The pattern of phenotypic expression can be helpful in determining the mode of transmission for a given trait. In fact, geneticists often study the expression of particular traits in family lineages, or pedigrees, in order to gain insight into the mode of expression for a given character trait. Not only can pedigree analyses provide insight into the mode of transmission, but importantly, they can be used to predict the genotype of particular individuals.
This tutorial examined some human genetic diseases, including cystic fibrosis, PKU, and sickle-cell disease. These diseases are found in individuals that are homozygous for the recessive allele, but as you learned, the heterozygous state has a well-documented advantage in the case of the sickle-cell disease and a probable advantage in the case of cystic fibrosis. Moreover, you learned that the phenotype can be affected by the environment; in the case of PKU, a person that is homozygous for the allele can escape the disease with a proper diet.
Not all genetic diseases that behave in a Mendelian fashion behave recessively. Huntington's disease is a degenerative disease of the nervous system. Individuals with either homozygous or heterozygous genotypes develop this disease, ensuring that the defective allele is expressed in all generations as a dominant character trait. Individuals carrying this dominant allele do not begin to show symptoms until late in life (after their child-bearing years), and so, natural selection cannot act directly to affect the frequency of this allele in the population. In other words, through their reproductive years, individuals with this detrimental allele are just as fit (likely to reproduce) as normal individuals.