1. Gene Linkage & Gene Maps

2. Thomas Hunt Morgan – studied Drosophila melanogaster (fruit fly)
- easy to maintain and breed; short life span
- determined it has 4 pairs of chromosomes

3. Gene linkage – when 2+ genes are on the same chromosome
- so therefore, fruit fly has 4 linkage groups
Linkage groups assort together b/c on the same chromosome

4. Example of Morgan’s Studies
Gray body, long wings in flies
P gen  GGLL x ggll  all gray, long in F1 (GgLl)
F1 crossed to get F2
– saw 3:1 ratio in F2 (3 gray,long : 1 black, short)
Conclusion – alleles for body color and wing length are on same chromosome

5. Further tests – saw some gray,short and black,long -- could not happen if G and L always go together
Morgan stated: alleles from 2 homologous chromosomes had exchanged places = crossing over

7. Chromosome Map
Alfred Sturtevant, student, determined gene locations on chromosome
Location depends upon % of crossing over
Genes that crossover 1% of the time are 1 map unit away

8. In which gene map is the probability of crossing-over between A and D greatest?
In which gene map is the probability of crossing-over between A and D the least?
In which map are genes C and D most closely linked?
In map D, which genes are least likely to cross over?