data <- scan(what = list(r50="",r21="",aminoacid=0), multi.line=F)
          no    no               1.697
         yes    no               2.032
          no   yes               2.211
         yes   yes               2.091
          no    no               1.601
         yes    no               2.017
          no   yes               1.673
         yes   yes               2.255
          no    no               1.830
         yes    no               2.409
          no   yes               1.973
         yes   yes               2.987

data


oex8_6 <- data.frame(data)
rm(data)

oex8_6$r50 <- as.factor(oex8_6$r50)
oex8_6$r21 <- as.factor(oex8_6$r21)

oex8_6.lm <- lm(aminoacid ~ r50 +r21 +r50:r21, data=oex8_6)

anova(oex8_6.lm)

 # examine the interaction

interaction.plot(oex8_6$r50,oex8_6$r21,
                 oex8_6$aminoacid,type="b")

 # check model assumptions

 # this code displays all 4 plots on a page
x11()
par(mfrow=c(2,2))  
plot(oex8_6.lm)
par(mfrow=c(1,1))  

 # this code displays just the top 2 plots, on a single page
x11()
par(mfrow=c(1,2))  
plot(oex8_6.lm, which =c(1,2))
par(mfrow=c(1,1))  

 # this code displays just the top 2 plots, on separate pages
x11()
plot(oex8_6.lm, which =c(1))
x11()
plot(oex8_6.lm, which =c(2))