Difference Between Linkage and Crossing Over: Linkage And Recombination

Linkage and crossing over are genetic processes that influence how traits are inherited. Linkage occurs when genes located close together on the same chromosome tend to be inherited together. While crossing over is the exchange of genetic material between homologous chromosomes during meiosis. This exchange breaks the linkage and creates new genetic combinations, contributing to genetic diversity. This discovery extended the principles of inheritance and variation.

This Story also Contains

1. What is Linkage?
2. What is Crossing Over?
3. Mechanism of Linkage
4. Mechanism of Crossing Over
5. Differences between Linkage and Crossing Over
6. Role in Evolution and Genetic Variation
7. MCQs on Genetics

The study of linkage and cross over has various practical applications. It helps in identifying disease-causing genes, improving plant and animal breeding, understanding inheritance of complex traits, and gene mapping. All these processes help in the advancement of genetic research and biotechnology.

What is Linkage?

Linkage is the tendency of genes that are located close to each other on the same chromosome to stay together during meiosis. This was discovered by Thomas Hunt Morgan through his work with fruit flies (Drosophila melanogaster). He observed that certain traits did not assort independently as Mendel’s Law of independent assortment. There are two kinds of linkages. First is complete linkage, in which genes are always inherited together. Second is incomplete linkage, where crossing over allows recombination between genes. Linkage groups refers to sets of genes that tend to be co-inherited and indicate the closeness on a chromosome. Hence, the process is useful in mapping genetic traits or understanding human genetic disorders.

What is Crossing Over?

Crossing over is the process where segments of genetic material are exchanged between a pair of homologous chromosomes during prophase I of meiosis I. It occurs during synapsis, the pairing of homologous chromosomes, and formation of chiasmata, which refer to points where crossing over will take place. This process results in the recombination of genetic material. It ultimately leads to the creation of new combinations of alleles and increased genetic diversity. This reshuffling of genetic information is very important for evolution and adaptation of species to changing environments.

Mechanism of Linkage

The physical basis of linkage is the linear arrangement of genes on chromosomes. Genes that lie closer to each other on the same chromosome are less likely to get separated by crossing over during one of the stages of meiosis. The recombination frequency between the different gene pairs helps in determining the distance between them, allowing construction of linkage maps. These maps estimate the physical distances based on genetic cross results and offspring patterns. These physical maps have become essential tools in genetics research for identifying the location of genes responsible for specific traits genetic disorders.

Mechanism of Crossing Over

Crossing over is a complex molecular process that involves several enzymes like recombinase These enzymes help in the exchange of segments between homologous chromosomes. It also provides a way for genetic diversity and appropriate segregation during meiosis. Unlike mutations, crossing over is not related to a change in DNA sequence. It is a natural and controlled mechanism that promotes genetic variation without altering the overall genetic component.

Differences between Linkage and Crossing Over

Linkage and crossing over are two related but opposite genetic processes. Linkage keeps genes together, while crossing over separates them exchanging genetic material. Here is a table that gives a detailed list of differences between linkage and crossing over:

Role in Evolution and Genetic Variation

Linkage and crossing over have a great effect on evolution and genetics. Linkage affects how traits are inherited. Especially, when genes are located close to each other on the chromosome and tend to be co-inherited. In contrast, crossing over enhances genetic variation by the production of new combinations of alleles. These variations are essential for adaptability and survival in changing environments. It provides genetic diversity that acts as raw material for natural selection and evolutionary processes.

Recommended Video on the Difference between Linkage and Crossing Over

MCQs on Genetics

Q1. Assertion: Linkage groups refer to a set of genes that are located on the same chromosome.

Reason: During meiosis, homologous chromosomes can exchange genetic material through the process of crossing over.

1. Assertion and reason are both true, and reason is an accurate account of assertion.

2. Both assertion and reason are accurate, but reason does not adequately explain assertion.

3. The assertion is correct, but the reasoning is incorrect.

4. Both the assertion and reason are incorrect.

Correct Answer: 1) Assertion and reason are both true, and reason is an accurate account of assertion.

Explanation:

On the same chromosome, there exist collections of genes called linkage groups. This indicates that the locations of these genes on the chromosome are contiguous. The explanation given is also true. Homologous chromosomes join together and can engage in a process known as crossing over or genetic recombination during meiosis, more specifically during prophase I. Genetic material is transferred between non-sister chromatids of homologous chromosomes during crossing over. Alleles may be rearranged and new gene combinations may be created as a result of this genetic material exchange between homologous chromosomes. Due to their lower likelihood of crossing over and genetic recombination, genes that are adjacent to one another on the same chromosome are more likely to be inherited together.

Hence, the correct answer is 1) Assertion and reason are both true, and reason is an accurate account of assertion.

Q2. Which type of linkage refers to genes that are located very close to each other on a chromosome?

1. Coupling linkage

2. Repulsion linkage

3. Complete linkage

4. Incomplete linkage

Correct Answer: 3) Complete linkage

Explanation:

Complete linkage refers to genes that are located very close to each other on the same chromosome. When genes are in complete linkage, they are inherited together as a single unit and are rarely separated by recombination during the crossing-over process that occurs during meiosis. This is because the physical distance between these genes on the chromosome is minimal, resulting in a low probability of genetic recombination events that would separate them. Thus, complete linkage is characterized by a strong association between the genes, and they are typically inherited as a linked group.

Incomplete linkage states that genes present on the same chromosome but get separated during crossing over in meiosis.

Coupling linkage means when genes from identical parents are inherited together.

Repulsion linkage occurs when a dominant allele from one gene is on the same homologous chromosome as a recessive allele from a linked gene.

Hence, the correct answer is 3) Complete linkage.

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• MCQs on Genetics

• Sex Determination in Drosophila

• Molecular Basis of Inheritance