In rice, a trihybrid of unknown pedigree was testcrossed to determine the positions of three genes (s, short; l, loose; r, susceptible) of interest. Below is the data collected from the testcross:

 s l+ r + 9

s l r 357
s + l r 11
s l r + 74
s l + r 61
s + l + r 66
s + l r + 79
s + l + r + 343

i). Determine the relative order and the map
distance between the three genes
ii). Is there any crossover interference? What
is the evidence?

 Solution

i) Recombinant frequencies for s l

RF = 9+11+61+79 1000 For s r RF = 9+11+74+66	= 0.16 = 16%
= 0.16 = 16%

1000
For l r

RF = 74+61+66+79 1000

= 0.28 = 28%

S R
28
16 16
Mapping:
l

ii) Expected number of double recombinants = 0.16 x 0.16 = 0.0256
0.0256 x 1000 = 25.6
Observed number =(9+11) = 20
Interference = 1- 20/25.6 = 21.9%