left-hand phenotype is represented by clear boxes/circles.
This is clearly seen in Generation 2 [Units 5 and 6]
If left handedness was shaded, then that would mean that all of the offspring in generation 3 would have to be shaded also.
As, ll x ll = 4 ll's - because it's recessive as said in the question.
Therefore left handedness musn't be shaded and thus must be clear, as the offspring are a mixture of shaded and non-shaded.. from the ratio most probably both parents are heterozygous Ll.