A frog that is dominant for its light green color mates with a brown frog and produces one small brown frog. How is this possible if the green color is dominate?

The dominant (light green) parent was heterozygote for the trait

Explanation:

According to Gregor Mendel in his law of dominance, an allele is said to be DOMINANT if it masks the phenotypic expression of another allele in a gene. The allele being masked is called RECESSIVE allele. In this case of a frog whose allele for light green color is dominant over the allele for brown color, the light green color allele (G) is dominant while the brown color allele (g) is recessive.

However, in a cross between that have light green frog and a brown frog, a small brown frog is produced. This is possible despite the green color being dominant because the genotype of the light green dominant parent is HETEROZYGOUS i.e. it contains both light green (dominant) allele and brown (recessive) allele.

Hence, when a gamete with recessive allele (g) is produced by the heterozygous light green frog (Gg), it mates with a recessive allele from the brown frog (gg) to produce a brown offspring (gg).