Need help in genetics! (linkage/mapping)?

I know the answers to these questions but I dont know how to get them, can someone please explain how?? Or if you know any website that will be helpful can you please post it? Thank you!





The genetics of Drosophila mostrificus, the only man-eating spcies of fruit fly known, has been investigated by Gregory Wendel of the UMBC Department of Entymology. Three linked genes have been found: rabid (r), mean (m), and gigantic (g).





A test cross was conducted using a heterozygous female. The progeny (1000 total) are linked below:





mg 280


m 90


g 150


wild type 30





r m g 10


r m 110


r g 70


r 260





1. The correct map order is:


a. Rabid -- Mean -- Gigantic


b. Mean -- Gigantic -- Rabid


c. Gigantic -- Rabid -- Mean





2. The distance between mean and gigantic is:


a. 20 map units


b. 27 map units


c. 30 map units


d. 42 map units


e. 50 map units

Need help in genetics! (linkage/mapping)?
Ok, to start this problem you need to look and find the 2 highest numbers and your 2 lowest numbers. Your highest numbers are ++r (+ denotes the wild type) and mg+. These are your parental genotypes. Your lowest numbers are +++ (wild type) and mgr so these are your double crossovers. Look and see which gene is different between the parental and the double crossovers and this is your middle gene. For example +++ has one difference from ++r and mg+ has one difference from mgr. In both cases, the difference is r, so your gene order has r in the middle. The outside genes can be switched back and forth, so mrg is the same as grm. The answer to your first question is C.


If this helps, email me and I will explain the other answers to you. Sometimes my explanations are a bit hard to understand; this is why I'm not going to go ahead and explain the other questions.