Methods of Gene Mapping | Botany Optional for UPSC PDF Download

Discovery of Linkage

The concept of linkage in genetics was first identified by English scientists William Bateson and R.C. Punnett in 1906, through their research on Sweet Pea (Lathyrus odoratus). However, it was further developed into a fundamental concept by Thomas Hunt Morgan in 1910, based on his extensive work with the fruit fly Drosophila melanogaster.

Definition of Linkage

Linkage refers to the phenomenon where specific genes remain closely associated during inheritance over multiple generations without undergoing any change or separation. This continuity is due to these genes being situated on the same chromosomes.

Characteristics of Linkage

Linkage in genetics exhibits the following characteristics:

• Linkage involves two or more genes that are physically located on the same chromosome in a linear arrangement.
• Linkage tends to reduce genetic variability.
• It can involve either dominant alleles or recessive alleles, which can be in a "coupling phase" (both dominant or both recessive alleles) or a "repulsion phase" (a combination of dominant and recessive alleles).
• Linkage typically occurs between genes that are physically close to each other on the chromosome.
• The strength of linkage is influenced by the distance separating the linked genes on the chromosome.

Genes in Linkage

• Linked Genes:
  • Linked genes are those genes that do not segregate independently during inheritance.
  • These genes are situated on the same chromosome.
  • When dealing with dihybrid crosses involving linked genes, the expected ratio of the offspring is 3:1.
• Unlinked Genes:
  • Unlinked genes, on the other hand, exhibit independent assortment during inheritance.
  • These genes are typically located on different chromosomes or are so far apart on the same chromosome that they behave as if they are on different chromosomes.
  • In dihybrid crosses involving unlinked genes, the expected ratio of the offspring is 9:3:3:1.

Theories of Linkage

• Differential Multiplication Theory (William Bateson):
  • Proposed by English scientist William Bateson, this theory suggests that linked genes remain together because they tend to replicate or multiply as a unit during cell division.
  • In other words, genes located on the same chromosome are physically close to each other and, therefore, replicate as a single unit, leading to their joint inheritance.
  • Bateson's differential multiplication theory was one of the early explanations for the phenomenon of genetic linkage.
• Chromosomal Theory (Thomas Hunt Morgan):
  • Proposed by American geneticist Thomas Hunt Morgan, the chromosomal theory of inheritance is a fundamental concept in genetics.
  • This theory states that genes are located on chromosomes, and the behavior of genes during inheritance can be explained by the movement and segregation of chromosomes during cell division.
  • Specifically, linked genes are those that are physically located on the same chromosome, and their inheritance patterns are influenced by the behavior of that chromosome during meiosis.
  • Morgan's work with Drosophila melanogaster provided strong experimental support for the chromosomal theory of inheritance, and it revolutionized our understanding of genetics.

Kinds of Linkage

Linkage refers to the tendency of certain genes to be inherited together due to their physical proximity on the same chromosome.
There are different kinds of linkage, classified based on various factors:

On the Basis of Crossing Over

• Complete Linkage:
  • Complete linkage occurs when genes located on the same chromosome are inherited together over generations without any crossing over taking place.
  • This is a rare phenomenon but has been reported in male Drosophila (fruit flies).

Incomplete Linkage

• Incomplete linkage is characterized by the tendency of genes on the same chromosome to occasionally separate due to crossing over during meiosis.
• As a result of crossing over, recombinant progeny are produced alongside parental types.

On the Basis of Chromosomes Involved

• Autosomal Linkage:
  • Autosomal linkage refers to the linkage of genes located on autosomes (non-sex chromosomes).
  • In this type of linkage, genes on the same non-sex chromosome are considered.
• Allosomal or Sex Linkage:
  • Allosomal or sex linkage refers to the linkage of genes located on the sex chromosomes (X or Y).
  • Genes on the sex chromosomes, which determine an individual's sex, can exhibit this type of linkage.

