Abnormalities range from single-gene disorders to chromosomal disorders. According to studies conducted in Newcastle, Baltimore, and Montreal, six to eight percent of genetic diseases among children were attributed to single-gene defects while .2 to .4 percent was attributed to chromosomal disorders. Some genetic disorders are common and harmless while some disorders are serious.

Genetic disorders are classified into three different categories: chromosomal disorders, Mendelian or monogenic disorders, and complex diseases traits.

Chromosomal Disorder
The abnormal arrangement or lack or excess of one or more chromosomes is called chromosomal disorder. In this disorder, the DNA undergoes largest changes and also includes an entire extra genome copy, an extra chromosome, duplications, or deletions of chromosomal portions, and other defects. A chromosomal abnormality causes mental retardation, behavioral disorders, growth disturbances, tumors, malformations, anatomic development, and spontaneous abortions.

Mendelian or Monogenic Disorders
Mendelian and monogenic disorders are primarily determined by a single mutant gene. These disorders exhibit the inheritance pattern and are further classified into autosomal dominant, autosomal recessive, or X-linked. Monogenic disorders occur in about 10 per 1000 live births with 7 in 1000 dominant, 2.5 in 1000 recessive, and 0.4 in 1000 X-linked.

Some of the common autosomal dominant disorders include: familial hypercholesterolemia, Marfan's syndrome, myotonic dystrophy, and tuberous sclerosis to name a few. Some of the autosomal recessive disorders include: deafness, sickle cell anemia, albinism, cystic fibrosis, and phenylketonuria. X-linked disorders include: hemophilia A, ocular albinism, Fabry's disease, color blindness, and testicular feminization.

Complex Disease Traits
Multiple gene interaction with multiple environments or exogenous factors causes complex disease traits or multifactorial genetic diseases. Although, these complex traits, such as gout, diabetes mellitus, and inheritance pattern are complex, the risk of comparatively less than in monogenic disorders. Common chronic diseases in adults such as peptic ulcer, hypertension, schizophrenia, and coronary heart disease and birth defects such as congenital heart disease and spina bifida fall under complex disorder traits.

Drugs, Environment, and Family Factors
Genetic disorders caused due to abnormal response to drugs such as malignant hyperthermia has affected 50 percent of the U.S. population. Addition to drugs, various environmental factors can further aggravate certain genetic disorders. Example, ultraviolet ray exposure affects patients with xeroderma pigmentosum while aspartame utilization causes special risks in persons with phenylketonuria.
Many diseases are known to run in the family where even if one parent suffers from a dominant gene disorder, then it is highly possible that it will pass on to the child. If both parents carry recessive genes, then the child may or may not be affected or may act as a carrier. When a mother carries X-linked genes, then very likely her son may get affected and when the father carries an X-linked gene, all daughters will acts as carriers. The common disorders include diabetes, heart diseases, extra finger or toes, and shorter arms and legs.

Genetic Disorders among Ethnic Groups
Monogenic and complex diseases traits depend on various factors such as family history, and ethnic and geographic origin of a family. Some examples of genetic disorders that occur in high-frequency in specific ethnic groups include: diabetes mellitus among native North American populations, congenital neprhosis among Finns, familial Mediterranean fever among Armenians, adult lactose deficiency among the Chinese, cystic fibrosis among Europeans, and familial hypercholesterolemia among South African whites and French Canadians.

Diagnosis and Prevention
Most monogenic disorders often go undiagnosed contributing to the high occurrences of genetic disorders. With the appropriate information on the disorder given, the physician establishes correct diagnosis using biochemical or molecular testing. Issues such as new mutation, expression variation, phenotype/genotype correlations, and lack of penetration are given more importance. Family members should be counseled on prenatal diagnosis, carrier testing, and reproductive counseling thereby to understand and prevent common genetic disorders.

The last decade has seen various advancements in human genetics. Still, the roles of genes are complex and central in terms health and disease. Treating a genetic disorder is both difficult and most often not completely effective. Though exciting breakthroughs have been achieved, the issue of genetic disorders continues to pose a serious challenge to the scientific community.

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