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.

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) Incomplete linkage

Rano : 1:2:1

(i) Complete Linkage
(ii)
 
(iii) Observed ratio 1:1 

Deletions are observable during meiotic pachytene stage and polytene chromosome. The unpaired loop formed in the normal chromosomal part at the time of chromosomal pairing. Such loops are called as deficiency loops and it can be seen in meiotic prophase.

Temperature, X-rays, Gamma rays and UV rays.

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.

Synaptonemal complex is a highly organised structure or filaments that facilitates the synapsis and chiasma formation during crossing over mechanism.

Crossing over is a precise process that includes stages like synapsis, tetrad formation, cross over and terminalization.

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

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

1. Single cross over: Formation of single chiasma and involves only two chromatids out of four.
2. Double cross over: Formation of two chiasmata and involves two or three or all four strands.
3. Multiple cross over: Formation of more than two chiasmata and crossing over frequency is extremely low.

Pairing of homologous chromosomes during zygotene stage of prophase I of meiosis I is called synapsis. Synapsis of three types:
1. Procentric synapsis: Pairing starts from middle of the chromosome.
2. Proterminal synapsis: Pairing starts from the telomeres.
3. Random synapsis: Pairing may start from anywhere.

(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.