Hi there!
With mendelian genetics and inheritance, it assumes that there are two alleles (a variant of a gene) for every trait, one from each parent. These two alleles can be dominant or recessive. This would result in different exhibitions of traits - as long as there is only one dominant allele, then the dominant trait is exhibited, even if there is the recessive allele. However, if there are both recessive alleles, then it is the recessive trait which is exhibited.
When a person is a carrier of a trait, in this case a genetic disease, it means that they carry the allele for the disease, but don't exhibit it. This would mean that the allele would be recessive, however they would also have a dominant allele which "overrides" this disease.
Hope this helps!
