This question involves a single gene coding for plant seedling. The green allele (C) is dominant over the white allele (c). This means that the green allele will mask the phenotypic expression of the white allele in a heterozygous state (Cc).
In this experiment where plate 1 only contained white seedlings instead of all green, this illustrates that all the offsprings were recessive. This is because the parental genotypes were both recessive for the color trait i.e. cc.
Note that, the recessive trait can only be expressed when the recessive alleles are present in a gene. Therefore, the parental genotype would have been cc × cc, in order to give rise to all offsprings with the recessive trait (white colour).
