ABSTRACT
Our model regarding the relationship between genotypes and phenotypes thus far
has been overly simplistic. We have assumed that each genotype translates into one
definite phenotype, and while this is a useful model for studying some genetic disor-
ders, it is not realistic for many other disorders. A good example of where this simple
model breaks down is with breast cancer. At least one gene has been found where
the presence of certain alleles seems to be associated with developing breast cancer.
The relationship here is complicated since not everyone who has this gene develops
breast cancer by a certain age, hence we speak of susceptibility genes rather than dis-
ease genes. Taking this perspective greatly extends the applicability of finding genes
associated with disorders.
