Conclude from ratios of progeny in a dihybrid cross whether two genes are linked.

Linked genes may be defined as the genes that are present on the same chromosomes and transfer to the next generation as well. The linked gene are responsible for the formation of recombination in the progeny. The genes that follow the independent assortment are present on different chromosome.

If the ratio of progeny is 9:3:3:1, then the genes are not linked. The deviation from this ration explains that the genes are linked. The parents number are more in than recombinant in case of linkage. The recombination frequency can never exceeds 50%.  

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that is wrong, its not thymus.