The only oral anticoagulant available was
warfarin
Well, you failed to mention which one is the recessive trait in this instance. But, let’s say red hair is recessive, denoted by r and brown hair is dominant denoted by R. Mom is rr and dad is RR. All children are going to be Rr and so heterozygous. They will all carry the trait for red hair but will express brown hair themselves, so 0% probability of having a child with red hair from the initial parents
An example of a parallel choice on a dichotomous key is happy or sad and old or young. Either one of those answers.
