The document discusses genetic linkage and mapping in Drosophila melanogaster. It describes how Thomas Morgan showed that two genes, white eye (w) and miniature wing (m), are linked on the X chromosome through test crosses. Recombination between linked genes occurs through crossing over during meiosis. Genetic mapping uses recombination frequencies between genes to determine their relative positions and distances on chromosomes in map units (centimorgans). A three-point test cross can detect double crossovers and more accurately determine map distances between closely linked genes.

4. Crossing-over

15. Calculation of chi-square χ 2 = 1,489.99 3,236 3,236 Total 398.79 322,624 -568 809 241 Black-vestigial 458.44 370,881 609 809 1,418 Back-normal 290.76 235,225 485 809 1,294 Grey-vestigial 342.00 276,676 -526 809 283 Grey-normal D 2 /E D 2 Deviation (D) Expected number (E) Observed number (O) Phenotypes

27. 5 v + cv ct + 8 3 v cv + ct 7 94 v + cv + ct + 6 89 v cv ct 5 40 v + cv + ct 4 45 v cv ct + 3 592 v + cv ct 2 580 v cv + ct + 1 Number observed Genotype Class

30. Recombinants (double crossovers) 5 v + ct + cv 8 Recombinants (double crossovers) 3 v ct cv + 7 Recombinants (single crossovers in region v – ct) 94 v + ct + cv + 6 Recombinants (single crossovers in region v – ct) 89 v ct cv 5 Recombinants (single crossovers in region ct – cv) 40 v + ct cv + 4 Recombinants (single crossovers in region ct – cv) 45 v ct + cv 3 Parentals 592 v + ct cv 2 Parentals 580 v ct + cv + 1 Type Number observed Genotypes Class

32. Genetic linkage map of the v ct cv region of the chromosome