Blonde hair (b) is recessive and brown hair(B) is dominant parent has brown hair (Bb) their first child has blonde hair(bb) their second child has brown hair(BB) what must the other parent have (hair color and allele combination)
The other parent must have brown hair (BB or Bb).
The allele combination for the other parent can either be homozygous dominant (BB) or heterozygous (Bb) for brown hair. Both of these combinations will result in a child with brown hair (BB or Bb).