Arabidopsis thaliana is a green‑leafed plant, but a variegated type has green‑and‑yellow leaves. STATE: To investigate the mechanism of plant variegation, researchers created a recessive mutation that suppresses variegation in the DNA of the variegated Arabidopsis. Are the data consistent with a single recessive mutation with 3:13:1 ratio of variegated to green plants? When the green mutants were crossed with the variegated types, the first‑generation plants were all variegated, thus confirming the recessive aspect of the mutation. The second generation consisted of 84 green plants and 216 variegated plants.

Required:
Find the test degrees of freedom (as a whole number), and the test P-value by chi‑square goodness-of-fit test.