Linkage and Recombination: Definition, Differences, Diagram, Example

Linkage and recombination are key concepts in genetics that explain how genes are inherited together or separately. Linked genes are located close to each other on the same chromosome and tend to be inherited together, while recombination occurs during meiosis, allowing genes to exchange segments and create genetic diversity. In this article, linkage and recombination, Morgan's experiment, linkage concepts, examples of linked genes in humans and other organisms, importance of linkage maps, types of linkage, linkage and crossing over, recombination, and tools and techniques in the study of linkage and recombination are discussed. Linkage and Recombination is a topic of the chapter Principles of Inheritance and Recombination in Biology.

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This Story also Contains

1. What Is Linkage and Recombination?
2. Morgan's Experiment
3. Linkage Concepts
4. Examples of Linked Genes in Humans and other Organisms
5. Importance of Linkage Maps
6. Types of Linkage
7. Linkage and Crossing Over
8. What is Recombination?
9. Tools and Techniques in the Study of Linkage and Recombination

What Is Linkage and Recombination?

Linkage refers to the tendency of genes located sufficiently close to each other on the same chromosome to be inherited together. On the other hand, recombination is the process whereby there is an interchange of genetic material between homologous chromosomes in meiosis, thereby forming new combinations of alleles.

All these concepts are of great importance to be understood in issues like inheritance patterns and genetic diversity including gene mapping onto chromosomes. Linkage explains why some characteristics are inherited together most of the time and some recombination has to take place so that at least some genetic variation necessary for evolution and adaptation is provided.

The history of studies in linkage and recombination began with the basic principles of inheritance laid down by Gregor Mendel. Concrete proof of occurrence came from experiments with fruit flies in the early twentieth century by Thomas Hunt Morgan. On the chromosomal theory of inheritance, this work by Morgan thus laid the foundation and paved the way for modern genetics.

Morgan's Experiment

At the beginning of the 20th century, a series of experiments on the fruit fly Drosophila melanogaster was conducted by Thomas Hunt Morgan. His work presented solid evidence for the chromosomal theory of inheritance. Morgan focused on trying to determine how particular characteristics were passed from one generation to another and which ones seemed to be inherited as groups more often than would be expected based on Mendelian genetics.

Key Findings

• Discovery of Genetic Linkage: Morgan found that some characteristics did not assort quite as independently as Mendel's laws would have predicted. He took up an approach to test these observations—classic cross-breeding experiments on the fruit fly Drosophila melanogaster. For instance, Morgan realised that genes governing eye colour and wing shape in Drosophilas are indeed linked, and because of this, on the same chromosome.

• Introduced the Concept of Recombination Frequency: Morgan postulated that the process of crossing-over in meiosis could 'unlink' linked genes. He introduced the concept of recombination frequency, which is taken to be the probability of crossing over occurring between two genes. This method uses the recombination frequency as an estimate of the distance between genes on a chromosome.

• Genetic Maps from Recombination Data: From these recombination frequencies, Morgan and his fellow students worked out the diagrams of chromosomal gene relative positions, called genetic maps. Taken together, these genetic maps presented scientists with schematic linear arrays of genes and permitted the prediction of the chances of a grouping of characteristics being inherited together. This was a major breakthrough in genetics. It gave a way of mapping genes responsible for particular characteristics or conditions.

Also Read:

• MCQ Practice on Linkage and Recombination
• Principles of Inheritance and Variation
• Genetics
• Molecular Basis of Inheritance

Linkage Concepts

Linkage is an occurrence where two or more genes happen to be on the same chromosome and thus are likely to be inherited together due to an absence of independent assortment during meiosis. That is to say, linked genes are genes that occur on the same chromosome and thus act differently from genes found on different chromosomes. That is, they conduct themselves in a way that does not fulfil Mendel's law of independent assortment. This is because the genes will be situated close to one another on the same chromosome and thus will have a higher tendency to be passed on as a block while gametes are being formed.

A linkage group refers to a set of genes located on the same chromosome. In this way, the entire genome of the organism gets divided into some linkage groups—equal in quantity to the organism's number of chromosomes. Humans thus have 23 linkage groups—one per chromosome. The genes in a linkage group will tend to be inherited together until they are separated due to recombination. The identification of linkage groups is hence very important in building genetic maps and interpreting patterns of genetic inheritance.

Examples of Linked Genes in Humans and other Organisms

• The genes for the red colour of hair (MC1R gene) and freckles, which have also been linked to variants of the MC1R gene, will generally be passed together in linkage from one generation to the other. This is why they are developmentally always coupled in an individual.

