Translocation
The transfer of a segment of chromosome to a nonhomologous chromosome is called translocation. Translocation should not be confused with crossing over, in which an exchange of genetic material between homologous chromosome takes place. Translocation occurs as a result of interchange of chromosome segments in non-homologous chromosomes. There are three types:
i. Simple translocation
ii. Shift translocation
iii. Reciprocal translocation
i. Simple translocation
A single break is made in only one chromosome. The broken segment gets attached to one end of a non-homologous chromosome. It occurs very rarely in nature.
ii. Shift translocation
Broken segment of one chromosome gets inserted interstitially in a non-homologous chromosome.
iii. Reciprocal translocations
It involves mutual exchange of chromosomal segments between two non homologous chromosomes. It is also called illegitimate crossing over. It is further divided into two types.
a. Homozygous translocation: Both the chromosomes of two pairs are involved in translocation. Two homologous of each translocated chromosomes are identical.
b. Heterozygous translocation: Only one of the chromosome from each pair of two homologous are involved in translocation, while the remaining chromosome is normal.
Translocations playa major role in the formation of species.

  
types of aneuploidy.

The diagrammatic representation of position of genes and related distances between the adjacent genes is called gene mapping. 
Uses 
(i) Used to determine gene order.
(i) Identify the locus of a gene.
(i) Calculate the distance between genes.
(i) Useful in predicting results of dihybrid and trihybrid crosses.
(i) Allows the geneticists to understand the overall genetic complexity of particular organism.

Zea mays is a monoecious plant with
(i) Male inflorescence - the tassel with staminateflowers
(ii) Female inflorescence - ear or cob with pistillate flowers. 
(iii) Unisexuality in maize is through selective abortion of stamens - in ear florets and Pistils - in tassel florets. 
(iv) A substitution of two single gene pairs 'ba' for barren plant and 'ts' - for tassel seed makes the differences between monoecious and dioecious (rare) maize plants. 
(v) The allele for barren plant (ba) when homozygous makes the stalk staminate by eliminating silk and ears. 
(vi) The allele for tassel seed (ts) transforms tassel into a pistillate flower. 
(vii) The development of both inflorescence is the result of selective abortion of pistils or stamens. 

Crossing over is a process that includes four stages.
1. Synapsis
(i) Pairing between two homologous chromosomes is initiated during sub stage zygotene stage of prophase I of meiosis I
(ii) Homologous chromosomes are aligned side by side resulting in the formation of bivalents. It also occurs in zygotene sub-stage of prophase I of meiosis I tetrads.This pairing phenomenon is called synapsis.
 2.Tetrad Formation
(i) Each homologous chromosome of a bivalent begin to form two identical sister chromatids.
(ii) They remain held together by a centromere at this stage each bivalent has four chromatids. This stage is called tetrad stage.
3. Cross Over
After tetrad formation, crossing over occurs in pachytene stage.
The non-sister chromatids of homologous pair make a contact at one or more points. These points of contact are called Chiasmata.
At chiasma, X-shaped structures are formed, where breaking and rejoining of two chromatids occur. This results in reciprocal exchange of equal and corresponding segments between them synapsis and chiasma formation are facilitated by protein filaments called synaptonemal complex.
4. Terminalisation
After crossing over, chiasma starts to move towards the terminal end of chromatids known as terminalisation. As a result, complete separation of homologous chromosomes occurs.

(i) Three point test cross.
(ii)

If we analyse the loci of two alleles at a time, starting with AB, since AB and ab are parental genotypes, the recombination will be Ab/aB.
The recombinant frequency for these two alleles can be calculated as follows.
\(RF = \frac{Total\ no.\ of \ recombination}{Total\ no.\ of\ progenies}\times100\)
\(\mathrm{RF}=\frac{114+5+4+116}{1200} \times 100 \)
\(\mathrm{RF}=\frac{239}{1200}=19.91 \)
For A and C loci, the recombinants are Ac / aC - RF will be as follows.
\(\mathrm{RF}=\frac{114+134+116+128}{1200} \times 100 \)
\(\mathrm{RF}=\frac{482}{1200}=40.1 \)
For B and C loci, the recombinants are Bc / bC- RF will be as follows.
\(R F=\frac{124+5+4+128}{1200} \times 100 \)
\(R F=\frac{261}{1200}=21.75 \)
All the loci are linked, because all the RF value is less than 50%. In this A - and C loci show highest RF value, they must be farthest apart. Therefore B locis must lie in between them.
The order of genes should be abc. A genetic map can be drawn as follows.
(iii)
 

(i) Incomplete linkage

Rano : 1:2:1

Normal chromosome of Drosophila melanogaster (2n)    = 8
Trisomic condition (2n+ 1)                                                = 9
Monosomic condition (2n-l)                                              =  7

(a) B - C - D - E - F - G - H - I - J - K (or) A- B - C - D - E - F - G - H - I - J.
(b) A- B - C - D - E - F - E - F - G - H - I - J - K.
(c) A - B - E - D - C - F - G - H - I - J - K.

