(c).  Linkage will not result in variations among siblings. Morgan carries out several dihybrid crosses in Drosophila to study genes that were sex-linked.

Morgan came to know that the genes were located on the X-chromosome and also observed that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type. 

It indicates that due to the physical association of the two genes. There will be no variations among siblings. Independent assortment of genes means that allele pair separates during the formation of gametes independently. It means that traits are transmitted to offspring independently of one another.

Crossing over is the exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination. A mutation is the sudden inheritable change in the genetic material of an organism which transfers to next generation.