The genetic material in most living organisms is in the form of DNA. It is vast in size with numerous nucleotides arranged in long chains. In order to fit into tiny microscopic cells, the genetic material is tightly interwound. It is circular DNA in prokaryotes while it is long-armed chromosomes in eukaryotes. The small segments of DNA coding specific characteristics are genes. Genes are hereditary units and carry information from parents to offspring. Genes lie in a specified position on chromosome arms called gene locus. Linkage and recombination explain gene inheritance in living organisms.
In 1910, very little knowledge about chromosomes, genes, and inheritance was known. By that time, it was a known fact that inheritance is through certain factors called genes as discovered by Mendel. Also, some research has shown the presence of chromosomes in the cells. However, the link that genes are borne on chromosomes was provided by the experiments of Thomas Hunt Morgan on Drosophila melanogaster, commonly called “fruit flies”.
Morgan conducted breeding experiments on fruit flies. The wild-type flies had red eyes while some mutants under Morgan's observation had white eyes. The first white-eyed fruit fly under Morgan's observation was a male fly and he crossed it with a red-eyed female to understand the inheritance of the whiteeyed character. The resulting F1 progeny were all red-eyed flies. These results indicated that the white-eyed trait is recessive.
He further repeated crosses between F1 flies and backcrosses also. Morgan found surprising results. Some progeny consisted of few red-eyed and few whiteeyed flies. The white-eyed character was found only in male flies. In Drosophila species, male flies have XY sex chromosomes while females have XX sex chromosomes.
With the obtained results, it was evident that the inheritance of the white-eyed character was related to the inheritance of the Y chromosome and the X chromosome has nothing to do with that.
Morgan further concluded that genes responsible for the inheritance of whiteeyed characteristics are on the Y chromosomes and since male flies have XY sex chromosomes, they have a chance of inheriting and expressing white-eyed mutation.
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Suppose that two genetic sequences coding for different characteristics are present closely on the same chromosome and the chances of them undergoing recombination or crossing over are less. They have more chances of being inherited together over generations.
In such cases, the two genes are inherited together and are called linked genes and the phenomenon is called linkage.
Linkage was first proposed by Bateson and Punnet during their experiments on the Sweet pea plant when they observed certain characteristics inherited together. However, their observation was explained by stating the fact that Sweet pea has only a limited number of chromosomes and each bears many genetic sequences.
The evidence for linkage was provided by the breeding experiments of T.H. Morgan.
The physical factor behind linkage is the distance between the genes.
Two genes closely lying on the chromosome have fewer chances to undergo cross-over or recombination during the meiosis of cell division. And hence they are inherited together.
While those genes separated by a distance on the chromosome have more chances of undergoing recombination or crossing over, pose fewer chances of being inherited together.
The physical distance between genes is defined by centimorgan (cM).
Inheritance of linked genes does not follow the Mendel law of independent assortment. Mendel's law state that alleles of different genes inherit independently of one another.
Complete linkage is when genes are inherited together and the inheritance of one gene guarantees the inheritance of another gene. The distance between them is far less so that the chance of them undergoing recombination in meiosis is very less.
Incomplete linkage has few chances of two genes being inherited together. There are chances of them being separated by the cross-over in meiosis during gamete formation.
Eukaryotic organisms reproduce sexually by producing gametes through meiosis. Female gametes are eggs while male gametes are sperms.
In the first phase of meiotic cell division, chromatid arms of homologous chromosomes overlap and temporarily fuse forming crossovers.
Homologous chromosomes in an organism are two chromosomes having the same length, the centromere position and genes corresponding to the same loci form pairs. Each homologous chromosome pair has one chromosome inherited from the mother and the other from the father.
Therefore, crossover results in an exchange of genetic material between two chromosomes leading to recombination.
Homologous recombination − It is a natural type occurring between homologous chromosomes during meiotic cell division.
Non-homologous recombination − Occurs between non-homologous (dissimilar) chromosomes.
Site-specific recombination − It is a genetic exchange at specific short nucleotide sequences. It requires the action of site-specific enzymes.
Replicative recombination − It is a genetic exchange resulting in a copy of a new DNA segment. It is mostly observed in transposable elements.
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Inheritance of linked genes has the least chance of crossing over. This preserves the parental gene combinations without allowing new combinations in a population.
Linkage poses difficulty for breeders to breed all desired characteristics in a single plant.
Recombination leads to the generation of variations that lead to the formation of new characteristics which over generations eventually end up in the formation of new species.
Linkage and recombination describe the inheritance in living organisms. Linkage is the inheritance of two separate genes together. It is the distance between genes that make them either completely linked or incompletely linked. Morgan's experiments on Drosophila provided a clear idea about linkage in genes. Diploid organisms form haploid gametes through meiotic cell division. The homologous chromosomes undergo crossing over during the meiosis. This allows the exchange of genetic material or recombination between the maternal and paternal chromosomes, thereby leading to variations. Variations are responsible for the diversity in living organisms.
Q1. Why did Morgan choose Drosophila for his experiments?
Ans. Drosophila has a short life cycle and is easy to grow in the lab. They show high levels of sexual dimorphism and produce a large number of offsprings in a single reproductive cycle. All these reasons made Drosophila a chosen organism for Morgan's studies.
Q2. What is meant by gene mapping?
Ans. Gene mapping is the process of finding the locus of all genes and the distance between them. Prior to the development of advanced molecular biology tools linkage analysis was the method used to map genes.
Q3. What is recombination frequency?
Ans. Recombination frequency measures genetic linkage between two genes lying on a chromosome. It is used to construct linkage maps.
Q4. What conclusion does recombination frequency draw?
Ans. Recombination frequency directly reflects the physical distance between the genes being studied. The higher the recombination frequency the farther the two genes while the lower it is the closer the two genes are.
Q5. Who proposed the Chromosomal theory?
Ans. Chromosomal theory of inheritance was proposed by Boveri and Sutton stating that genes are carried on chromosomes.
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