When you have a genetic disease, where one allele is dominant over the other, then there are three types of genotypes you can have - DD homozygous dominant (assuming the disease is recessive, this person would be normal); Dd - heterozygous (this person would be normal but will be able to pass on the disease trait to their offspring, and we call that a carrier) and dd - homozygous recessive, which would be the person that has the disease.
