This could be possible if the wife is a carrier of the FMO3 allele. Therefore the two carrier parents must have passed the defective alleles to the child, who now possessed the gene for the FMO3, and said to be dominant for the fish odor.
In genetics a Carrier is an individual who inherited a defective allele (FMO3) , but do not show the manifestations of the allele, or symptoms of the diseases attributed to it. Therefore the wife is a Carrier for the allele if the child could show this symptoms of fish odor.
