Linkage is the tendency of certain genes located in same chromosome that gets inherited without any change in sequence.
They disobey Mendel’s law of independent assortment as linked genes while forming genome won’t segregate as independently as stated in the law.
Chromosomal theory of linkage states :
1. linked genes are situated in same chromosome.
2. They are found in linear fashion.
3. The strength of linkage is determined by distance between the linked genes ,more close the genes more will be chances of linkage.
4. They remain in their original combination during course of inheritance.
Theories of linkage
Coupling and repulsion theory.
Given by : Punnet and Bateson
When dominant genes are linked they always try to couple and when its not they repel one another.
Here blue and long pollened then red round pollened became at greater statistics due to coupling .
Here, opposite is observed because dominant blue and long pollen gene is repelling one another.
1. Complete linkage.
The linkage where 100 % parental characters are restored in progeny generation.
For instance experiment by T.H Morgan by crossing red eyed normal winged Drosophila with white eyed vestigial winged Drosophila.
Cross for it :
Inference : only parental combination was seen zero recombinants.
2. Incomplete linkage.
Linkage where parental combination is restored in majority not completely while in minority we get parental combination of genotype.
For example : cross by Hutchinson in Maize taking coloured full seed and colourless shrunken seed.
Here : 48% coloured full seeded progeny
48% colourless shrunken seeded progeny
But , 4% coloured shrunked seeded and 4% colourless full seeded progeny seen.
Inference : genes for colour and shape of seed were not completely linked due to certain distance apartness having certain chances of crossing over for recombinant genotype.
1. Parental characters get preserved.
2. Due to incomplete linkage prediction of ancestors is possible.
3. Stops variation so, adaptability halts.
4. With that, no evolution occurs.
Crossing over is the process of exchange in genetic material between non sister chromatids of homologous chromosome during prophase –I of meiotic cell division.
Here, homologous chromosome gets paired up. This occurs in zygotene sub stage of prophase in meiosis . the pair is termed as bivalents.
2.Formation of tetrad
The Ribo nucleoprotein between sister chromatids dissolve and 4 strands get clearly seen in pachytene sub phase.
3.Crossing over and chaismata formation.
In Pachytene only crossing over occur , where certain strand of non sister chromatids get exchanged by splicing via Endo nuclease then, fixed by DNA ligase enzyme. The Ribo protein in between non sister now dissolves except in zone of exchange called chiasmata. This happens in diplotene sub phase.
The point of chaismata reach terminal ends as repulsion continues . this occurs in last sub phase called diakinesis.
Types of crossing over :
1. Single : only one region of crossing over. Most common type.
2. Double : two chiasmata forms between same non-sister chromatids or the other one of same homologous chromosome. It can be in two strand or 3 strand or all strand.
3. Multiple : more than two point of crossing over . most rare type.
Factors affecting crossing over :
1. Sex : like for male Drosophila female silkworm no crossing over occur in them
2. Age of parent
4. Distance between the genes: greater is the distance for crossing over more are the chances of it.
5. Electro magnetic radiation : it can ionize genes increasing crossing over frequencies.
6. Chemicals : like Colchicine in fact prevents pairing for crossing over.
1.Intra specific variation is occurred.
2. Hybrid disease resistant progeny can be obtained.
3.They are proof for linear arrangement of gene in chromosome.
4.Supports sexual reproduction.
5. Plays vital role in Evolution .
FOR MORE ON IT :