On the Basis of Genes Involved

Linkage can also be categorized based on the types of alleles present on the linked genes:

• Coupling Phase:
  • In the coupling phase, dominant alleles and recessive alleles of different genes located on the same chromosome are linked together.
  • For example, if the alleles for Trait A (dominant) and Trait B (recessive) are located on the same chromosome, they would be in the coupling phase.
  • Example:
    Trait A: TR
    Trait B: tr
    This represents the coupling phase.
• Repulsion Phase:
  • In the repulsion phase, dominant alleles of the same gene are linked with recessive alleles of other genes on the same chromosome.
  • For instance, if the alleles for Trait X (dominant) and the alleles for Trait Y (recessive) are linked on the same chromosome, they would be in the repulsion phase.
  • Example:
    Trait X: Tr
    Trait Y: tR
    This represents the repulsion phase.

Linkage Group

A linkage group is a term used in genetics to describe a linearly arranged group of genes that are closely located on the same chromosome and are typically inherited together. In other words, these genes tend to be passed from one generation to the next without significant recombination or crossing over events.

Examples of linkage groups include:

• In the fruit fly Drosophila melanogaster, there are four linkage groups, corresponding to the four pairs of chromosomes in this species.
• In humans, there are 23 linkage groups, which correspond to the 23 pairs of chromosomes in the human genome.

Significance of Linkage

Linkage has several important implications and significance in genetics:

• Reduction of New Gene Combinations: Linkage reduces the likelihood of new combinations of genes forming in gametes during meiosis. Genes within a linkage group tend to stay together, limiting the mixing of alleles from different genes.
• Maintenance of Traits: Linkage helps maintain the association of parental, racial, or specific traits together in offspring. This means that traits that are physically close on the same chromosome are often inherited together.
• Beneficial for Breeding: In agriculture and breeding programs, understanding linkage is valuable. It allows breeders to select for specific combinations of genes or traits by considering the linked genes together.
• Role in Hybridization: Linkage plays a crucial role in determining the nature and scope of hybridization. When certain genes are linked, they are more likely to be inherited together in hybrids, impacting the traits of the resulting offspring.

Crossing Over

Discovery of Crossing Over

The phenomenon of crossing over was described by Frans Alfons Janssens in 1909. Janssens made this discovery by observing cross-like arrangements in meiosis and proposed crossing over as a genetic process.

Meaning of Crossing Over

Crossing over, also known as chromosomal crossover, is the genetic process that involves the exchange of genetic material between homologous chromosomes. This exchange results in the formation of recombinant chromosomes, which carry combinations of genetic traits that are different from the original parental chromosomes.

Characteristics of Crossing Over

• Crossing over occurs between non-sister chromatids of homologous chromosomes. One chromatid from each of the two homologous chromosomes is involved in the crossing over process.
• It leads to the recombination of genes, creating new combinations of linked genes on the chromosomes.
• The frequency of crossover events can vary from 0% to 50% depending on the genes and chromosomes involved.
• Crossing over typically results in two recombinant types (crossover types) and two parental types (non-crossover types) of chromosomes.
• The exchange of genetic material during crossing over is reciprocal, meaning that both homologous chromosomes involved in the crossover exchange equal segments or genes.

Stage at Which Crossing Over Occurs

• Crossing over in meiosis occurs during the pachytene stage of the prophase of meiosis I. The pachytene stage is also known as the recombination stage. It is during this stage that homologous chromosomes are in the tetrad stage, with each homologous pair consisting of four chromatids. Crossing over takes place between non-sister chromatids within these tetrads.
• In summary, crossing over is a genetic process where genetic material is exchanged between homologous chromosomes, leading to the formation of recombinant chromosomes with new combinations of genetic traits. This process occurs during the pachytene stage of meiosis I.

Types of Crossing Over

Crossing over is a genetic process that involves the exchange of genetic material between homologous chromosomes during meiosis.
There are mainly two types of crossing over:

• Somatic or Mitotic Crossing Over:
  • Somatic or mitotic crossing over occurs in somatic cells during the process of mitosis.
  • It is relatively rare and has limited genetic significance.
  • Examples of this type of crossing over have been reported in organisms like the fruit fly and the fungus Aspergillus.
• Germinal or Meiotic Crossing Over:
  • Germinal or meiotic crossing over takes place in germinal cells during meiosis, specifically during the formation of gametes (sperm and egg cells).
  • This type of crossing over is universal and has significant genetic implications.

