Role of Genetics in Rare Diseases

Reviewed by: HU Medical Review Board

Roughly 7,000 rare diseases have been identified so far. Of those, 8 out of 10 have a genetic cause. Understanding the root causes of disease can help us better understand what leads to certain health conditions. It can also help us learn how to prevent and treat rare conditions.1

Genetics 101

What does it mean when a disease is caused by a genetic change? Humans inherit their chromosomes from their parents. Chromosomes contain genes and genes contain DNA (deoxyribonucleic acid). Each baby is supposed to get 46 chromosomes, 23 from each parent. Think of chromosomes as a set of instructions that tell the body how to work.2,3

Sometimes these instructions are wrong. A person may be missing a chromosome or have an extra one. Some of the genes or pieces of DNA on the chromosome may be missing or arranged incorrectly. These changes are called mutations. A person can have many genetic mutations or just one. Sometimes genetic mutations cause health problems, but not always.2,3

Some examples of diseases caused by genetic mutations:3-5

  • Sickle cell disease is caused by a mutation in a gene on chromosome 11.
  • Fragile X syndrome is caused by a change in a gene on the X chromosome.
  • Turner syndrome occurs when a person has only 1 X chromosome instead of 2.
  • Williams syndrome occurs when someone is missing a small part of chromosome 7.
  • Cystic fibrosis occurs when someone inherits 2 copies of the CFTR gene, which is found on chromosome 7 (7q31.2).

Some mutations are inherited and some appear for the first time for reasons doctors may not understand.

Genetic testing and counseling

The entire set of someone's DNA is called their genome. The genome contains about 3 billion pairs of DNA. The Human Genome Project had "read" (sequenced) almost the whole human genome by 2003. We had to wait until 2022 for technology to advance enough for scientists to read one entire human genome.6

Having more human genomes fully mapped will help us better understand diseases. It will also help explain how genetic changes contribute to all diseases. More diverse genome maps will also help experts understand how DNA differs from person to person.6

Even with incomplete information about the human genome, genetic testing has helped many people with rare conditions find answers. Genetic testing looks for changes, or mutations, in your DNA. A person may seek genetic testing to:7

  • Learn whether they have a genetic condition that runs in their family
  • Learn about the chances of passing a genetic condition to current or future children
  • Diagnose a possible genetic condition
  • Understand and guide a cancer prevention or treatment plan

You will also receive genetic counseling before and after testing.

Some people decide not to get tested after learning more. You may choose this option if:7

  • Knowing the results will not:
    • Help you
    • Help your family
    • Change your treatment
  • The results may make you anxious
  • The tests may be too expensive

Types of genetic tests

No one genetic test can find all your possible genetic conditions. The different types of genetic tests include:7

  • Single gene testing may be done if there is a known genetic mutation in a family or if your doctor believes you have a specific condition due to your symptoms. Single gene testing does just what the name says. It looks for changes in 1 gene. It can confirm diseases such as Duchene muscular dystrophy or sickle cell disease.
  • Panel testing looks for changes in many genes. The panels, or groups of tests, are often based on a type of medical concern like epilepsy or unusually short stature. Panels can also assess a group of genes linked to a higher risk of developing certain cancers.
  • Genomic testing looks at large sections of or all of a person's DNA, not just 1 chromosome or gene. This may be ordered for someone with a complex health condition. Examples include rare heart conditions or unusual childhood developmental delays.
  • Karyotype and chromosomal microarray tests look at changes to chromosomes instead of genes.
  • Gene expression tests look at whether genes are turned on or off (expressed) and their levels in different types of cells.

Newborn screening

Newborn screening is what we call a series of tests that are run on babies soon after birth. Many babies look healthy and have no family history of serious disease, so screening tests may be the only way to know they need early help. These tests check for certain serious conditions, including:8-10

  • Phenylketonuria
  • Spinal muscular atrophy
  • Severe Combined Immunodeficiency
  • Pompe disease
  • Mucopolysaccharidosis Type 1
  • X-linked Adrenoleukodystrophy
  • Rare heart conditions

Screening includes a blood drop test (blood spot screening), a blood oxygen reading (pulse oximeter), and a hearing test. These screenings help babies get treatment faster so they stay healthier and possibly avoid serious complications or death.7,9

Learn more about what to expect with genetic counseling and testing and the most common tests for rare diseases.

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