Genetic disorders can either be expressed as recessive or dominant. For the dominant genetic disorders to be observable, it is enough if only one of the parents pass on the abnormal gene to their offspring therefore there are two possible genotypes.
On the other hand if the disorder is recessive, the person with the disorder must have taken the abnormal gene from both parents and therefore their genotypes are known which makes it easier for the scientists' studies.
The genotype is the part of the genetic makeup of a cell, and therefore of any individual, which determines one of its characteristics (phenotype).
One's genotype differs subtly from one's genomic sequence, because it refers to how an individual differs or is specialized within a group of individuals or a species. So, typically, one refers to an individual's genotype with regard to a particular gene of interest and the combination of alleles the individual carries (see homozygous, heterozygous).
