Rare Diseases Genetic Disorders How Genetic Disorders Are Inherited Understanding Patterns of Inheritance By Cherie Berkley, MS twitter Cherie Berkley is an award-winning journalist and multimedia storyteller covering health features and news content for Verywell. Learn about our editorial process Cherie Berkley, MS Medically reviewed by Medically reviewed by Elizabeth Molina Ortiz, MD, MPH on September 25, 2020 linkedin Elizabeth Molina Ortiz, MD, MPH, is a board-certified specialist in family medicine and is the former medical director of a community health center. Learn about our Medical Review Board Elizabeth Molina Ortiz, MD, MPH on September 25, 2020 Print Table of Contents View All Patterns of Inheritance Genetic disorders are precisely what they sound like: Diseases caused by a mutation of a gene. When such diseases are inherited (rather than the result of a random mutation), it means they are passed along to a child from one or both parents according to a specific patterns of inheritance. These patterns are determined by the gene involved, whether only one or both parents have the gene, which chromosome it is on, and other factors. The presence of a mutation doesn't always translate to the disease it's associated with. For example, Huntington's disease, breast cancer, and autoimmune diseases are associated with specific genes, but a person who inherits them won't necessarily develop these conditions. On the other hand, some genetic mutations, such as those linked to hemophilia, will always manifest the disorder. Furthermore, the environment can have an effect on the degree to which a gene mutation is expressed, which explains why in some cases family members with the same genetic mutation may experience an inherited disorder somewhat differently. cdascher / Getty Images Patterns of Inheritance The various patterns of inheritance are attributed to the Austrian scientist Gregor Mendel, who discovered them while working with garden pea hybrids in the 1800s. Mendel sometimes is referred to as the father of modern genetics; likewise, the patterns of inheritance for single-gene diseases are often described as Mendelian. According to Mendel's work, there are five distinct patterns of inheritance: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial. Two primary factors influence the likelihood a person will inherit a genetic disorder:Whether one copy of the mutated gene (from either parent) is passed down or whether two copies (one from both parents) are passed downWhether the mutation is on one of the sex chromosomes (X or Y) or on one of 22 other pairs of non-sex chromosomes (called autosomes) Chromosomal Abnormalities That Can Lead to Complications or Disorders Autosomal Dominant In autosomal dominant disorders, only one copy of a mutated gene is necessary and males and females are equally likely to be affected. Children who have a parent who has an autosomal dominant disorder have a 50% risk of inheriting the disorder. Sometimes, however, these disorders result from a new mutation and happen in people with no family history. Examples of autosomal dominant disorders include Huntington's disease and Marfan syndrome. Genetic Testing for Huntington's Disease Autosomal Recessive In autosomal recessive disorders, both copies of a mutated gene—one from each parent—are present. A person with only one copy will be a carrier. Carriers will not have any signs or symptoms of the disorder. They can, however, pass the mutation to their children. If families in which both parents carry the mutation for an autosomal recessive disorder, the odds of the children having the disorder are as follows: 25% risk of inheriting both mutations and having the disorder50% risk of inheriting only one copy and becoming a carrier25% risk of not inheriting the mutation at all Examples of autosomal recessive disorders include cystic fibrosis, sickle cell disease, Tay-Sachs disease, and phenylketonuria (PKU). Living With Sickle Cell Disease X-Linked Dominant X-linked dominant disorders are caused by mutations in genes on the X (female) chromosome. In females, who have two X chromosomes, it takes a mutation in only one of the two copies of the gene for a disorder to manifest. In males (who have one X chromosome and one Y chromosome), a mutation in only one copy of the gene in each cell is enough to cause the disorder. Most times, males have more severe symptoms of an X-link disorder than females. However, one feature of X-linked inheritance is that fathers cannot pass on these traits to their sons. Fragile X syndrome is an example of an X-linked dominant disorder. X-Linked Recessive In X-linked recessive disorders, the mutated gene occurs on the X chromosome. Because males have one X chromosome and one Y chromosome, a mutated gene on the X chromosome is enough to cause an X-linked recessive disorder. Females, by contrast, have two X chromosomes, so a mutated gene on one X chromosome usually has less effect on a female because the non-mutated copy on the other largely cancels out the effect. However, a female with the genetic mutation on one X chromosome is a carrier of that disorder. From a statistical standpoint, this means 50% of her sons will inherit the mutation and develop the disorder, while 50% of her daughters will inherit the mutation and become a carrier. Examples of X-linked recessive disorders are hemophilia and red-green color blindness. An Overview of Genetic Testing for Hemophilia A Mitochondrial Mitochondria are structures called organelles that exist in each cell of the body where they convert molecules into energy. Each mitochrondrion contains a small amount of DNA: A mutation of that DNA is responsible for mitochondrial disorders. Mitochondrial disorders are passed down from mothers: Only females can share mitochondrial mutations with their offspring because egg cells contribute mitochondria to the developing embryo; sperm cells do not. Conditions resulting from mutations in mitochondrial DNA can appear in every generation of a family and can affect both males and females. An example of a mitochondrial inherited disorder is Leber hereditary optic neuropathy, a form of sudden vision loss. Signs of Neurological Mitochondrial Disorders Other Inheritance Patterns In addition the the five maine patterns of inheritance there are a few others sometimes recognized by geneticists. Y-Linked Disorders Because only males have a Y chromosome, only males can be affected by and pass on Y-linked disorders. All sons of a man with a Y-linked disorder will inherit the condition from their father. Some examples of Y-linked disorders are Y chromosome infertility and cases of Swyer syndrome in which a male's testicles do not develop normally. Codominance Codominant inheritance involves a relationship between two versions of a gene. Each version of a gene is called an allele. If the alleles inherited by a parent don't match, the dominant allele usually will be expressed, while the effect of the other allele, called recessive, is dormant. In codominance, however, both alleles are dominant and therefore phenotypes of both alleles are expressed. An example of a codominance condition is alpha-1 antitrypsin deficiency. Was this page helpful? Thanks for your feedback! Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit Article Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Genetic Alliance; The New York-Mid-Atlantic Consortium for Genetic and Newborn Screening Services. Washington (DC): Genetic alliance; 2009 Jul 8. 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. Washington (DC): Genetic Alliance; 2009 Jul 8. APPENDIX E, INHERITANCE PATTERNS. Genetic Alliance; District of Columbia Department of Health.Washington (DC): Genetic Alliance; 2010 Feb 17. What are the different ways in which a genetic condition can be inherited? U.S. National Library of Medicine. If a genetic disorder runs in my family, what are the chances that my children will have the condition? U.S. National Library of Medicine: National Institutes of Health. Aug. 17, 2020. Fact Sheet 12 | Mitochondrial inheritance. Center for Genetics Education. Sept. 30, 2015. Cleveland Clinic. Leber hereditary optic neuropathy (sudden vsion loss). Dec. 29, 2014. National Organization for Rare Disorders. Swyer syndrome. 2019. Codominance. National Human Genome Research Institute: National Institutes of Health. Additional Reading What are the different ways in which a genetic condition can be inherited? U.S. National Library of Medicine: National Institutes of Health. Fact Sheet 10: X-Linked Dominant Inheritance. Center for Genetics Education. NSW Government Health. U.S. National Library of Medicine: National Institutes of Health. If a genetic disorder runs in my family, what are the chances that my children will have the condition? Genetics Home Reference. Bethesda, Maryland; updated November 7, 2017.