(1) Morgan used Drosophila melanogaster for his experiments.
(2) He carried out several dihybrid cross experiments to study sex-linked genes of Drosophila.
(3) Crosses between yellow-bodied, whiteeyed female and brown-bodied, red-eyed males were done in P1 generation. Brownbodied and red-eyed forms were wild.
(4) Morgan intercrossed their F1 progeny and noted that two genes did not segregate independently of each other and F2 ratio deviated very significantly from Mendelian 9 : 3 : 3 : 1 ratio.
(5) When genes are grouped on the same chromosome, some genes are strongly linked. They show very few recombinations (1.3%).
(6) When genes are loosely linked, i.e. located away from each other on chromosome, they show more (higher) recombinations (37.2%).
(7) For example, the genes for yellow body and white eye were strongly linked and showed only 1.3 per cent recombination (in cross-I).
(8) White-bodied and miniature wings showed 37.2 per cent recombination (in cross-II). Cross I shows crossing over between genes y and w.
(9) Cross II shows crossing over between genes white (w) and miniature wing (m). Here dominant wild type alleles are represented with (+) sign.
(10) Parental combinations occur more due to linkage and new combinations less due to crossing over.
