The following table summarises the results of the 2022-2023 Drosophila three-point cross involving the loci white eyes (w), miniature wings (m) and singed bristles (sn). The data is also available in the Excel file 'Drosophila Counts 2022-2023'. We strongly recommend working in Excel during this exercise.
Phenotype Count
+ + + 584
w m sn 324
w + + 227
+ m sn 150
+ m + 134
w + sn 196
+ + sn 134
w m + 92
The aim of today's tutorial is to use the above data to establish the genetic map for these three loci. We have been told that all three loci are on the X chromosome; however, as scientists, we shouldn't simply take someone else's word for it - we need to test whether our data does, in fact, indicate linkage.
As a first step, we will therefore conduct a chi-square test in order to test our data against the null hypothesis of independent assortment (i.e., "no linkage"). If the null hypothesis is rejected, we can assume that there is linkage between the three loci. If you are unsure how to do a chi-squared test, be sure to look at the talk titled "An introduction to Chi Squared", or look at any basic Statistics for Biologists textbook ("Practical Statistics for Field Biology" by Jim Fowler can be freely accessed online via UCL Library services) or simply look it up online.
For the chi-square test, we need to compare the number of flies counted in each
What is the expected number of flies for each class assuming that the loci are not linked? Give your answer to 3 decimal places, for example 1.234
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