Lecture 5- Chromosomes and linkage Flashcards
Drosophila melanogaster - fruit fly. Why use it?
- At least 144 drosophila species
- easy to rear
- rapid generation time
- model organism for geneticists since early 1900s
- genome sequenced before humans
Morgan and chromosomes, 1903 & 1909 -10
1903- it was suggested that genes must lie on chromosomes
1909 - 10 first proof form the fly room - white eye mutants
In humans ___ pairs of chromosomes but _____ genes
23 pairs of chromosomes
~30,000 genes
In drosophila ___ pairs of chromosomes but _____ gene
4 pairs of chromosomes but ~ 14,000 genes
independent assortment in peas/ cross hetero with a homo recessive….
equal probability of getting all 4
If WwGg X wwgg doesn’t = 1/4 then
two genes do not assort independently - they must be linked (i.e. on the same chromosome)
Recombination (crossing over)
non-independent assortment of linked genes. Explains Morgans findings. The parental types are more common than the recombinants.
How does recombinant DNA occur?
1) enzyme nicks one strand of DNA at homologous sequence
2) recombination enzyme inserts the cut strand into second module, which is nicked in the process
3) Ligase anneals nicked ends in new combinations
34) Molecules resolve into recombinants
First genetic map
1911, Sturtevant in one night!
The recombination fraction is a..
measure of the distance between two genes. Units = centiMorgans (cM)
They’re additive
What was discovered between additive map distances and observed recombinant fraction?
additive map distances is much greater than observed recombination fraction.
additive map distances is much greater than observed recombination fraction.WHY???
DOUBLE RECOMBINANT.
Double recombinant:
if recombinant is 0.35. double recombinant is 0.35*0.35 = 0.12.
So 0.35 - 0.12 = 0.23 of recombinants are detected.
recombination can produce new
fitter genotypes
Recombination can also cause..
the break up of co-adaptive gene combinations and speed up rate of evolution in sexual organisms
