A disease that is inherited from one or the other parent, sometimes both, through germ line DNA. Huntington's disease is a dominant single gene ( the gene is defective ) disease that appears in people that are over 40, generally. Sickle cell trait is a recessive gene that has one amino acid difference on the B hemoglobin that causes red blood cells to sickle fully when homozygous and confers some protection against malaria when heterozygous.