Linkage And Recombination – Principles Of Inheritance And Variation Class 12 NCERT

CBSE Class 12- Principles Of Inheritance And Variation- Linkage And Recombination: Linkage and recombination are the phenomena that describe the inheritance of genes. Linkage and Recombination both are related to the genetic information inherited from parents to offspring. Linkage is the tendency of genes present close to each other on a chromosome to be inherited together more frequently than expected by chance. Recombination is the process by which genetic information from 2 parent chromosomes is mixed and exchanged during meiosis, resulting in new combinations of alleles in the offspring.

Morgan’s Experiment

Thomas Hunt Morgan studied the fruit fly Drosophila melanogaster to understand how sexual reproduction created differences. For the following reasons, he preferred to work with fruit flies or Drosophila melanogaster:

• It was simple to grow it in a lab using a synthetic medium.
• Demonstrate a brief lifespan (two weeks).
• Several offspring could result from a single mating.
• Distinct sexes for men and women.
• There are various kinds of hereditary differences.

Morgan created a dihybrid cross using males and females with red eyes and brown bodies and white eyes and yellow bodies. Surprisingly, self-crossing of the F1 generation led to an F2 generation that was not 9:3:3:1 in ratio. In terms of peas, the results showed a departure from Mendel’s dihybrid cross.

What is Linkage?

In his research with flies, Morgan discovered that two genes didn’t follow the law of division. The percentage of parental combination should be higher than the non-parental combination if the two genes are on the same chromosome. The term “linkage” refers to the actual physical joining of genes. The gene is said to be related if many features are found on the same chromosome.

However because several characters are found on the same chromosome, the Mendelian principle of independent assortment can be used to explain the linkage. In dihybrid crosses, genetic recombination can be seen (non-parental combination). Whether genes are closely or loosely coupled to chromosomes affects the likelihood of recombination.

A Sturtevant Morgan student identified the chromosome’s place of linkage. By the gene mapping procedure, he discovered the location based on the frequency of recombination. The link map is frequently employed in the Human Genome Project.

Types of Linkage

Linkage typically falls into one of two categories: complete or incomplete.

• Complete Linkage: A complete linkage occurs when a group of characters regularly appears together throughout the course of more than two generations. Only two types of gametes are produced as a result of this perfect connection. Example: Melanogaster Drosophila
• Incomplete Linkage: New gene combinations are created in the progeny or offspring when there is an incomplete linkage. This happens as a result of the connected genes crossing over or forming a chiasma.

Linkage and Crossing Over

• The linkage gene is passed down from one generation to the next and has a tendency to remain on the chromosome. Many linkages are made throughout the crossing-over.
• In the crossover, the linkage often declines while the parental type and age of the connection rise.
• A new variety is created through crossover, whereas a new variety is kept alive through connection.
• In a linked gene, cross-over occurs during prophase I of meiosis, when parts of sister and homologous chromosomes are exchanged.

Significance of Linkage

• Because of linkage, breeders are unable to combine advantageous traits in a single variety. Breeders of plants and animals find it difficult to combine various features as a result.
• By lowering the likelihood of gene recombination, linkage aids in the preservation of parental traits. It thus helps the organism maintain parental, racial, and other traits.

What is Recombination?

The interchange of genetic materials between different species, also known as genetic recombination or genetic reshuffling, results in the production of offspring with combinations of traits that differ from those that originate in either parent. In eukaryotes, meiosis-induced genetic recombination can produce a new genetic data sequence that can be approved from the parents to the offspring. Recombination typically happens in nature.

Genetic recombination in eukaryotes comprises the linking of homologous chromosomes during meiosis. The transfer of information across chromosomes may be used to monitor this.

Types of Recombination

Two different types of recombination exist:

• Homologous Recombination: During meiosis, this kind of recombination takes place between chromosomes with comparable sequences.
• Non-Homologous Recombination: This happens between unrelated chromosomes.
• Site-specific Recombination: This is seen in extremely brief episodes that typically have commonalities.
• Mitotic recombination: Interphase is when mitotic recombination takes place. However, this kind of recombination is typically bad and can cause cancers. Radiation exposure to the cells causes an increase in it.

One of the following three procedures results in recombination in the prokaryotic cells:

• Conjugation
• Transformation
• Transduction

Recombination of Linked genes

To further understand how related genes recombinant, let’s use the example of freckles and red hair. People have freckles and red hair because the genes for both are located on the same chromosome. It is uncommon for the DNA to be divided between the two genes during homologous recombination. Although homologous recombination happens frequently, the likelihood of the DNA coding for these two genes splitting up is relatively low, hence the two traits are typically inherited together. As a result, genes frequently pass down in pairs.

Difference Between Linkage and Recombination

Characteristics	Linkage	Recombination
Definition	Recombination through crossing over occurs more frequently when the distance between two genes increases and the linkage between them decreases.	The probability of recombination decreases as the distance between two sites grows smaller.
Genes	Linkage helps to keep certain genes together in the same chromosome.	Process of recombination mixes genes between chromosomes.
Occurrence	When two genes are very close to one another, it occurs.	When two genes are spread out sufficiently, it happens.
Law	It does not follow the law of independent assortment.	It follows the law of independent assortment.

Conclusion

We have read about linkage and recombination and how they differ, as well as about the various kinds of linkage and recombination, Morgan’s example, and other topics. Eye color and sex are both governed by the same chromosome. As a result, some genes are passed down through chromosomes from parents to children. In his research with fruit flies, Morgan found that while there were over a thousand fruit flies with red eyes, there was only one male fly with white eyes. Linkage describes the actual physical connection between genes. There are two categories of linkage: incomplete linkage and complete linkage. Based on the existence or absence of the non-parental combination, this division is made.

FAQs on Linkage and Recombination

Q1: Who is the father of linkage?

Answer:

T.H. Morgan is known as the father of linkage.

Q2: What is the history of linkage?

Answer:

In 1905, the first experiment to show linking was conducted. At the time, it was unclear why particular qualities seemed to run in families. Genes are physical structures that are connected by physical distance, according to later research. A centimorgan is a common unit of genetic linkage (cM).

Q3: What is the importance of recombination?

Answer:

Hereditary recombinations give a consistent DNA homogenization inside the species and, in this manner, the species’ respectability as a rudimentary design answerable for the protection and ascend in the degree of biological solidness of creatures in developing genealogies.

Q4: What are two causes of recombination?

Answer:

There are two particular hereditary components that lead to recombination i.e. independent assortment and crossing over.