Genetic

Hard-wired health concerns

A genetic disorder is an adverse health condition resulting from one or more abnormalities in an individual’s DNA. At the molecular level, an abnormality can be as profound as the duplication or omission of an entire chromosome, or as subtle as a single-base mutation in only one gene. There are more than 6,000 known genetic disorders, generally classified by their genomic origin and degree of complexity.

Chromosomal abnormalities can be inherited from one or both parents, or can occur without a family history of the disorder. Subtypes include:

• Aneuploidy – Fewer or more chromosomes than normal; includes Down syndrome (one extra chromosome #21), Turner syndrome (only one X chromosome in females)
• Deletion – Part of a chromosome is missing
• Inversion – Results from chromosome breakage and reattachment in the reverse orientation during cellular meiosis
• Duplication – A portion of the chromosome is duplicated, adding genetic material
• Translocation – Displacement of a chromosome segment from one location to another on the same or a different chromosome
• Mosaicism – Two cell lines exist within an individual that contain different chromosome patterns, due to one cell line having undergone any of the above genetic events

Single-gene disorders are caused by mutations in one gene. These are classified into subtypes according to chromosomal location and the dominant or recessive nature of the affected gene:

• Autosomal disorders stem from mutations within autosomes (not X or Y sex chromosomes); in dominant mutations, only one allele is needed for the disorder to manifest; recessive mutations require both alleles to be present, although just one can make an individual a carrier of the disorder
• Sex-linked disorders result from mutations within the X or Y chromosome and can also be either dominant or recessive; X-linked disorders and far more common than Y-linked

Examples of single-gene disorders include:

• Cystic fibrosis (autosomal recessive)
• Albinism (autosomal recessive)
• Hemophilia (sex-linked recessive)
• Sickle cell anemia (autosomal recessive)

Multifactorial genetic disorders, also known as polygenic disorders, are a result of abnormalities in multiple genes, combined in many cases with environmental factors during early development. Examples include:

• Alzheimer’s disease
• Arthritis
• Cancer
• Diabetes
• Heart disease
• High blood pressure
• Obesity

Several methods of laboratory genetic testing are currently used, with the type of test depending on the type of disorder or abnormality being assessed. The most commonly used methods include: cytogenetic (to detect chromosomal structure anomalies); molecular (DNA sequencing); and biochemical (analysis of protein function).