Imagine that you are working in a fruitfly ( Drosophila ) genetics laboratory. T
ID: 34426 • Letter: I
Question
Imagine that you are working in a fruitfly (Drosophila) genetics laboratory. Two new mutations have been recently discovered. Warty eyes (w) is recessive to normal eyes (W) at the warty locus. Hairy (h), the presence of extra sensory bristles, is recessive to normal (H). Assume that you have a pure breeding strain of each recessive phenotype (wwHH and WWhh) and a third strain that is pure breeding for both mutants (wwhh). Diagram the experimental design of crosses that you would use to test the null hypothesis that these two loci segregate independently; the alternative hypothesis is that they are linked. Design your crosses using the three strains described above. Make sure that you show exactly what experimental results you would use to test the null hypothesis of independent assortment. Please note: there is no recombination between loci on the same chromosome in male Drosophila because there are no crossovers (chiasmata) during meiosis in male Drosophila.
Explanation / Answer
Experiment- The experiment is to perform fruit fly crosses and determine their phenotype.
We have to determine if the observed phenotype of drosophila is close enough to the expected ratio for the loci segregating independently.
Taking the given strains into consideration in order to diagram the experimental design of crosses to test the null hypothesis we can consider three crosses with their expected phenotypic ratios as:
Crosses:
wwHH X WWhh - 9:3:3:1
wwHH X wwhh - 3:1
WWhh X wwhh - 3:1
Null hypothes- These two loci segregate independently
Alternative hypothesis-They are linked
To determine which hypothesis is selected we use chi square test.
X2 = E (O-E)2/O
Here, X2 = chi square
E= Summation of
O= observed value
E= Expected value
Now state clearly the chi square for phenotype and calculate.
Result- If the chi square comes less than critical value of 4 then null hypothesis is selected and if it comes more than 4 then alternative hypothesis is selected.