Mendel crossed y/y; r/r (yellow wrinkled) peas with y/y; r/r (green smooth) peas and selfed the f1 to obtain an f2. in the f2 what proportion of the yellow wrinkled individuals were pure-breeding? the first number is the number that are pure breeding.

The answer is 3/36. The proportion of yellow, wrinkled peas are pure-preeding. This means that they are homozygous for this trait. There are 9 yellow, wrinkled peas. What proportion of these are R/R; Y/Y. I'm not writing out all 16 genotypes for the cross, but you'll see that only one individual is R/R; Y/Y. So the proportion of yellow, wrinkled seeds that are pure-breeding is 1/9. If you do a punnet square for the F1 cross, you'll observe the 9:3:3:1 ratio, described above, in F2.