Two white sheep are bred to one another. Over the span of several years they produce three white offspring and a black one. What is a possible explanation for this?

Both parents are heterozygous

Explanation:

Assuming the allele for colour in the sheep is represented by S.

A white sheep will have a genotype of SS or Ss assuming S is dominant over s and a black sheep will have a genotype of ss.

Over the couse of several years, a cross betweem two white sheep (SS or Ss?) produced three white and one black offspring. For them to have produced one black offspring (ss), it thus means that the two white sheep are heterozygous:

Ss x Ss = SS (white), Ss (white), Ss (white) and ss (black).

The best explanation is that both parental sheep are heterozygous for the trait.