Linkage and crossing over are alternatives of each other | STUDENT BABA


We all know humans have 23 linkage groups and they are linked and then crossed over. After learning about linkage and crossing over one question which strikes is "Are linkage and crossing over alternatives of each other ?". Well, no need to stress about it because i got a solution to your answer.

Linkage and crossing over are alternatives of each other:

Yes, linkage and crossing over are alternatives of each other as linkage is means that the genes stay together in the same chromosome and crossing over means where the segments of the non-sister chromatids are exchanged during the process of meiosis. 

What is linkage ?

There are two genes present on a chromosome, when they combine with each other is called linkage. There are numerous genes on the chromosomes and they all are linked each other which is said to be a linkage group.

 Linkage
Linkage

Types of linkage:

There are two types of linkage groups:-

Incomplete linkage:

The genes which can be separated by crossing over are called incompletely linked genes and this phenomenon where their inheritance takes place is called incomplete linkage. These genes are widely located on the chromosomes.

Complete linkage:

The genes which cannot separate by crossing over are called completely linked genes and this phenomenon where their inheritance takes place is called complete linkage. These genes are closely located on the chromosomes.

Types of sex linkage

Whilst the transfer of sex chromosomes with body characters of the offspring from its parents is called sex linked inheritance or sex linkage. There are two types of sex linkages:-

Incomplete sex linkage:

These genes are present on the homologous regions of X and Y chromosomes. Since crossing over can occur in these homologous regions they do not combine or inherit because of this they are called incompletely sex linked genes and this phenomena of inheritance is called incomplete sex linkage. Eg, colour blindness, nephritis, etc.

Complete sex linkage:

These genes are present on the non-homologous regions of X and Y chromosomes. Since crossing over does not take place in these regions they combine or inherit and these genes are called as completely sex linked genes and this phenomena of inheritance is called complete sex linkage. Eg, Haemophilia, Myopia, etc.

What is crossing over ?

When the genetic material is being transferred to non-sister chromatids of the homologous chromosome during mitosis and new allelic combinations are to be seen in the daughter cells is called crossing over.

 Crossing over
Crossing over

Steps of crossing over:

There are four steps for the process of crossing over:-

Synapsis:

The pairing taken place on the homologous chromosomes is called synapsis. The chromosomes come close to each other in order to form a pair and this pair of the homologous chromosomes is called bivalent.

 Synapsis
Synapsis

Tetrad formation:

Two chromosomes present on the homologous chromosomes align in such a way that they are next to each other, they are two sister chromatids so it ends up looking like a group of four and hence a tetrad is formed.

Tetrad formation
Tetrad formation

Crossing over:

Crossing over leads to recombination by the break down activity carried out by endonuclease. The endonuclease breaks down the two non sister chromatids present on the bivalent, these breaks are at certain intervals and the process of crossing over takes place

The intervals where the breaks are made are called chiasma or chiasmata.

Terminalization:

When the chisasmata starts to move from it's initial point to the end, this process is called terminalization.

 Terminalization
Terminalization

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