• In the fruit fly, Drosophila melanogaster, the work Morgan did on linkage used the genes controlling body colour and wing shape. Long-winged flies are typically grey-bodied and the short-winged flies are often black-bodied. The two characteristics are associated with each other because their controlling genes are on the same chromosome.

• In plants like corn, genes controlling kernel colour and kernel texture are typically linked and their patterns of inheritance can be predicted.

Importance of Linkage Maps

There is a need for linkage maps in genes. They are tools to provide an indication, on a chromosome, of the relative locations of genes for how frequently cross-overs occur between them. They help in:

• Gene Discovery and Disease Research: Linkage maps are critically needed to establish chromosomal locations of specific genes responsible for particular traits or genetic disorders. Through family and population studies, scientists have been able to determine genes that contribute to diseases such as cystic fibrosis, Huntington's disease, and various kinds of cancers.

• Breeding Programs: Linkage maps in agriculture assist in the breeding programs of plants and animals by selecting desirable characters. For example, linkage maps will help a breeder in selecting crop varieties that possess higher yields, resistant to diseases, or have better nutritional values.

• Understanding Evolution: Linkage maps can tell real evolutionary processes in the way genes and their corresponding traits seem to be passed on to succeeding generations together.

• Genetic Counseling: Linkage analysis enables the genetic counsellor to identify the risks of inherited conditions within families. Thus, it would be possible to give a higher accuracy of risk about the passage of a genetic disorder if genetic markers that are linked could be identified.

Types of Linkage

The degree of linkage of genes may vary depending upon the physical distance between them on a chromosome and result in different inheritance patterns known as complete and incomplete linkage.

Complete Linkage

When the genes are physically so close to each other on a chromosome they are always inherited together. That is, the alleles of the genes are passed to the offspring as a unit always. There will be no recombination between the genes in such a case. Very few genes exhibit complete linkage, for even genes that are physically very close normally have at least some recombination over several generations.

A classic example of complete linkage is found in certain gene pairs of the fruit fly, Drosophila melanogaster, which determine body colour and wing size in some mutant strains. In such cases, genes are so tightly linked that they do not separate by recombination.

It makes genetic analysis and breeding programs easy for the fact that the linked characteristics will always be inherited together, but it limits the generation of genetic diversity since alleles between these genes that are linked cannot mix.

Incomplete Linkage

In these cases, genes are bunched closely enough on a chromosome to stay together through most, but not all, crossing-overs. Recombination can occur between the genes and new allele combinations end up in the offspring. The probability of recombination between two genes increases with an increase in the actual distance between them on the chromosome.

A good example of incomplete linkage comes from the genes controlling flower colour and pollen shape in sweet peas (Lathyrus odoratus). In that case, while the two genes are physically linked and thus tend to pass as a pair from parent to offspring, some recombination occurs, so that some offspring end up with new combinations of flower color and pollen shape.

The recombination frequency between incompletely linked genes is less than 50%. This frequency reflects the physical distance of genes on a chromosome and so this may be used, for example, to estimate gene distances.

Incomplete linkage is very vital in genetic mapping since it allows one to measure the relative positions of genes in a chromosome. Frequencies of recombination therefore make geneticists establish highly detailed linkage maps. These are essential in the identification of genes related to particular genes and diseases.

Linkage and Crossing Over

How linkage and crossing over work together is important in understanding genetic variation and the ultimate inheritance of multiple genes.

Homologous chromosomes come very close together during meiosis, particularly during prophase I. Genetic material segments can be exchanged between these two homologues; the process is called crossing over. Since crossing over has resulted in new allelic combinations that were not found in either of the parents, this is an important mechanism by which genetic variation is achieved in sexually reproducing organisms.

These crossing-over events take place at locations in the chromosomes called chiasmata, which appear under the microscope during crossing-over as the expression of an exchange of genetic material. Following is the detailed mechanism:

• Pairing up of homologous chromosomes forms a tetrad, consisting of four chromatids, two each from the homologous chromosomes.

• Within these tetrads, non-sister chromatids break and rejoin at corresponding positions, creating chiasmata. Thus, crossing over is an exchange of segments between non-sister chromatids (one maternal and one paternal), mixing their alleles.

• Chromosomes then separate, but exchanged pieces have introduced new combinations of alleles into the gametes, either the sperm or the egg cells.

• The more frequently crossing over occurs between two genes, the more distance there is physically between them on a chromosome. This relationship can be measured as the recombination frequency and stated as a percentage. For example:

• Genes that are near each other on a chromosome tend to be inherited together; because of this, they have a lower recombination frequency—they have less physical distance where crossing over could occur.

