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.
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.
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