UNDERSTANDING GENETIC DISEASES

What is a Genetic Disease?
Genetic diseases (also called genetic disorders ) occur when changes to DNA (referred to as a genetic mutation) result in an alteration in the encoded protein that prevents or alters its function or prevent protein expression entirely. Genetic mutations can be passed down from parents. But they can also occur randomly or can be due to environmental factors. Some genetic mutations are harmless, but others can result in genetic diseases that affect our health.

Without genetic diseases, research in gene therapy would not be as advanced as it is today. On this page we will discuss how genetic mutations can cause genetic disease and how those genetic mutations come about.

Genetic diseases

Genetic diseasesa disease caused by a mutation (or problem) in one or more genes

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(also called genetic disorders) occur when changes to DNA (referred to as genetic mutations) result in an alteration in the instructions that prevent or alter protein function or prevent protein expression entirely. Genetic Mutationa change in a DNA sequence

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can be passed down from parents. But they can also occur randomly or can be due to environmental factors. Some genetic mutations are harmless, but others can result in genetic diseases that affect our health.
There are three main types of genetic diseases1:

  • Single-gene diseasesalso called monogenic diseases, in which a change occurs in one gene only

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     (also called monogenic diseases), where a change occurs in one gene only—this is currently a major focus of research in gene therapy1
  • Chromosomal diseasesdiseases in which chromosomes (or parts of chromosomes) are missing or changed

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    , where chromosomes (or parts of chromosomes) are missing or altered1
  • Complex genetic diseasesdiseases in which changes occur in two or more genes

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    , where changes occur in two or more genes1
Gene therapy is being studied as a way to treat the effects of these mutations.2
Image of gene mutation Image of gene mutation
Gene-ius Questions
One example of a genetic disease is Hemophiliaan inherited disease where blood does not clot properly

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, an inherited blood disorder in which the blood does not clot properly. In hemophilia, a gene that codes for a protein involved in blood clotting is mutated. This gene resides on the X chromosome and the disease occurs mainly in males.3
Example of genetic disease inheritance

What causes genetic diseases?
Although genetic diseases are usually inherited from a person's parents, there are instances of genetic diseases occurring spontaneously. This means that a child can be born with a genetic mutation without either parent having the genetic change (or mutation) themselves. This is called a de novo occurrence of the disease. When a person acquires a genetic disease that is inherited, there are several ways it can happen.

Although genetic diseases are usually inherited from a person's parents, there are instances of genetic diseases occurring spontaneously. This means that a child can be born with a genetic mutation without either parent having the genetic change (or mutation) themselves. This is called a De novofrom birth

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occurrence of the disease.

When a person acquires a genetic disease that is inherited, there are several ways it can happen.4
 Next Level Knowledge

Different ways a single-gene disease can be inherited4:

Remember that, even though diseases often fit into categories like the ones below, every disease is different. If you have questions about a specific disease, you should speak to your doctor or another healthcare professional.

Autosomal Dominant

  • Individuals carrying 1 mutated copy of a gene in each cell will be affected by the disease 
  • Each affected person usually has 1 affected parent
  • Tends to occur in every generation of an affected family

Autosomal Recessive

  • Affected individuals must carry 2 mutated copies of a gene
  • Parents of affected individual are usually unaffected, and each carry a single copy of the mutated gene (known as carriers)
  • Not typically seen in every generation of an affected family

Mitochondrial

  • Only females can pass on mitochondrial conditions to their children (maternal inheritance)
  • Both males and females can be affected
  • Can appear in every generation of an affected family

X-linked Dominant

  • Females are more frequently affected than males
  • Fathers cannot pass X-linked traits to their sons (no male-to-male transmission)

X-linked Recessive

  • Males are more frequently affected than females
  • Families with an X-linked recessive disease often have affected males, but rarely affected females, in each generation
  • Both parents of an affected daughter must be carriers
  • Mothers of affected sons are carriers (fathers cannot pass X-linked traits to their sons)
Cancers also may be caused by mutations at the genetic level. Most genetic mutations that cause cancer are Acquired mutationsa genetic change that happens after conception; this change is passed on to cells that come from the cell that acquires the change

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. These cancers may be caused by exposure to things in the environment, such as cigarette smoke or radiation. In these cancers, genes are mutated later in life and are found in only certain types of cells. Lung cancer is an example. These cancers are not considered genetic diseases because they are not inherited.5,6  

A small portion of cancers are Germline mutationsa gene change in a body's reproductive cell that is incorporated into the DNA of a person’s offspring

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, which are inherited.  These mutations are present from the time sperm fertilizes an egg (in the zygote) or in the sperm or egg themselves. In contrast to acquired mutations, germline mutations are present in every cell in the body.5 Hereditary retinoblastoma is an example of a disease caused by a germline mutation.7

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References

1. National Institutes of Health. Medline Plus. Genetic disorders. Accessed March 4, 2020. https://medlineplus.gov/geneticdisorders.html 2. National Institutes of Health. Genetics Home Reference. Help me understand genetics. Accessed March 4, 2020. https://ghr.nlm.nih.gov/primer 3. Centers for Disease Control and Prevention. What is hemophilia?  Accessed March 16, 2020. https://www.cdc.gov/ncbddd/hemophilia/facts.html 4. Genetic Alliance; The New York–Mid-Atlantic Consortium for Genetic and Newborn Screening Services. Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals. Genetic Alliance Monographs and Guides; 2009. https://www.ncbi.nlm.nih.gov/books/NBK115563/pdf/Bookshelf_NBK115563.pdf 5. American Society of Clinical Oncology. Cancer.Net. The genetics of cancer. Accessed March 26, 2020. https://www.cancer.net/navigating-cancer-care/cancer-basics/genetics/genetics-cancer 6. Jackson M, Marks L, May GHW, Wilson JB. The genetic basis of disease. Essays Biochem. 2018;62(5):643-723. 7. National Institutes of Health. Genetics Home Reference. Retinoblastoma. Accessed March 4, 2020. https://ghr.nlm.nih.gov/condition/retinoblastoma

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