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. All genes of a chromosome are not linked together. Genes that are far away from each other are likely to be separated during a process called homologous recombination. A chromosome is always a single piece of DNA. Genes are the segments of DNA which are arranged alongside a chromosome. A single chromosome can have hundreds or even thousands of genes. Most sexually reproducing organisms have two copies of each chromosome, which are named as homologous chromosomes. Humans have 23 chromosome pairs. Homologous chromosomes have the same genes which are arranged in the same order, but they do have different DNA sequences. Different versions of the same gene are named as alleles. Homologous chromosomes contain different alleles. Alleles are important because they are responsible for the differences in inherited characteristics from one individual to another. For example, different alleles of the same genes can make the eyes blue, green, or brown. The structure of the chromosome helps the DNA tightly wrapped around histones which are proteins that appear like spools. During the formation of gametes, chromosomes go through a process which is called homologous recombination. Recombination increases genetic diversity. The location of the chromosome breakpoints is random, and each gamete receives a copy of each of the recombinant chromosomes. All of this jumbling and mixing always allows for a nearly infinite number of allele combinations.
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.
Researchers tend to use linkage to find the location of a gene on a chromosome. By looking at how often different genes are inherited together, researchers can create maps of the distance between them. Since each gamete gets one of two possible versions of a chromosome, two unlinked genes will be inherited together 50% of the time. Unlinked genes may be on different chromosomes, or so, on the same chromosome that they are often separated by recombination. If two genes are inherited together more than 50% of the time, there is evidence that they are linked on the same chromosome. The closer the genes are, the more frequently they will be inherited together. When scientists end up discovering a new mutation, they go looking for linkage to other genes to determine the location of the mutation on a chromosome and help identify the mutated gene.
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 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.
Crossing over is the swapping of genetic material that occurs along the germ line. During the formation of egg and sperm cells, it is also known as meiosis. The paired chromosomes from each parent align so that similar DNA sequences from the paired chromosomes come and cross over one another. Crossing over results in a shuffling of genetic material and it is also an important cause of the genetic variation among offsprings.It is a biological occurrence that happens during the process of meiosis, when the paired chromosomes of the same type are lined up. In meiosis, they are lined up on the meiotic plates. Those paired chromosomes have some biological mechanism that sort of keeps them together. These things are called chiasmata, which is actually where strands of the duplicated homologous chromosomes break and recombine with the same strand of the other homolog. Two Chromosomes lined up, one strand of one Chromosome 1 will break and it will reanneal with a similar breakage on the other Chromosome 1. So then, the new chromosome that will happen will have part of the maternal Chromosome 1 and the paternal Chromosome 1, where maternal and paternal stands for where that person got their Chromosomes 1s from their one or their two. Therefore, the child that is formed out of one of those Chromosome 1s now has a piece of his or her grandmother's Chromosome 1 and a piece of his or her grandfather's Chromosome 1. It is this crossing over that lets recombination across generations of genetic material happen. It also allows us to use that information to find the location of genes. Organisms that divide only asexually, without the chance of a recombination suffer from a condition which is called Muller’s Ratchet. Each generation of that species contains as many genetic mutations as the previous generation, if not more. When the progeny are genetically identical to one another, there is no scope for genetic errors that can be corrected, or for new and beneficial combinations to arise again. Crossing over increases the variability of a specific population and it prevents the accumulation of deleterious combinations of alleles, while also allows some parental combinations to be passed on to the offspring. There is a balance between maintaining potentially useful allelic combinations as well as providing the opportunity for variation and change.
1. What is a Linkage Map?
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?
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:
Incomplete linkage or partial linkage.
3. What is the Role of Chiasma in Crossing Over?
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.