Linkage and Crossing Over: Gregor John Mendel demonstrated in his experiment that characters are determined by certain factors. Such factors are stable and segregate independently at the time of gamete formation. He was, however, unaware of the location of these factors in the cell and thus could not identify the physical counterparts of these factors.

The development of new and improved techniques subsequently helped in the discovery of chromosomes, chromosomal theory of inheritance, and the process of cell division. The significant workings of geneticists on the chromosomal theory of inheritance put light on the phenomenon of Linkage Crossing Over. Let’s go deep into the article to study the characteristics, types, and significance of linkage and crossing.

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What is Linkage?

Definition of Linkage: Since the number of genes in an organism is much larger than the number of pairs of chromosomes, therefore each chromosome pair must contain several genes. This means that during cell division, chromosomes move as a unit, and all the genes of a chromosome move together and do not assort independently. Thus, linkage can be defined as the tendency of certain genes of the same chromosome to be inherited together during chromosomal inheritance.

William Bateson and R.C. Punnett discovered the phenomenon of Linkage. However, Morgan formulated the concept of linkage by his workings of inheritance in Drosophila melanogaster.

Key Terms Related to Linkage

Find below the key terms related to linkage:

1. Linked genes: The genes that do not show independent assortment and are inherited together with other genes as they are closely placed on the same chromosome are called linked genes.
2. Non-linked genes: The genes that are located farther apart from each other on a chromosome or the two chromosomes of a homologous pair are called non-linked genes. These genes show independent assortment.
3. Linkage Group: All the genes located on a homologous pair of chromosomes collectively form a group called the linkage group. The number of linkage groups in an organism is the same as the haploid number of chromosomes or the number of pairs of chromosomes in the organism.
4. Linkage Value: It is the degree of intensity with which the two genes are linked together. The linkage value depends on the distance between the linked genes on the same chromosome.
5. Linkage Map: It is a genetic map of genes located on a chromosome that helps to determine how often two gene loci are inherited together.

Types of Linkage

Linkage can be classified on the basis of different parameters of inheritance. These are discussed as follows:

I. Based on crossing over: The linkage can be of two types:

1. Complete linkage: The two adjacent genes located on the same chromosome do not separate and are inherited together over generations due to the absence of crossing over. It is rare, but Morgan demonstrated the complete linkage in male Drosophila through his experiment.
2. Incomplete linkage: It is the phenomenon in which particular and well-tested characters, instead of appearing together for generations, give rise to a new combination in the F₂ generation. A dihybrid produces four types of gametes; however, the two parental types occur more frequently than the non-parental types. Incomplete linkage leads to the formation of new combinations due to the exchange of chromosome segments during crossing over. Incomplete linkage is common in organisms.

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II. Based on the chromosome involved: The linkage can be classified into two types based on the types of chromosomes:

1. Autosomal linkage: It is the linkage of genes located on the autosome other than the sex chromosome. Humans have 22 autosomal linkage groups.
2. Allosomal linkage: It is the linkage of genes located on a sex chromosome. There are two types of sex chromosomes found in humans; X and Y chromosomes.

III. Based on Genes Involved: Depending on dominant and recessive alleles in a pair of genes, the linkage can be categorised into two phases:

1. Coupling or Cis phase: It is the phase or linkage in which either dominant alleles or recessive alleles of both the genes are present together on the same chromosome and are inherited together.
2. Repulsion or Trans phase: It is the phase when the dominant allele of a trait or gene is linked or paired with the recessive allele of another gene of the same chromosome.

Examples of Linkage

Bateson and Punnett perform a cross between the dominant pea plant with blue flowers and long pollen with a recessive pea plant having red flowers and round pollen to exhibit the incomplete linkage during the cross. Besides maize plants, most organisms show the phenomenon of incomplete linkage.

