Linkage and Crossing Over Types of Linkage

Linkage : Tendency of two or more genes to be inherited together during inheritance is known as linkage.

what is linkage ? 

The concept of linkage was given by Morgan in 1910.
He studied peculiar inheritance of white eye gene of Drosophila and drawn following conclusions.
1) Genes which are located on the same chromosome tend to stay together during inheritance.
2) Genes are arranged in linear fashion on the chromosomes.
3) The intensity of linkage between two genes is inversely proportional to the distance between
4) There are two aspects of linkage, coupling phase and repulsion phase.
5) Linked genes do not show independent segregation.

what is linkage 

Linkage explanation : The tendency of two or more genes to be inherited together is called as linkage.

As a consequence linked genes do not show independent segregation.
As a result the ratio obtained in F, and test cross generations are significantly different from the expected ratios of 9:3:3:1 and 1:1:1:1
The effect of linkage was clearly noticed in test cross generation.
The frequencies of parental characters were found to be more than expected than those of new character combinations.
In 1922 C.B. Hutchinson studied seed colour and appearance in maize.
In maize, a dominant gene ‘C’ produces coloured seeds while its recessive allele ‘c’ produces coulourless seeds.
Another dominant gene ‘Sh’ produces full seeds whereas its recessive counterpart ‘sh’ produces shrunken seeds.
When plants having coloured full seeds (CCShSh) were crossed with colourless shrunken (ccshsh), the F1 seeds were found as coloured full (CcShsh ).
When this F1 plant is test crossed with recessive parent ie colourless shrunken, he observed (48.2 %) colouredfull seeds, (48.3 %) colourless shrunken, (1.7 %) coloured shrunken and (1.8 %) colourless full seeds.
The test cross ratio deviated from the normal ratio of 1:1:1:1.
The two character combinations were referred as parental combinations as they were present in parents.
The remaining two phenotypic classes were called as non parental combinations or recombinant types as they combined from two parents.
From the above experiment it appeared that the two dominant genes C and Sh have strong affinity for each other.
That’s why the frequencies of coloured full and colourless shrunken phenotypes were greater than expected.

What is linkage and its types?

Types of linkage : There are two types of linkage are found

types of linkage

  • Complete linkage : When only parental character combinations are recovered it test cross progeny. It is called as complete linkage.

  •  Incomplete linkage : When recombinant types are also recovered in progeny in addition to the parental types in the test crosses it is called as incomplete linkage.

linkage and crossing over

Linkage is also classified as :

1) Coupling Phase : When dominant alleles of the linked genes are present in the same chromosomes it is called as coupling phase.

2) Repulsion phase : When dominant alleles of one gene is present with recessive allele of second gene then it is called as repulsion phase. C sh coloured shnunken c Sh

what is linkage and crossing over

Factors affecting crossing over :

1) The frequency of crossing over between two genes is positively associated with the distance between them.
2) The heterogametic sex shows relatively lower recombination frequencies than the homogametic sex of the same specius.
3) The frequency of recombination declines with age of the famales.
4) The frequency of crossing over increases at lower as well as higher temperatures.
5) The frequency of crossing over is affected by presence of metallic ions. E.g. When Drosophila flies are fed with high calcium diet they showed lower frequency of crossing over.
6) Use of antibiotics leads to increase in frequency of crossing over in Dros “phila flies.
7) When the Drosophila females are irradiated with x rays an increase in recombination was reported.
8) Some plasmagenes are known to reduce the frequency of recombination. Eg. Tifton cytoplasm in Bajra.
9) Chromosomal changes reduce the frequency of crossing over.
10) Centromere tends to reduce crossing over between the genes.

What is linkage and crossing over?

linkage and crossing over :-

Theories of crossing over :
1) Classical theory: (Sharp)
This theory was putforth by sharp in 1934.
This is also called as two plane theory.
According to this theory a chaisma is formed when non-sister chromatids of homologous chromosomes become associated during synapsis.
Chaisma formation is the cause of crossing over.
Each chaisma does not lead to crossing over.
Crossing over occurs during diplotene.
The available evidence does not support this hypothesis and it is of historical importance.

Chaisma Type theory (Janssens)
It was proposed by Janssens in 1909.
It is also called as one plane theory.
According to this theory crossing over results from breakage in on-sister chromatids of homologous chromosomes followed by reunion of the chromatid segments.
Chaismata are produced due to crossing over.
Crossing over occurs during pachytene stage.
Throughout entire bivalent only sister chromatids are associated with each other.
Each chaisma is the consequence of a crossing over event.
As a result 1:1 ratio is expected between the frequencies of chaisma and crossing over.

Copy choice theory : (Belling)
This theory was prosposed by Belling in 1933.
The genes present in a chromosome are the first to be replicated.
They are subsequently connected with each other through the synthesis of the remaining part of the chromosomes.
The homologous chromosomes are likely to be coiled with each other so that the newly syathesized genes may sometimes become joined with those of neighboring segrnent of the homologous chromosome.
It gives rise to recombinant of chromatids.
This theory is unrealistic in nature.

Linkage Group and Linkage Map
Linkage group : All the genes that are linked together from a linkage group.
Genes of a linkage group may be represented on a single strainght-line in the same order in which they are normally present in the concened chromosome.

Linkage map : When the two linked genes are depicted on a line using recombination frequencies the line is called as Linkage map, genetic map or chromosoine map.

Map unit : It is that distance in chromosome which permits one per cent recombination between two linked genes.
A map unit is also called as Morgan. (I centi morgan – 0.1 Map unit)
The sequence of linked genes can be determined by testerosses.
The number of linkage groups of a specius is equal to its gametic chromosome number.
Garden pea =7, Maize – 10.
The frequency of recombination between two linked genes can not exceed 50%.

Crossing over : The exchange of genetic material between nonsister chromatids of homologous chromosomes is called as crossing over.

The crossing over occurs during pachytene stage.

Chaisma : The points of crossing over is called as chaisma.

A) Single crossing over : When only one chaisma is formed during crossing over, it is called as single crossing over.
B) Double crossing over : When two chaismata are formed during crossing over then it is termed as double crossing over.
C) Triple crossing over: When three chaismata are formed during crossing over it is termed as triple crossing over.
D) Multiple crossing over : When more than three chaismata are formed during crossing over, it is termed as multiple crossing over.

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