A red-haired woman marries a brown-haired man, and all the children are brown haired. Explain this genetically.

All children of a red-haired woman and a brown-haired man had brown hair.

Thus, brown hair gene is dominant over the red hair gene.

According to the law of dominance, in the presence of dominant allele, the recessive allele is not able to express itself, hence in heterozygous condition, the dominant allele will always express itself.