A. Incomplete Linkage in Maize: This experiment of chromosomal inheritance is demonstrated by Hutchinson and can be described as follows:

1. A cross is made between the maize plant having coloured and full seeds with another maize plant having colourless and shrunken seeds.
2. Coloured and full seeds are dominant over colourless and shrunken seeds as all the plants of the F₁ generation have coloured and full seeds.
3. A test cross between recessive plant (colourless and shrunken seeds) and F₁ hybrid (heterozygous dominant) leads to the formation of the following four different types of seeds with a new combination:
   • Coloured and full seeds
   • Colourless and shrunken seeds
   • Coloured and shrunken seeds
   • Colourless and full seeds

However, the parental combinations are about 96.4% and new combinations about 3.6%.

This shows that paternal characters located on the same chromosome are linked together except for a small percentage of new combinations that arise due to the recombination (exchange between the non-sister chromatids of a homologous pair of chromosomes) during crossing over.

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B. Complete linkage in male Drosophila: This experiment is demonstrated by Morgan. It can be described in the below-listed points:

1. A dominant Drosophila with a grey body and long-winged is crossed with a recessive Drosophila with a black body and vestigial wings.
2. The hybrids of the F₁ generation will be like those of dominant Drosophila, exhibiting the feature of grey body and long-winged.
3. A cross is then performed between the hybrids of the F₁ generation and recessive parent.
4. Four kinds of offspring in equal numbers were expected as a result of independent assortment. However, the offspring formed were phenotypically similar to the original parents used in the experiment and did not show any recombinant forms.
5. The results of the cross justify that the two closely placed genes of a chromosome are linked together.  Hence, the grey body and long-winged characters appear together. On the other hand, the black body character is linked with vestigial wings.

Theories of Linkage

The following two theories have been formulated for the linkage:

A. Differential multiplication theory: This theory was proposed by Bateson in 1930. This theory stated that gametes possessing parental combinations could multiply more rapidly compared to those having non-parent combinations. This leads to the formation of more offspring with parental combinations, and only a few organisms have new combinations. This theory is no longer accepted by cytologists since it does not have any cytological basis.

B. Chromosomal theory of linkage: This theory is proposed by Castle and Morgan. The theory is based on the following two principles:

1. The genes located on the same chromosome are inherited together and are called linked genes. In contrast, those located on the different chromosomes are inherited independently and are called non-linked genes.
2. The degree of linkage depends on the distance between the genes—the shorter the distance, the stronger the linkage.

Significance of Linkage

The significance of linkage is mentioned below:

1. Linkage reduces the chances of the formation of a new combination of genes and therefore restores the parental genes in the upcoming generations.
2. Linkage plays an important role in determining the scope of hybridisation and selection in plants.

What is Crossing Over?

Definition: Crossing over is the recombination of genes due to the exchange of genetic material between the homologous chromosomes of a pair. It is the mutual exchange of segments of genetic material between the non-sister chromatids of two homologous chromosomes so as to produce recombination or a new combination of genes.

F. Janssens was the first person to discover chiasma formation and the related process of crossing over. Morgan found the phenomenon of linkage and recombination.

Types of Crossing Over

Below we have provided the types of crossing over:

Single Crossing Over	The chromatids break and unite at one place (one chiasma is formed)
Double Crossing Over	The chromatids break and unite at two places (two chiasmas are formed)
Multiple Crossing Over	The chromatids break and unite at multiple places (multiple chiasmas are formed)

Theories of Crossing Over

There are two theories to explain the relationship between crossing over and chiasma formation:

1. Classical theory: It is also called a two-plane theory. This theory was proposed by L.W. Sharp. This theory stated that chiasma is the cause of crossing over but not the result of crossing over. The formation of chiasma occurs before the genetic crossing over.
2. Chiasma type theory: It is also called one plane theory. It was proposed by Jannsen and later developed by Belling and Darlington. According to this theory, chiasma is the result of crossing over, and crossing over precedes the chiasma formation. This theory is widely accepted.

