The details of crossing and linkage between chromosomes and results of new structures of in new chromosomes.

1. LINKAGE AND
CROSSING
OVER

2. CONTENT
LINKAGE
 Discovery of Linkage
 Meaning of Linkage
 Characteristics of Linkage
 Genes in Linkage
 Theories
 Kinds of Linkage
 Significance
CROSSING OVER
 Discovery of Crossing
over
 Meaning of Crossing over
 Characteristics of Crossing
over
 Types of Crossing over
 Factors
 Significance
 Difference between
Crossing over and Linkage

3. DISCOVERY OF LINKAGE
• The principle of linkage was
discovered by English
Scientists William Bateson
and R.C.Punnet in 1906in
Sweet Pea .However, it was
put forward as a regular
concept by Morgan in1910
from his work on
Drosophila melanogaster.
THOMAS HUNT
MORGAN

4. MEANING OF LINKAGE
• Linkage is the
phenomenon of
certain genes staying
together during
inheritance through
several generations
without any change or
separation due to their
being present on same
chromosomes.

5. CHARACTERISTICS OF LINKAGE
• Linkage reduces variability
• It usually involves those genes which are
located close to each other
• Linkage involves two or more genes which are
linked in same chromosomes in a linear
fashion

6. GENES IN LINKAGE
There are 2 types;
1.LINKED GENE:
These genes do not show
independent assortment .It
occurs in same
chromosome. Dihybrid ratio
of linked gene is 3:1
2.UNLINKED GENE:
These gene show
independent assortment
Dihybrid ratio is 9:3:3:1.

7. THEORIES OF LINKAGE
DIFFERENTIAL MULTIPLICATION
THEORY(WILLIAM BATESON)
 CHROMOSOMAL THEORY
(THOMAS HUNT MORGAN)

8. DIFFERENTIAL MULTIPLICATION
THEORY
• This theory was put forward by Bateson in
1930.
• He said that after the segregation of genes
during gametogenesis certain genes multiply
faster tham others.

9. CHROMOSOME THEORY
• The genes which show Linkage are located in
same chromosome.
• Morgan and Castle associate genes with
chromosome and formulated postulates.
• The distance between linked gene in the
chromosome determine the strength of
linkage.
• The genes lie in a linear manner in the
chromosomes.

10. KINDS OF LINKAGE
ON THE BASIS OF CROSSING OVER
1. Complete linkage
2. Incomplete linkage
ON THE BASIS OF CHROMOSOME INVOLVED
1. Autosomal linkage
2. Allosomal /Sex linkage
ON THE BASIS OF GENE INVOLVED
1. Coupling phase
2. Repulsion phase

11. SIGNIFICANCE OF LINKAGE
• The phenomenon of linkage has one of the
great significance for the living organisms in
that reduces the possibility of variability in
gametes unless crossing over occurs.
• It helps keeping the parental, racial and
specific traits together.
• Linkage plays an important role in determining
the nature and scope of hybridization.

12. CROSSING OVER

13. DISCOVERY OF CROSSING OVER
Frans Alfons Janssens who
described the
phenomenon of crossing
over in 1909.He has
observed cross-like
arrangements in meiosis
And proposed crossing
over as a genetic process.

14. MEANING OF CROSSING OVER
• Crossing over or (chromosomal cross over) is
the exchange of genetic material between
homologous chromosomes that results in
recombinant chromosomes.

15. CHARACTERISTICS OF CROSSING OVER
 Crossing over leads to re-combinations or new
combinations between linked genes.
 Crossing over occurs between non-sister
chromatids .One chromatid from each of the two
homologues chromosomes is involved in crossing
over.
 The value of crossover or recombinants may vary
from 0-50%.
 Crossing over generally yields two recombinant
types or crossover types and two parental types
or non-crossover types.

16. STAGE AT WHICH CROSSING OVER
OCCURS
• The meiotic crossing over
takes place during the pachytene
stage of the prophase of meiosis 1.
Pachytene stage is also known
as recombination stage.
crossing over occurs when
homologus chromosomes are
in the four chromatid or tetrad
Stage in Pachytene.

17. TYPES OF CROSSING OVER
There are 2 types;
1. Somatic or Mitotic Crossing over
2. Germinal or Meiotic Crossing over
This is further divided into 2 types:
a) Equal Crossing over
b) Unequal Crossing over

18. FORMULATING AND TESTING
GENETIC HYPOTHESIS
• Genetic Linkage:
The tendency of the genes(DNA
Sequences) to stay together in a chromosome is
called genetic linkage. The genes linked together
in a chromosome are called the Linkage Group.
• Crossing Over

19. STATISTICAL TREATMENT OF LINKAGE
DATA
• Testing the Null Hypothesis:
Let us assume that two inbrect strains of
mice(B6 and C3H for example)carry
distinguishable alleles (symbolized by c and c
respectively)at each of two fictitious loci Xy1
and Gh3.
An F1 hybrid between B6 and C3H will be
heterozygous at each locus with a genotype of:
Xy1c/ Xy1b, Gh3c/Gh3b

21. By definition ,linkage means that theF1 hybrid will
produce a greater number of gametes carrying a
parental set of alleles,either
Xy1b Gh3b or Xy1c Gh3c
Than a recombinant set of alleles either;
Xy1b Gh3c or Xy1c Gh3b
Let us say that 100 offspring from the F1 hybrid
have been typed to test for linkage between Xy1
and Gh3 with result that 62 carry parental allele
combinations and 38 carry non parental allele
combinations.

22. There is a problem with a general hypothesis that states
“genes A and B are linked” in that there is no precise
prediction of what to expect in terms of data from a
breeding experiment. This is because linkage can be
very tight so that recombination would be expected
rarely, or linkage can be rather loose so that
recombination would be expected frequently.
The strength of linkage, if indeed the genes under
analysis are linked, is known as the outset of the
experiment.
In contrast, there is a precise prediction of what to
expect from the so called “null hypothesis” of no linkage
between genes A and B.
The prediction of this null hypothesis is that alleles at
different genes will assort independently leading to a
50:50 ratio of gametes with parental or recombinant
combinations of allele.

23. FACTORS AFFECTING CROSSING
OVER
• Temperature
• Chromosomes Effect
• Maternal age Effect
• Centromere Effect
• Mutation Effect
• Sex
• Nutritional and chemical Effect

24. SIGNIFICANCE OF CROSSING OVER
• Crossing over helps in establishing the
concept of linear arrangements of genes.
• It is an important factor in sexual
reproduction.
• It helps in plant breeding.
• The frequency of crossing over helps in
mapping of chromosomes i.e.
determining the location of genes on the
chromosomes.

25. DIFFERENCE BETWEEN LINKAGE AND
CROSSING OVER
LINKAGE
1. It involves individual
chromosomes.
2. It keeps the genes
together
3. It reduces variability.
CROSSING OVER
1. It involves exchange of
segments between
non- sister chromatids
of homologous
chromosomes.
2. It leads to separation
of linked genes.
3. It increases variability
by forming new gene
combinations.

26. SUMMARY

27. REFERENCES