19 Chromosomes and Linkage Flashcards
(13 cards)
Why were Drosophila used in experiments?
Easy to rear
Rapid generation time
Genome sequenced before humans
What did Morgan discover?
1903 - it was suggested that ‘factors’ (genes) must lie on chromosomes
1909-1910 - first proof from the fly room of white eye mutants
What does crossing red-eyed and white-eyed flies show?
That the eye colour gene must lie on the x chromosome
- Showed that genes are on chromosomes
What experiment did Morgan do that was similar to Mendel’s?
Purple(p)/Red(R) eyes
Vestigial(v)/Normal(N) wings
Crossed pRvN x ppvv expecting equal numbers of all 4 possible outcomes
- Did not observe this
- Was a lot more of pRvN and ppvv
- Shows the two genes do not assort independently and they must be linked
- Recombination allows the other two combinations to be expressed
- The parental types are more common than the recombinants
Who made the first genetic map?
Sturtevant in 1911
How is the recombination fraction calculated?
Number of recombinants / total number
What is the recombination fraction?
A measure of the distance between two genes
Units = centiMorgans (cM)
Recombination fractions are additive
Why might additive map distances be greater than the observed recombination fraction?
Double recombinants cause underestimation of the distance as they are not detected
- Due to having the same combination of alleles as non recombinants
How is the proportion of double recombinants calculated?
Square the recombination fraction
How do you work out the proportion of recombinants detected?
recombination fraction - proportion of double recombinants
What can gene maps be used for?
Mapping disease causing genes
What recombination fraction would be expected if two genes were unlinked?
0.5 because of independent assortment
What is the evolutionary significance of recombination?
Recombination can produce new, fitter genotypes that selection can act on
Can also cause the break up of co-adaptive gene combinations
Can speed up rate of evolution in sexual organisms
