Difference Between Linkage And Crossing Over

To understand the phenomena of linkage and crossing over, one must understand the basics in genetics that is the genes and the chromosomes. A gene is a region in DNA that is responsible for encoding function. In other words, genes determine what an organism looks like, its features, appearance, and also its behavior in the environment. A chromosome is a DNA molecule that consists of a part or all the genetic material of the organism. 


The structure of the chromosome helps the DNA tightly wrapped around histones which are proteins that appear like spools.


Linkage 

When the DNA sequence undergoes the meiosis phase of sexual reproduction, the tendency of the DNA sequences that are close together on the chromosome that is to be inherited together takes place. In other words, linkage can be described as a close association of DNA sequences or other genes on the same chromosome. The closer genes are on the chromosomes, the possibility of them being inherited together increases. 


In 1905, the experiment to exhibit Linkage was carried out. Pea plants were crossbred and the pollen shape and color of the flowers appeared to be linked together. Later in 1911, Thomas Hunt Morgan studied heredity in fruit flies. It seemed to be that the color of the eye of the fly was closely associated with the sex of the fly. Hence, he concluded that the two traits were linked together. 


Similarly, two genes that are far away from each other on the chromosome have more tendency to get separated at the time of recombination. Recombination is a process that recombines the DNA during meiosis. Therefore, the strength of the genetic linkage depends upon how near the genes are on the chromosome.  


Crossing Over 

Sometimes we can notice genetic variations amongst offsprings. This happens because of crossing over. During the process of meiosis, when there will be the formation of egg and sperm cells, the paired chromosomes from each parent position themselves such that the similar DNA sequence from these paired chromosomes crosses over each other. This, in turn, results in the mixing up of genetic material and hence brings about genetic differences in the offsprings. 

This phenomenon is best explained with an example as stated below.

Supposedly, if we have two chromosomes A’s lined up, a single strand of one chromosome A will break. This will rearrange itself with a similar breakage on the other chromosome A. Then a new chromosome formed will have a part of maternal chromosome A and paternal chromosome A. These maternal and paternal means from where the individual got their chromosome A’s from (here we are talking about the original chromosome derived from). 


Subsequently, the offspring formed out of one of the chromosomes A’s also has a piece of their grandmothers and grandfathers chromosome A. This type of crossing over leads to the recombination of generations of genetic material. Hence we can locate the genes using this information.


Difference Between Linkage and Crossing Over 

Sl.No

Linkage

Crossing Over

1

This helps the genes in a chromosome to come together.

There will be a separation of genes in the chromosome and hence will lead to separation into different gametes.

2

The probability of linkage between two genes increases when the distance between them reduces.

The probability of crossing over between two genes reduces when the distance between them decreases. 

3

With linkage, one can expect the parental trait to get inherited and maintained in the offspring.

Crossing over leads to alterations and reduces the chances of parental traits being inherited in the offspring. 

4

With sexual reproduction, linkage reduces the chance of variabilities.

With sexual reproduction, cross over increases the chances of variabilities. 

5

Linkage decreases with age.

Crossing over increases with age.

FAQs (Frequently Asked Questions)

1. What is a Linkage Map?

Ans: A linkage map is also known as a genetic map. This is a table maintained for species or the population that is experimented which shows the position of the genes that are relative to each other in terms of frequency to recombine, rather than a specific distance along each chromosome. These maps were first developed by Alfred Sturtevant who was a student of Thomas Hunt Morgan. 


A linkage map notes the frequencies of recombination of genes during the crossover. The more the frequency of recombination between two genetic markers, the farther they are assumed in a chromosome. Similarly, with the reduced frequency of recombination between the markers, one can assume that the distance between them is less. 

2. Why is Genetic Linkage Important?

Ans: The analysis of genetic linkage is necessary to detect the location of disease genes on a chromosome. Based on this analysis one can tell that the genes which are physically close on a chromosome remained linked during meiosis.


Linkage is classified into:

  • Complete linkage

  • Incomplete linkage or partial linkage. 

3. What is the Role of Chiasma in Crossing Over?

Ans: When a pair of homologous chromosomes crossover, between them a structure called Chiasma is formed and this physically links the homologous chromosomes at the time of meiosis.