Hint: Genetic Linkage is the tendency of the alleles of a gene which are closely located together on a chromosome to get inherited together during the process of meiosis in sexually reproducing organisms.
Linkage is a divergence from the Mendelian theory of self-assortment. However, during meiosis the homologous chromosomes pair up and exchange corresponding sections of DNA which leads to change in location of gene and genes that were on the same chromosome can finish up on different chromosomes. This process is known as genetic recombination. The recombination rate depends on the physical distance between the two loci. If alleles are closer they have lower rates of recombination than those that are located far apart. The distance between two alleles on a chromosome can be determined by calculating the percentage or recombination between two loci.
Linkage can be complete or incomplete. In complete linkage the two loci are so close to each other that they are virtually never separated when crossing over occurs.the closer the two loci are the less likely they are to separate from each other.
In male Drosophila complete absence of recombinant types is observed which can be due to absence of crossing over. This can also be understood as the genes which were originally present on a chromosome are still present on the same chromosome even after recombination and crossing over events. And this leads to production of only parental phenotypes.
For example,the male Drosophila and female silkworm also called Bombyx mori shows 100% parental genotype which shows that the gene for grey body colour is completely linked with long wings. In this dihybrid cross, F2 phenotypic ratio is 3:1 and test cross ratio is 1:1 just like the monohybrid cross.
Note: Linkage is useful in finding out the patterns of inheritance of genetic disorders and other psychological disorders like dementia, anxiety etc. Linkage also decreases the likelihood of gametes variability in absence of crossing over.