Linkage, Recombination and Crossing Over

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Linkage, Recombination and Crossing Over B. Sc. Zoology/Chemistry (Subsidiary Botany): Degree- I: CMSC, LNMU Dr. K. Anwer, CMSC, LNMU LINKAGE AND RECOMBINATION Linkage and recombination are phenomena that describe the inheritance of genes. A linkage is a phenomenon where two or more linked genes are always inherited together in the same combination for more than two generations. The recombination frequency of the test cross progeny is always lower than 50%. Therefore, if any two genes are completely linked, their recombination frequency is almost 0%. The phenomenon of linkage was studied by the scientist T.H. Morgan using the common fruit fly or Drosophila melanogaster. LINKAGE AND CROSSING OVER Linkage is the tendency of genes present in the chromosome to stay intact and transfer to the next generation whereas crossing over is the exchange of chromosomal sections to disrupt built links and form new linkages. Linkages generate parental types and increases in age as the linkage increases. Crossing over generates recombination and it decreases with age and the occurrence of crossing over between two genes is decreased if they are placed closely, whereas the linkage strength between two genes increases if a chromosome is located in the vicinity. Linkages assist in maintaining a newly improved variety, crossing over, on the other hand, forms as a source of variations to produce new varieties. Crossing over, in other form, is the exchange of segments observed in homologous chromosomes between non-sister chromatids and takes place during the late pachytene stage of the prophase I in the cell division process of meiosis and always takes place within linked genes. The recombination of linked genes that crossing over produces plays a significant role in evolution. LINKAGE AND RECOMBINATION Morgan noted that while crossing a set of characters, two genes did not follow Mendel’s law as they did not divide as per the law. The likelihood of achieving a parental combination if two genes are situated on the same chromosome was relatively higher in the subsequent generation in comparison to the non-parental combination. The physical connection of genes was referred to as linkage. Genes are said to be linked when genes for different traits are located in similar chromosomes and hence are tied to each other. It is a deviation from the Mendelian principle of independent assortment that is appropriate to be applied to the genes that are situated on different chromosomes. The term genetic recombination described the non- parental gene combinations in a dihybrid cross. *Figure showing Thomas Hunt Morgan’s Drosophila melanogaster genetic linkage map. This was the first successful gene mapping work and provides important evidence for the chromosomal theory of inheritance. The map shows the relative positions of alleles on the second Drosophila chromosome. The distances between the genes (centimorgan, cM) are equal to the percentages of chromosomal crossover events that occur between different alleles. .

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