Kinds of Germinal Crossing Over

Within germinal or meiotic crossing over, there are two main categories:

• Equal Crossing Over:
  • Equal crossing over involves the exchange of segments between chromosomes that are of equal size.
  • It can be further categorized into three types based on the number of points at which crossing over occurs:
    • Single Crossing Over: Chromatids break and reunite at one point only.
    • Double Crossing Over: Chromatids break and reunite at two points within the same tetrad.
    • Multiple Crossing Over: Chromatids break and reunite at multiple points in the tetrad. This occurs rarely.

• Unequal Crossing Over:
  • In unequal crossing over, the segments exchanged between chromatids are of unequal size.
  • As a result, one chromosome receives an extra gene, while the other chromosome gets one gene less.

Mechanism of Crossing Over

Crossing over is a crucial genetic process that occurs during meiosis, specifically in prophase I. It involves the exchange of genetic material between homologous chromosomes.
The mechanism of crossing over can be broken down into several steps:

1. Synapsis

• During the Zygotene stage of prophase I, homologous chromosomes come together and align side by side. This pairing of homologous chromosomes is called synapsis.

2. Tetrad Formation

• Within this paired state, the two chromatids of each chromosome are referred to as dyads.
• A group of four homologous chromatids, consisting of two dyads from two synapsed homologous chromosomes, is known as a tetrad.
• In each tetrad, there are two sister chromatids from one chromosome and two non-sister chromatids (one from each homologous chromosome).

3. Exchange of Chromatid Segments

• The key event in crossing over involves the exchange of genetic material between two non-sister chromatids within a tetrad.
• Specifically, two non-sister chromatids break at equivalent locations.
• The broken ends then transpose and join the corresponding broken ends of the opposite chromatid. This exchange of segments completes the crossing over process.
• The chromatids that remain unchanged are referred to as parental or non-crossover chromatids.
• The chromatids that have undergone genetic exchange are called recombinants.

4. Terminalization

• Following the completion of crossing over, the pachytene stage (where crossing over occurs) transitions into the diplotene stage of meiosis I.
• The forces that held the homologous chromosomes together in synapsis come to an end.
• The points where separation does not occur between chromatids are referred to as chiasmata.
• Chiasmata are mobile and move along the chromosome.
• The gradual separation of chromatids from the centromere toward the chiasma, with the chiasma moving like a zipper towards the end of the tetrad, is known as terminalization.

Factors Affecting Crossing Over

Crossing over is a complex genetic process that can be influenced by various factors. Some of the factors affecting crossing over include:

• Maternal Age Effect: The age of the mother can influence the frequency of crossing over in offspring.
• Temperature: Temperature can have an impact on the occurrence of crossing over in some organisms.
• Nutritional and Chemical Effects: Diet and exposure to certain chemicals can affect the rate of crossing over.
• Chromosomal Effects: The structure and composition of chromosomes can influence the likelihood of crossing over.
• Centromere Effects: The position of the centromere on a chromosome can affect the occurrence of crossing over.
• Mutation Effects: Genetic mutations can alter the frequency and pattern of crossing over.
• Sex: In some species, the sex of the individual can influence the rate of crossing over.

Significance of Crossing Over

Crossing over plays a significant role in genetics and biology:

• Linear Arrangement of Genes: Crossing over helps establish the concept of the linear arrangement of genes on chromosomes.
• Chromosome Mapping: The frequency of crossing over can be used to map the location of genes on chromosomes, allowing for the creation of genetic maps.
• Sexual Reproduction: It is a crucial process in sexual reproduction, as it leads to the creation of genetic diversity among offspring.
• Evolution: Crossing over increases genetic variation, which is essential for the process of evolution and adaptation.
• Plant Breeding: In plant breeding, crossing over can be used to create new combinations of desirable traits in crops.

Difference between Linkage and Crossing Over