Mention the different types of crossing over?

Introduction:

The important of genetic exchange which is also known as crossing over , takes place during pachytene(third phase of prophase 1 of Meiosis 1 ) , but the process crossing over does not become until the transition to diplotene (4th phase of prophase 1 of meiosis 1).

The term crossing over was coined by Morgan.

Crossing over and double crossing over

Definition of crossing over:

The multistep process crossing over ,is defined as, an exchange of genetic material from one chromatid of a chromosome with the corresponding region of it's homologous chromatid of the other chromosome in the tetrad.

Crossing over is involves in the production of new combinations of genes.Crossing over is facilitated by the formation of the Synaptonimal complex.

In most organisms it is seen that crossing over during meiosis is essential for the proper segregation of the two duplicated homologs into separate daughter nuclei.

Crossing over does not takes place in male Drosophila and female silk worm, Bombyx Mori.

➡️Important characteristics features of Crossing over:

Some characteristics of genetic crossing over are discussed below:

At two levels ,crossing over happens

1.At gross chromosomal level, which is known as chromosomal crossing over.

What do you know about chromosomal crossing over

Chromosomal Crossing Over ,Author-Cbechtel16037 , Source , License-CC BY-SA4.0

2.At Deoxyribonucleic acid level, which is known as genetic recombination.

➡️In crossing over ,a mutual exchange of material between homolog in heterozygotes is reflected.

It appears that physical distance between genes on chromosome is closely associated with the frequency of the crossing over.

Hence the probability of crossing over between two genes increases if the physical distance between the genes along the chromosomes becomes larger.

This principle has an very important role in genetic mapping.The crossing over results basically from an exchange of genetic material between non sister chromatids by break and exchange following replication.

Types of crossing over :

➡️Somatic crossing over :

This is also known as Mitotic crossing over.

During mitotic cell division (M phase or mitosis) ,when in the chromosomes of a body or in the somatic cells of an organism, crossing over takes place , then it is regarded as somatic crossing over.

The somatic crossing over has been reported in the somatic cells of Drosophila by the scientist Curt stern and by G .Pontecorvo in the fungus Aspergillus nidulans.

Somatic crossing over is a very rare case and it has no genetical significance.

Germinal crossing over:

It is also known as Meiotic crossing over.This type of crossing over happens in the germinal cells during the process of gametogenesis,in which the meiotic cell division happens.

The meiotic crossing over is universal in its occurrence and has great genetic significance.

Single crossing over:

When only at one point of the chromosome pair chaisma (plural chaismata,a structure named after the Greek lettter chi , which is shaped like a cross.

It is results from physical exchange between chromatids of homologous chromosomes) occurs then this type of crossing over is regarded as single crossing over.

This type of crossing over helps in the production of two cross over chromatids and two non cross over chromatids.

↪️ Double crossing over:

When,in the same chromosome chaiasmata happen at two points then it is known as double crossing over.

In the process of double crossing over, the formation of each chaismata is dependent on the other and in it four possible classes of recombination occur.

In double crossing over following two types of chaisma may be produced

. Reciprocal chaisma

And complimentary chaisma

↪️Multiple crossing over:

When two genes are located far apart along a chromosome, more than one cross over can be formed between them in a single meiosis, then this type of crossing over is known as multiple crossing over.

This is rare type of crossing over.

If the distance between the genes increases then probability of multiple cross over increases.

Multiple crossing over complicates genetic mapping.

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