If the offspring of a cross between two autosomal traits show a phenotypic ratio of 0 dom/dom : 0 dom/rec : 1 rec/dom : 1 rec/rec, what is the genotype of the parents?

The parents are of genotype heterozygous dominant and homozygous recessive. Supposing the dominant allele is N and the recessive allele is n, one of the parents will be Nn while the other nn. The phenotypic (based on visible characteristics) ratio will be 1:1 for dom/rec and rec/rec as indicated in the diagram below.