The impact of genetic disease is often underestimated. The importance of genetic influence in common adult disorders such as heart disease, diabetes, and cancer is significant. The human genome contains approximately 3 billion base pairs and is estimated to contain 80,000 gene pairs. It is likely that every human being has 5-10 genes which do not function correctly, many passed from generation to generation. Single gene defects are caused by mutant genes.
Recessive disorders occur only when a pair of mutant genes is present at a homologous site.
Sex-linked disorders occur when the aforementioned genes are located on the sex chromosomes, either the X or the Y. Single gene disorders usually exhibit characteristic pedigree patterns, and the cause of the defect is a single major genetic error. Most single gene disorders occur with a frequency of 1 in 2000 or less. More commonly encountered single gene disorders include sickle cell disease, cystic fibrosis, phenylketonuria, thalassemia, Tay Sachs disease, and many others. Extremely rare and very unique recessive disorders may appear in children whose parents are genetically related. Chromosomal disorders are caused by a deficiency or excess of an entire chromosome or a chromosomal segment which upsets the normal genomic balance.