Abnormalities caused by hereditary factors are of three types: autosomal inheritance, X-linked inheritance, and defective chromosomes. The below table shows examples of problems resulting from the three types of genetic abnormalities, their causes, their characteristics, and what is known about the development course of individuals with the abnormality.

Abnormality Cause/Early Detection Characteristics Developmental Course
Autosomal Inheritance Cystic fibrosis Recessive gene/prenatal testing Glands controlling production of mucus, sweat, tears, and saliva function incorrectly and make breathing difficult. Coughing, recurring pneumonia, large appetite, small size, and enlarged fingertips are symptoms. Increased risk of severe respiratory infection; possible future candidate for gene therapy.
Phenylketonuria (PKU) Recessive gene located on chromosome #12/ possible prenatal DNA analysis and routine newborn screening Inborn error of metabolism; newborn lacks ability to process phenylalanine found in products like milk. If left untreated, can cause mental retardation. Dietary restriction of phenylalanine beginning in infancy minimizes any effects.
Sickle-cell anemia Recessive gene/prenatal screening and blood test after birth Shortage of red blood cells causes pain, damage to vital organs, possible death in childhood or early adulthood. Chronic illness; often treated medically but there is no cure.
Tay-Sachs disease Recessive gene/prenatal screening Progressive nervous system disease that allows a toxic product to accumulate in the brain as a result of enzyme deficiency. Leads to brain damage and death. Normal development until 6 months of age. Neurological deterioration includes seizures, blindness, mental retardation, and death by age 5.
Achondroplasia Dominant gene/prenatal testing Disproportionately short stature, relatively large head, short limbs, trident hand, normal intelligence. Possible delay in developmental milestone attainment; occasional deafness.
Marfan’s syndrome Dominant gene/no prenatal testing at this time; usually diagnosed by physical exam Tall, thin with hypermobile joints; long, thin spiderlike fingers; spinal curvature, dislocated eye lens. Prone to lung collapse with high incidence of heart and blood vessel defects; associated with ADHD and learning disabilities.
Neurofibromatosis Dominant gene; linked to chromosome #17/no prenatal tests; diagnosis based on physical exam Multiple “cafe-au-lait” spots on body; small nerve tumors on body and skin; some affected persons may have large heads, scoliosis, or variety of bone defects. No known treatment. Wide variability in expression; may be associated with mild mental retardation or learning disabilities.
X-Linked Inheritance Color blindness X-linked recessive gene Red-green color blindness. No known cure.
Abnormality Cause/Early Detection Characteristics Developmental Course
Hemophilia X-linked recessive gene Blood lacks important clotting factor. Blood-clotting factor is needed to stop bleeding. Frequent hospitalizations and chronic problems. No known cure.
Duchenne muscular dystrophy X-linked recessive gene/prenatal testing Normal development until 6 to 9 years of age; then muscular weakness appears and progresses. Progressive disease affecting all muscles including heart and diaphragm; usually results in death during young adulthood.
Fragile X syndrome X-linked fragile site/ chromosomal testing available pre- and postnatally The most common hereditary form of mental retardation in males, with associated physical features of prominent jaw, large ears and testes. Children may have associated behavioral problems, hyperactivity, and some autisticlike features.
Defective Chromosomes Cri du chat syndrome Deletion on top portion of #5 chromosome Microcephaly, widely spaced eyes, small chins, and high-pitched cry (“cat cry”); severe retardation. No known cure.
Down syndrome Extra chromosome of 21st pair/ prenatal testing Cognitive deficits, hypotonia, facial characteristics, short stature, congenital heart disease. Developmental progress appears to slow.
Klinefelter’s syndrome Extra X chromosome (45XXY)/prenatal testing Male child with inadequate testosterone production resulting in abnormal sexual development. Usually tall, slender, with breast development and small genitalia. Close to normal intelligence. Psychological and psychiatric abnormalities; small percentage has mental retardation, language delays. Medical treatment involves administration of male hormones.
Turner’s syndrome Missing X chromosome (XO or 45X)/prenatal testing Only disorder associated with survival despite loss of chromosome. All are female,very short, and have webbed necks, widely spaced nipples, and nonfunctional ovaries. Usually of normal intelligence, with visual perceptual difficulties. Majority of girls have learning disabilities; medical treatment with hormones.
XYY syndrome Extra Y chromosome/ prenatal testing Usually tall with normal sexual development but low intelligence. Aggressive behavior; severe acne. Associated with behavioral problems and learning disabilities. No known cure.