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.

Hyperploidy
Addition of one or more chromosomes to diploid sets are called hyperploidy. Diploid set of chromosomes represented as Disomy. Hyperploidy can be divided into three types. They are as follows:
(a) Trisomy
Addition of single chromosome to diploid set is called Simple trisomy (2n+ 1). Trisomies were first reported by Blakeslee (1910) in Datura stramonium (Jimson weed). But later it was reported in Nicotiana, Pisum and Oenothera. Sometimes addition of two individual chromosome from different chromosomal pairs to normal diploid sets are called Double trisomy (2n+ 1+ 1).
(b) Tetrasomy
Addition of a pair or two individual pairs of chromosomes to diploid set is called tetrasomy (2n+2) and Double tetras6my (2n+2+2) respectively. All possible tetrasomics are available
in Wheat.
(c) Pentasomy
Addition of three individual chromosome from different chromosomal pairs to normal diploid set are called pentasomy (2n+3).

(i) Triticale (Hexaploid) (6x) is the first man made cereal.
(ii) It is obtained from a cross between tetraploid wheat Triticum durum and rye (diploid).

(iii) Triticale hybrid plants, have characteristics of both macroni wheat and rye.
(iv) They combine the high-protein content of wheat a high content of amino acid lysine of rye.

  
types of aneuploidy.

(i) Multiple alleles are reported in Nicotiana which are responsible for self, incompatibility.
(ii) The gene for self-compatibility can be designated as S, which has allelic series S₁, S₂, S₃, S₄ and S₅.
(iii) The cross-fertilizing tobacco plants were not homozygous as S₁S₁ or S₂S₂.
(iv) But, were heterozygous as S₁S₂, S₃S₄, S₅S₆.
(v) When crosses were made between different S₁S₂ plants, pollen tube did not develop normally.
(vi) But effective pollen tube developed when crossing was made with other than S₁S₂.
(vii) When crosses were made between S₁S₂ (seed parents) S₂S₃ (pollen parents), two kinds of pollen tubes were distinguished.
(viii) Pollen grains carrying S₂ were not effective.
(ix) Pollen grains carrying S₃ were capable of fertilization.
(x) In a cross between S₁S₂ X S₃S₄, all the pollens were effective.
(xi) So four kinds of progeny resulted. S₁S₃/S₁S₄/S₂S₃/S₂S_4.

The widely accepted model of DNA recombination during crossing over is Holliday's hybrid DNA model.

Steps involved are:
1. Homologous DNA molecules are paired side by side with their duplicated copies of DNAs
2. One strand of both DNAs cut in one place by the enzyme endonuclease.
3. The cut strands cross and join the homologous strands forming the Holliday structure or Holliday junction.
4. The Holliday junction migrates away from the original site, a process called branch migration, as a result heteroduplex region is formed.
5. DNA strands may cut along through  the vertical (V) line or horizontal (H) line.
6. The vertical cut will result in heteroduplexes with recombinants.
7. The horizontal cut will result in heteroduplex with non recombinants.

(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

(a) Cross B produces more recombinants.
(b) The gene e and f are located sufficiently apart leading to crossing over resulting in recombinant varieties.

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.

Colchicine, an alkaloid is extracted from root and corms of Colchicum autumnale, when applied in low concentration to the growing tips of the plants it will induce polyploidy. Surprisingly it does not affect the source plant Colchicum, due to presence of anti-colchicine.

Comutagens:
The compounds which are not having own mutagenic properties but can enhance the effects of known mutagens are called comutagens.
Example: Ascorbic acid increase the damage caused by hydrogen peroxide.

Multiple alleles of a series always occupy the same locus in the homologous chromosome. Therefore, no crossing over occurs within the alleles of a series.
(i) Multiple alleles are always responsible for the same character.
(ii) The wild type alleles of a series exhibit dominant character whereas mutant type will influence dominance or an intermediate phenotypic effect.
(iii) Proterminal synapsis: Pairing starts from the telomeres
(iv) Random synapsis: Pairing may start from anywhere.

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 between non-sister chromatids of homologous chromosomes are called Chiasmata (singular-Chiasma). At chiasma, cross-shaped or 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.

If two linked genes are sufficiently apart, the chances of their separation are possible. As a result, parental and non-parental combinations are observed. The linked genes exhibit some crossing over. This phenomenon is called incomplete linkage. This was observed in maize. It was reported by Hutchinson.

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

(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.

Aneuploidy	Euploidy
Ploidy involving individual chromosomes within a diploid set.. E.g: Trisomy.	Ploidy involving entire sets of chromosomes E.g: Polyploidy

Sharbati Sonora is a mutant variety of wheat, which is developed from Mexican variety (Sonora 64) by irradiating of gamma rays.

When any of the three or more allelic forms of a gene occupy the same locus in a given pair of homologous chromosomes, they are said to be called multiple alleles.

(a) Locus: A specific location of genes on a chromosome.
(b) Centimorgan: Unit of distance in a genetic map.

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.

No. Though crossing over is a common process in germinal cells rarely it also occurs in somatic cells during mitosis. Such crossing over is called mitotic crossing over or somatic crossing over.

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