LINKAGE AND RECOMBINATION

Introduction. 

Inheritance of the gene is described by linkage and recombination. For more than two generations, the same gene of the same combination is inherited together in case of linkage.

In recombination, offspring are produced by the exchange of genetic materials. However, Mendelian genetics came up with the theory of chromosomal inheritance, which was rejected due to a lack of proof and technology. But later, many scientists proved Mendel’s results.

T.H. Morgan proved chromosomal inheritance which became the platform for the development of the concept of linkage and recombination. In the case of linkage, the DNA remains in the same chromosome for more than two generations. In recombination during the time of meiosis, the genetic material gets transferred during crossing over and leads to the formation of offspring.

MORGAN’S EXPERIMENT

In 1910, T. Hunt Morgan used fruit flies (Drosophila melanogaster) to explain the variation in offspring caused by sexual reproduction. He experimented with fruit flies and observed that there were more than 1000 fruit flies with red eyes but only one male fly with white eyes.

He even published an article in the name of “Sex limited Inheritance in drosophila”.

He chose fruit flies due to the reasons mentioned below.

• One can easily differentiate both the male and female drosophila
• One single mating is capable of producing many offspring
• It can easily cultivate in the synthetic medium in the laboratory
• It has a shorter lifespan of about 2 weeks.
• Different types of hereditary variations are available.

In his experiment, he found out that white colour eyes were only available in male flies; the reason that the white colour trait is found in the X chromosome. As females only have one X chromosome, they don’t have white eyes. It has proven the inheritance of parental traits differs for each sex.

He conducted the dihybrid cross with males (red-eye and brown body) and females (white eye and yellow body). When he conducted the self-crossing in F1, he didn’t get the ratio of 9:3:3:1 in the F2 generation.

DEFINE LINKAGE AND RECOMBINATION

LINKAGE

• While Morgan was experimenting with flies, he noted that two genes didn’t obey the law of division.
• If the two genes are located on the same chromosome, the percentage of parental combination should be more as compared to the non-parental combination.
• Linkage is referred to as the physical connection of genes.
• If different traits are present in the same chromosome, then the gene is said to be linked.
• But the Mendelian principle of independent assortment can be applied to the linkage as different traits are present in the same chromosome.
• Genetic recombination can be seen in dihybrid cross (non-parental combination)
•  The chances of recombination depends on genes, whether they are tightly linked or loosely linked to the chromosome. If the genes are tightly linked, the chances of recombination are low, or if it is loosely linked, the chances of recombination are more.
• The occurrence of the linked gene shows changes in the character in the next generation.
• Sturtevant Morgan’s student found the location of linkage in the chromosome. He found the site by frequency of the recombination through the gene mapping process.
• During the Human genome project, the link map is widely used.

TYPES OF LINKAGE

Based on the presence or absence of non-parental combination, the linkage is divided into two types

1. complete linkage and 
2. incomplete linkage

COMPLETE LINKAGE

If two or more genes are inherited from the parental traits, it is known as complete linkage. This gene won’t perform recombination. Linkage is taken by the gene located near the same chromosome.

Example: Male drosophila (long wings with the grey body)

INCOMPLETE LINKAGE

Linkage is performed by the non-parental gene located somewhere in the chromosome. The genes are located at a distance or located in the damaged chromosome that occurs during crossing over.

Example: Female drosophila 

LINKAGE AND CROSSING OVER

• The linkage gene tends to stay in the chromosome and is transferred from one generation to another generation. During crossing over, many linkages are formed.
• In linkage, the parental type and age of the linkage increase, whereas in crossing, on average, the linkage decreases
• crossover is the formation of a new variety, whereas linkage helps in maintaining the new variety
• Cross-over takes place in a linked gene where the segments are exchanged between homologous chromosomes and sister chromosomes in prophase I in meiosis.

RECOMBINATION

• Here, the exchange of DNA takes place between two chromosomes.
• Recombination is most common in eukaryotes and prokaryotes

There are four types of recombination they are

1. Homologous recombination
2. Non-homologous recombination
3. Site-specific recombination
4. Mitotic recombination

HOMOLOGOUS RECOMBINATION

Homologous recombination is carried out in meiosis, where recombination occurs in chromosomes of the same nucleotide sequence.

NON HOMOLOGOUS RECOMBINATION

The non-homologous recombination takes place in chromosomes that have a different sequence

SITE-SPECIFIC RECOMBINATION

The recombination occurs in the short sequence of the chromosome

MITOTIC RECOMBINATION

It occurs during interphase and may be harmful and cause tumours. This recombination takes place due to prolonged exposure to radiation. There are three types of recombination in prokaryotic cells. They are

1. conjugation
2. transformation
3. transduction

RECOMBINATION OF THE LINKED GENE

For example, freckles and red hair occur in the same gene. During homologous recombination, the chance of splitting up a gene is very low, so the same gene is inherited.

DIFFERENCE BETWEEN LINKAGE AND RECOMBINATION

Conclusion

In conclusion, we have read about linkage and recombination and its difference, the different types of linkage and recombination, Morgan’s example, and so on. The same chromosome determines both eye colour and sex. Therefore, some genes are carried from parents to the offspring through chromosomes. Morgan experimented with fruit flies and observed that there were more than 1000 fruit flies with red eyes but only one male fly with white eyes. There are four types of recombination – Homologous Recombination, Non-homologous Recombination, Site-Specific Recombination and Mitotic Recombination.

The physical connection of genes is called linkage. Linkage can be divided into two types – incomplete and complete linkage. This division is based on the absence and presence of the non-parental combination.