• The frequency of recombination will be higher for genes that lie farther apart because there is more chance of including a crossover event between them.

The measurement of the recombination frequencies is a good tool in genetics to draw linkage maps. The maps drawn depict the relative positions of genes on a chromosome because of their recombination frequency. Here's the way genetic mapping works:

These recombination frequencies will then be converted to map units or centiMorgans, with one centimorgan estimated as the distance between two genes having a 1% chance of recombination.

Knowing the recombination frequencies between several gene pairs permits the geneticist to determine their sequence and even roughly estimate distances between them. For example, if gene A and gene B have a recombination frequency of 10%, while gene B and gene C have a recombination frequency of 5%, it would suffice to conclude that gene B is closer to gene C than to gene A. The linkage maps are necessary for those genes determining certain characteristics or diseases, as they are used in breeding programs by selecting desirable characters. They are also applied in medical research, used in finding genes associated with genetic disorders, and in evolutionary biology to study the genetic relationships among different species.

What is Recombination?

Recombination is the process wherein there is an interchange of genetic material between homologous chromosomes during meiosis which leads to new combinations of alleles in the offspring.

Gene recombination allows for an increase in gene diversity among a population. Such a stretch of genetic diversity is exposed in a manner that allows evolution and adaptation. It allows alleles from each parent to combine and creates new, unique combinations in each generation.

Types of Recombination

Recombination is a process underpinning genetic diversity and DNA repair and has different types with different mechanisms and functions.

Homologous Recombination

This recombination occurs between homologous chromosomes and is the foundation of meiosis.

During prophase I of meiosis, every homologous chromosome forms a pair and creates a tetrad. The segments of the non-sister chromatids are exchanged at chiasmata. That is, the physical exchange of the segment of the chromosomes creates new combinations of alleles and increases genetic diversity in gametes. Homologous recombination conveys another vital job: the repairing of DNA. In particular, double-strand breaks are repaired using a template from a homolog to ensure accurate repair.

Non-homologous Recombination

Repair pathway for double-strand breaks in DNA that does not involve a homologous template.

In this process, a collection of proteins directly joins the broken DNA ends. It is therefore somewhat fast but fairly error-prone because insertions and/or deletions could result at the site of repair. It is important for genome stability, especially in cells that do not divide or when the action of a homologous template is not possible.

Site-Specific Recombination

This form of recombination occurs at specific sequences in DNA and requires a class of special enzymes called recombinases.

Recombinase enzymes are known to recognise specific DNA sequences and catalyze the exchange of DNA segments thereat. The most prominent applications include the use of site-specific recombination by viruses while their genome gets integrated into host DNA and by bacterial cells in promoting genetic rearrangement, for example, integration and excision of plasmids.

Mitotic Recombination

Although the frequency of crossing over is much higher during meiosis, the same event within somatic cells occurs during the cell division process called mitosis.

Mitotic recombination generally takes place due to the interchange of segments between homologous chromosomes during the S or G2 phase in the cell cycle. The event often responds to DNA damage and helps to repair damaged DNA. It may also result in somatic cells having genetic diversity. Unlike meiotic recombination, mitotic recombination does not result in gametes but may end up in a state where genetic mosaicism is partially developed.

Linked Genes and Recombination Genes that are near one another on the same chromosome are physically linked and tend to be inherited together unless they become separated by recombination.

As homologous chromosomes pair up during meiosis, they include the potential for exchanging segments with each other. Crossing over of this nature can thus break apart new allele combinations for linked genes.

Crossing over is essential for the recombination of linked genes, providing an opportunity for genetic diversity by forming new combinations of alleles passed down to subsequent generations.

Tools and Techniques in the Study of Linkage and Recombination

Genetic Markers: A specific DNA sequence that points to a location on a chromosome, thereby serving as a tag for mapping genes and studying their linkage.

Test Crosses and Back Crosses: Breeding experiments used in determining gene linkage. Test crosses are crossing an organism with a homozygous recessive individual. Back crosses are crossings of the offspring with one of its parents.

PCR (Polymerase Chain Reaction): Enlarges certain DNA sequences so more study of the genetic material can be done.

Gel Electrophoresis: Separates DNA fragments by size and allows for genetic marker analysis.

DNA Sequencing: It identifies the correct sequence of nucleotides that make up a DNA molecule, offering detailed genetic information.

Gene editing with CRISPR: This is a relatively new, powerful tool in gene editing that confers high precision.

Genomics: The study of genomes, which incorporates sequencing, analysis, and comparative study of complete genomes, seeking a better understanding of genetic variation and function.

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