Linkage vs Crossing Over
Linkage and crossing over are two processes that are considered to be exceptions of Mendel’s law of independent assortment. Mendel’s law is mainly used to describe the inheritance patterns of chromosomes, but it does not really describe the inheritance of individual genes. Therefore, in order to examine linkage and crossing over, genes on chromosomes must be considered.
Tendency of certain genes on the same chromosome to be inherited together is called linkage. Linkage occurs only when two genes are located close to each other on the same chromosome. Such closely located genes, which do not assort independently, are referred to as linked genes. Unlike independently assorting genes, the linked genes are transmitted together to the same gamete more often. If the two genes are far apart on the same chromosome, then they tend to assort independently and equally pass to the same or different gametes.
The process of exchanging material between homologous chromosomes and resulting recombinant genes is called crossing over. The process that produces recombinant genes by crossing over is called ‘recombination’. It occurs only during the prophase of meiosis I of meiotic division. Crossing over can produce gametes with totally different gene combinations not found in either parent alone. The percentage of crossing over varies with the organisms. When two genes are located very close on the same chromosome, the frequency of crossing over is low. When they are apart, the percentage of crossing over is very high.
Crossing over occurs very rarely near centromere or toward the telomeres. Crossing over is important in chromosomal mapping and it proves that the genes are arranged linearly on a chromosome.
What is the difference between Linkage and Crossing Over?
• Linkage is the tendency of inheriting genes together on the same chromosome, whereas crossing over is the process of exchanging genes between homologous chromosomes.
• Linkage occurs when two genes are closer to each other on the same chromosome. In contrast, crossing over occurs when two genes are located far apart on the same chromosome.
• Crossing over can disrupt the gene groups made by linkage.
• Unlike the linkage, crossing over only occurs during the prophase of meiosis I.
• Unlike the linkage, crossing over produces recombinant alleles.