Studies of Single Gene Disorders Have Sought to Draw Correlations

Essay

Studies of single gene disorders have sought to draw correlations between the genotypes at a disease locus and the phenotype of the single gene but the genotype-phenotype correlations are often poor. Even within families there may be significant variability in the phenotype of affected members (who are expected or known to have the same genotypes at the disease locus). List three factors that can explain why that should be so.