Genetics 101

What is genetics?

Genetics is the study of how genes and traits are inherited, or passed down, in a family. Certain characteristics—including height, hair color, skin color, and eye color—are determined by genes.  Other aspects of our health can be affected by inherited genes too. 

Some people or families have an abnormal trait caused by a gene mutation that can be passed from parents to their children. An abnormal trait can be something that has a more minor effect like color blindness, or it can have a major effect on long-term health, such as your risk for certain types of cancer, heart disease, or the way some organs work.

To determine the likelihood of inheriting a genetic disorder, you can schedule a genetic consultation. A genetic counselor can assess the possibilities of inheriting a genetic or chromosomal condition that could affect an individual’s health.

How genes work

Generally every person has cells with a set of 46 chromosomes. These chromosomes consist of strands of genetic information called DNA. Genes are sections within DNA that act as instructions for how our bodies should build the proteins they need to grow and work.

Everyone has two copies of each gene, one from each parent. Almost all genes are the same between people, but less than one percent of genes are slightly different from one person to the next. Different versions of a gene are called alleles. Some alleles result in an abnormal version of the gene, which can either produce an abnormal protein or produce an abnormal amount of a normal protein.

Each pair of autosomal (non-sex) chromosomes contains a gene copy from each parent. If one gene is abnormal, but the other gene produces enough protein so that no disease develops, then the abnormal gene is considered recessive. The other gene is referred to as dominant because it can produce a enough protein so that the body functions normally.

Genetic disorders

There are many diseases in which genes play an important role. There are three major types of genetic diseases: single-gene defects, chromosomal disorders, and multifactorial disorders.

Single-gene disorders are caused by a problem in one specific gene. There are six types of single-gene disorders depending on how the genes are inherited:

  • Autosomal dominant
  • Autosomal recessive
  • X-linked dominant
  • X-linked recessive
  • Y-linked inheritance
  • Maternal inheritance

The appearance of a genetic disorder is called a phenotype. 

Chromosomal disorders result from a change in the number or structure of chromosomes. Down’s syndrome is one of the dozens of identified chromosomal disorders. It is caused by the presence of an extra copy of chromosome 21.

Multifactorial disorders are typically the result of genetic factors combined with lifestyle and environmental factors. Certain medical conditions such as heart disease, diabetes, and obesity have many contributing factors in addition to genetic causes. 

Last reviewed: 
March 2018
Alternative Names: 
Inherited disorders

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