State the chromosome theory of inheritance. Explain linkage and recombination using Morgan's work on Drosophila.

1. 1 · State the theory. Genes are located on chromosomes, which pair and segregate during meiosis just as Mendel's factors do (Sutton and Boveri).
2. 2 · Give the parallels.
   • Both occur in pairs.
   • Both segregate at gamete formation.
   • Different pairs assort independently.
3. 3 · Explain Morgan's choice and cross. Drosophila: short life cycle, many offspring. Dihybrid crosses of X-linked genes gave F2 ratios far from 9 : 3 : 3 : 1.
4. 4 · Define linkage and recombination with values. Body colour and eye colour: 1.3%, tightly linked. Eye colour and wing size: 37.2%, loosely linked. Crossing over in meiosis makes recombinants.
5. 5 · Show crossing over. [Diagram: a pair of homologous chromosomes, non-sister chromatids crossing, parental and recombinant chromatids]
6. 6 · Mention gene mapping. Sturtevant used recombination frequency as a measure of distance to map genes.
7. 7 · Link to real life. Breeders use gene maps to find markers close to useful genes in crops like rice.
8. 8 · Close in one line. So the closer two genes are, the stronger their linkage and the fewer the recombinants.

Scoring tip: Quote Morgan's two values, 1.3% and 37.2%. Numbers show the examiner you know the evidence.