Many polyploids are more vigorous and more adaptable than diploids.
(i) Many ornamental plants are autotetraploids and have larger flower and longer flowering duration than diploids.
(ii) Autopolyploids usually have increase in fresh weight due to more water content.
(iii) Aneuploids are useful to determine the phenotypic effects of loss or gain of different chromosomes.
(iv) Many angiosperms are allopolyploids and they play a role in an evolution of plants.

1. Exchange of segments leads to new gene combinations which plays an important role in evolution.
2. Studies of crossing over reveal that genes are arranged linearly on the chromosomes.
3. Genetic maps are made based on the frequency of crossing over.
4. Crossing over helps to understand the nature and mechanism of gene action.
5. If a useful new combination is formed it can be used in plant breeding.

(a) Double tetrasomy
(b) Double monosomy
(c) Monoploidy
(d) Diploidy
(e) Polyploidy
(f) Trisomy

Mis-sense Mutation	Non-sense Mutation
The mutation where the codon for one amino acid is changed into a codon for another amino acid is called Mis-sense or  non-synonymous mutations.	The mutations where codon for one amino  acid is changed into a termination or stop codon is called Non-sense mutation.

(I) It is used to determine gene order, identify the locus of a gene and calculate the distances between genes.
(II) They are useful in predicting results of dihybrid and trihybrid crosses.
(III) It allows the geneticists to understand the overall genetic complexity of particular organism.

S.No.	Linkage	Crossing over
1.	The genes present on chromosome stay close together.	It leads to separation of linked genes.
2.	It involves same chromosome of homologous chromosome.	It involves exchange of segments between non-sister chromatids of homologous chromosome
3.	It reduces new gene combinations.	It increases variability by forming new gene combinations. Leading to formation of new organism.

The two dominant alleles or recessive alleles occur in the same homologous chromosomes, tend to inherit together into same gamete are called coupling or cis configuration. If dominant or recessive alleles are present on two different, but homologous chromosomes they inherit apart into different gamete are called repulsion or trans configuration.

(i) Sutton and Boveri independently proposed the chromosome theory of inheritance.
(ii) Somatic cells of organisms are derived from the zygote by repeated mitotic division. These consist of two identical sets of chromosomes. One set is received from female parent and the other from male parent which constitute the homologous pair.
(iii) Chromosomes retain their structural uniqueness and individuality throughout the life cycle of an organism.
(iv) Each chromosome carries specific determiners or Mendelian factors which are now termed as genes.
(v) The behaviour of chromosomes during the gamete formation provides evidence to the fact that genes are located on chromosomes.

Crossing over results in the formation of new combination of characters in an organism called recombinants. In this, segments of DNA are broken and recombined to produce new combinations of alleles. This process is called recombination.
The widely accepted model of DNA recombination during crossing over is Holliday's hybrid DNA model.

Chromosome 1: Tandem duplication.
Chromosome 2: Intercalary deletion.

Sometimes, the chromosome number of somatic cells changed due to addition or elimination of individual chromosome or basic set of chromosomes. This condition in known as ploidy.

(a) Ethyl methanesulphonate (EMS)
(b) Mustard gas
(c) Maganous salt
(d) Formaldehyde

The diagrammatic representation of position of genes and related distances between the adjacent genes is called genetic mapping.

After crossing over, chiasma starts to move towards the terminal end of chromatids. This is known as terminalization. As a result, complete separation of homologous chromosomes occurs.

During zygotene stage of prophase I of meiosis I, homologous chromosomes are aligned side by side resulting in a pair of homologous chromosomes called bivalents.

Genes located close together on the same chromosome and inherited together are called linked genes.

Tendency of genes to remain together during separation of chromosomes is called linkage. Linkage are of 2 types - complete linkage and incomplete linkage.