Mechanism of Crossing Over

The mechanism of crossing over involves the following steps:

1. Synapsis: The pairing between the homologous chromosomes (synapsis) takes place during zygotene. This pair of homologous chromosomes is called synapsis.

2. Tetrad formation: The two chromatids of a chromosome are referred to as dyads. A group of four homologous chromatids (two dyads) of two synapsed homologous chromosomes is known as a tetrad. The two chromatids of the same chromosome are called sister chromatids. The two chromatids, one of the one chromosome and the other of its homologue, are termed non-sister chromatids.
A highly organised structure of filaments is formed between the paired homologous chromosomes at the zygotene stage of meiosis-I called the synaptonemal complex. It helps in keeping the homologous chromosome in a closely paired state.

3. Exchange of Chromatid segments: The two non-sister chromatids come in contact at certain points. This is the region where the exchange of genes between the two non-sister chromatids of a tetrad takes place. The places where homologous chromosomes are held together and exchange bits of chromatids are known as chiasma. In synapsis, the non-sister chromatids of homologous chromosomes break and recombine. This leads to the formation of chiasmata. The exchange of fragments is stimulated by the development of recombination nodules during the pachytene stage. The unchanged part of the chromatid is called non-crossover, and the changed parts are called recombinants.

4. Terminalisation: The chromatids separate progressively from the centromere towards the chiasma and get separated from each other. It is called terminalisation. Terminalization of chiasma begins in the diplotene stage after crossing over, and completion takes place in the diakinesis stage.

Significance of Crossing Over

The phenomenon of crossing over is of great significance that can be discussed as follows:

1. It provides an inexhaustible store of genetic variability in sexually reproducing organisms.
2. Since crossing over helps in the development of new characteristics. Therefore it is of paramount importance in plant breeding.
3. The new gene combination produced during crossing over plays an important role in microevolution.
4. The frequency of crossing over is helpful in the mapping of chromosomes.
5. Crossing over also justifies the linear arrangement of genes.

Difference between Linkage and Crossing Over

The difference between crossing over and linkage are mentioned below:

Linkage	Crossing Over
Linkage is the tendency to keep the genes together over several generations.	Crossing over leads to the separation of linked genes by the exchange of genes between non-sister chromatids of a homologous pair of chromosomes.
It involves a single chromosome.	It involves a pair of homologous chromosomes.
It reduces the chances of variations.	It promotes the variation in organisms by recombination of genes.

Summary

Genetics is the study of heredity and variations. Linkage and Crossing over are the two main principles of inheritance and variations. The concept of linkage deals with the detailed study of gene location on the chromosome and their inheritance that reflect in the phenotype of an organism. Linkage keeps the two or more linked genes together over generations during inheritance. Linkage can be complete or incomplete. The early experiments to understand the phenomenon of linkage were studied using Drosophila and sweet pea. Crossing over is the phenomenon of the exchange of genes through the recombination of sister chromatids. It is of great importance due as it leads to variations in the organisms.

FAQs on Linkage and Crossing Over

Following are the frequently asked questions on linkage and crossing over:

Q.1: What is linkage?
Ans: Linkage is the tendency of certain genes of the same chromosome to be inherited together during chromosomal inheritance.

Q.2: What are the two types of linkage?
Ans: Based on the phenomenon of crossing over, the linkage is of the following two types:
1. Complete linkage
2. Incomplete linkage

Q.3: What is the difference between linkage and crossing over?
Ans: Linkage is the phenomenon of keeping the genes of a chromosome together during inheritance and therefore reduces variation in organisms. Whereas crossing over promotes the exchange of genes between a pair of the homologous chromosome, which results in variations among organisms.

Q.4: What happens if no crossing over occurs?
Ans: The lack of crossing over greatly reduces the possibility of new genetic combinations in an organism and therefore reduces the chances of variations in organisms.

Q.5: What is a germinal crossing over?
Ans: The crossing over that takes place in reproductive cells during meiosis is called germinal crossing over.

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