Q.What is linkage? Explain, with reference to Morgan's work on Drosophila, how the presence of linked genes on the same chromosome causes deviation from Mendel's Law of Independent Assortment.

Linkage is the tendency of genes located close together on the same chromosome to be inherited together rather than assorting independently, since they physically travel on the same DNA molecule; discovered in plants by Bateson and Punnett and in animals by T. H. Morgan. Complete linkage occurs when genes are so closely spaced that crossing over never separates them (e.g., genes on the X chromosome of male $Drosophila$), so parental trait combinations always pass intact to offspring. Incomplete linkage occurs when genes lie farther apart on the chromosome, giving crossing over a real opportunity to separate them and generate new (recombinant) trait combinations (e.g., grain colour and shape in maize). All linked genes on one chromosome form a linkage group, and the number of linkage groups in a species equals its haploid chromosome number (4 in $Drosophila$, 7 in garden pea, 23 in humans).

Crossing over is the physical exchange of corresponding DNA segments between non-sister chromatids of homologous chromosomes, occurring during the pachytene stage of prophase I of meiosis, through the sequential steps of synapsis, tetrad formation, crossing over and terminalisation; it is essentially universal in sexually reproducing organisms (with the notable exception of male $Drosophila$) and is evolutionarily important because it generates fresh allele combinations. Morgan demonstrated all of this experimentally in $Drosophila$: when tracking genes for yellow body and white eye (closely linked), only about 1.3% of offspring were recombinants, whereas tracking the more distant white-eye and miniature-wing genes gave about 37.2% recombinants — showing directly that recombination frequency increases with the physical distance separating two genes on a chromosome. A special case, sex-linkage, applies this same logic to genes on the X and Y chromosomes: complete sex linkage for genes on non-homologous regions (no crossing over possible), and incomplete sex linkage for genes on the small homologous region shared by X and Y (crossing over does occur there).