crossing Over
i. Single Crossing Over
In this type of crossing over only one
chiasma is formed all along the length of
a chromosome pair. Gametes formed by
this type of crossing over are called
single cross over gametes .

34. (ii) Double Crossing Over:
In this type two chiasmata are formed along the
entire length of the chromosome leading to
breakage and rejoin of chromatids at two points.
(iii) Multiple Crossing Over:
In this type more than two chiasmata are formed
and thus crossing over occurs at more than two
points on the same chromosome pair.

35. Difference between linkage and Crossing Over
Crossing Over
• It leads to separation of
linked genes.
• It involves Non sister
chromatids of homologous
chromosomes.
• It increase variability by
forming new gene
combinations.
Linkage
• It keeps the genes
together.
• It involves individual
chromosomes.
• It reduces variability.