1. Linkage refers to the tendency of genes located on the same chromosome to be inherited together. Linked genes remain in their original combination during inheritance. 2. Crossing over occurs during meiosis when exchange of genetic material occurs between non-sister chromatids of homologous chromosomes, resulting in new combinations of genes. 3. Recombination produces new variations in offspring through the formation of recombinant chromosomes from crossing over. This contributes to genetic variation and evolution.

1. LINKAGE
AND
CROSSING OVER

2. LINKAGE
The genes are located at particular
position on chromosome called locus /
loci.
All genes on a chromosome are called
linked genes and phenomenon is
called as linkage.
Morgan:-linkage is tendency of
parental gene combinations to remain
together & inherited to same gamete.

3. LINKAGE
Bateson & Punnet in 1906 :- crossed sweet pea (
Lathyrus odoratus ), male – red flower, round
pollen, female – blue flower, long pollen. F1 –
blue flower long pollen. F2 – 7 (blue long) : 1
(blue round) : 1 (red long) : 7 (red round)
instead of usual 9:3:3:1 ratio. They given
coupling and repulsion theory. Parental
combinations tend to be inherited together
called coupling and combinations from two
different parents tend to be inherited into
different gametes called repulsion. They
conclude this is due to linkage.

4. LINKAGE THEORY - T. H. Morgan, W. E. Castle
Linked genes are located on a single chromosome.
They are arranged in linear fashion.
Distance between linked genes is inversely
proportional to strength of linkage.
Linked genes tend to remain in original
combination during the course of inheritance.
Two types of arrangements cis – if dominant allele
of one or more genes are located on one
chromosome and their recessives on homologous
chromosomes. Trans – if dominant allele of one
pair and recessive allele of second pair are on one
chromosome and counterpart on respective
positions on homologous one.

5. TYPES OF LINKAGE
Complete linkage :- if two genes are inherited in
original combinations for two or more generations.
Incomplete linkage :- if genes produce some
percentage of new combinations.
SIGNIFICANCE OF LINKAGE
 Scope for nature of hybridisation and selection.
 Maintain parental combination and reduces
chances of recombination.
LINKAGE MAP
It is the graphical representation of genes showing their
location and relative distance on a chromosome. It helps
to determine the linkage or crossing over strength.

6. CROSSING OVER
It is the recombination of genes due
the exchange of segments between
homologous chromosomes during
pairing.
TYPES OF CROSSING OVER
Single cross over – only 1 chiasma,
likewise double & multiple cross over.

7. MECHANISM OF CROSSING OVER
Pairing of homologous chromosomes takes
place in ZYGOTENE stage of Prophase I of
meiosis during gamete formation due to
mutual attraction of allelic genes. It is
called as SYNAPSIS.
The paired chromosomes are called as
BIVALENT.
Each splits into two sister chromatids
attached at centromere, now called
TETRAVALENT.

8. MECHANISM OF CROSSING OVER
The non sister chromatids twist over each
other by chiasma formation during
PACHYTENE stage. Number of chiasma
may be more than one.
At chiasma, the chromatids separate and
rejoin to other chromatid during
deplotene.
The chromatids separate by
TERMINALISATION during diakinesis.
The new chromosomes are called as
recombinant chromosomes.

9. SIGNIFICANCE OF CROSSING OVER
Provide direct proof for linear
arrangement of genes on
chromosomes.
Source of variation and
evolution.
Help in construction of linkage
map.

10. RECOMBINATION
It is the process of formation of
recombinant chromosomes due
to crossing over. The offspring
having such chromosome(s) is
called as a recombinant
offspring. As we got new varieties
in dihybrid cross described
earlier. (see dihybrid cross).

11. CHROMOSOMAL ABERATION
Any change in chromosomal
structure that change
number, position or sequence
of genes is called as
chromosomal aberration or
chromosomal mutation.

12. TYPES OF CHROMOSOMAL ABERATION
Deletion / Deficiency :- loss of a part or
more.
Duplication :- addition of genes or lost part
of another chromosome.
Inversion :- lost parts reunite in reverse
order.
Translocation :- transfer of a part of a
chromosome to different location of same or
different chromosome.
Reciprocal Translocation :- exchange of
parts between two chromosomes.

14. CHANGES IN CHROMOSOME NUMBER
Aneuploidy :- addition or loss of few
chromosomes to the usual diploid sets.
Monosomy :- loss of one chromosome
(2n – 1)
Nullisomy :- loss of two / one pair of
chromosome (2n – 2)
Trisomy :- addition of one chromosome
(2n + 2)
Tetrasomy :- addition of one pair of
chromosomes (2n + 2)

15. CHANGES IN CHROMOSOME NUMBER
Euploidy :- addition or loss of complete set
(n) of chromosomes to the usual diploid sets.
Haploidy / Monoploidy :- loss of one
set (2n – n = n)
Triploidy :- addition of one set (2n +
n = 3n)
Tetraploidy :- addition of two sets (2n
+ 2n = 4n), and so on…
all others except haploidy are called as
polyploidy