The key difference between autosomal dominant and autosomal recessive disorders is that, in autosomal dominant disorders, one altered copy of a gene is enough to cause the disease while, in autosomal recessive disorders, both altered copies of the gene are needed to cause the disease.
Autosomal dominant and autosomal recessive disorders are inherited genetic disorders. In both disorders, the affected gene is present in an autosome (non-sex chromosome). In autosomal dominant condition, one altered copy of the gene in a cell is sufficient for a person to be affected by the disease. In autosomal recessive condition, both recessive copies of the gene in a cell are needed for a person to be affected.
CONTENTS
1. Overview and Key Difference
2. What are Autosomal Dominant Disorders
3. What are Autosomal Recessive Disorders
4. Similarities Between Autosomal Dominant and Autosomal Recessive Disorders
5. Side by Side Comparison – Autosomal Dominant vs Autosomal Recessive Disorders in Tabular Form
6. Summary
What are Autosomal Dominant Disorders?
In autosomal dominant condition, the mutated copy of the gene is a dominant one. This mutated copy is located on one of the non-sex chromosomes. A person needs only one mutated copy of the gene to be affected by this type of disorder. The affected person has a 50% chance of having an affected child with one mutated copy (dominant) of the gene. The same person also has a 50% chance of an unaffected child with two normal copies (recessive) of the gene. Each affected person usually has one affected parent in autosomal dominant disorders.
Figure 01: Autosomal Dominant Disorder
Autosomal dominant conditions sometimes show reduced penetrance. It means although only one copy is needed to develop the disease, not all individuals who inherit the mutation will be affected by the disease. For example, in conditions such as tuberous sclerosis, people may be minimally affected. But they have a higher risk of having severely affected children.
Autosomal dominant disorders affect both males and females. These disorders have late onset of sign and symptoms. The dominant disorder is not transmitted by unaffected family members. The popular examples of autosomal dominant disorders are Huntington disease, tuberous sclerosis, Myotonic dystrophy and neurofibromatosis.
What are Autosomal Recessive Disorders?
An autosomal recessive disorder is a genetic disorder in which two copies of an abnormal gene are responsible for the development of a disease. The recessive inheritance means both mutated copies of a gene must be abnormal (recessive) to cause the disease. The parents of a child that is suffering from an autosomal recessive disorder usually do not have the disease. These unaffected parents are carriers. The parents carry a mutated copy of the gene that can be passed to their children. The parents have a 25% chance of having an unaffected child with two normal copies of a gene, a 50% chance of having an unaffected child who is also a carrier, and a 25% chance of having an affected child with two recessive copies of a gene.
Figure 02: Autosomal Recessive Disorder
In this disorder, the affected gene is located on a non-sex chromosome. Autosomal recessive disorders are not typically observable in every generation of an affected family. Sickle cell disease and cystic fibrosis are common examples of autosomal recessive disorders.
What are the Similarities Between Autosomal Dominant and Autosomal Recessive Disorders?
- Both are genetic conditions.
- In both conditions, affected genes present on an autosome (non-sex chromosome).
- Both conditions are inherited.
- They both cause severe diseases.
What is the Difference Between Autosomal Dominant and Autosomal Recessive Disorders?
In autosomal dominant disorder, one altered copy of the gene in a cell is sufficient for a person to be affected by a disease. In contrast, in autosomal recessive condition, both copies of the gene are needed for a person to be affected by a disease. So, this is the key difference between autosomal dominant and autosomal recessive disorders. Moreover, in autosomal dominant disorders, a person with an autosomal dominant condition has a 50% chance of having an affected child with one mutated copy (dominant) of the gene. On the other hand, in autosomal recessive disorders, the unaffected parents have a 25% chance of having an affected child with both mutated copies (recessive) of the gene. Thus, this is another important difference between autosomal dominant and autosomal recessive disorders.
The below infographic shows more differences between autosomal dominant and autosomal recessive disorders in tabular form.
In autosomal disorders, the affected gene is present on a non-sex chromosome. A single copy of the disease-associated mutation is enough to cause the disease in autosomal dominant disorder. Two copies of the disease-associated mutation are needed to cause the disease in autosomal recessive disorder. Thus, this is the key difference between autosomal dominant and autosomal recessive disorders.
1. “Autosomal Dominant: MedlinePlus Medical Encyclopedia.” MedlinePlus, U.S. National Library of Medicine, Available here.
2. “Genetic Disorder.” Wikipedia, Wikimedia Foundation, 11 Mar. 2021, Available here.
1. “Autosomal dominant” By Simon Caulton – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “2926 Autosomal Recessive Inheritance-new” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia
It is important to understand the basic laws of inheritance to appreciate how conditions are passed on in a family. An accurate family health history is a valuable tool to illustrate how conditions are passed down through generations.
A person has two copies of almost every gene, one copy from mom and one copy from dad. Scientists have studied human genes to learn how they normally work and how changes in genes can change how they work. Some changes are very minor and do not affect the way a gene works. These changes are often called single nucleotide polymorphisms (SNPs, pronounced “snips”) or gene variants. Other changes, called mutations, affect how a gene works and can lead to disease.
For some conditions, family members with the same mutation may not have the same symptoms. For other conditions, individuals with different mutations can have similar characteristics. This is because gene expression is influenced by genes, as well as by the environment.
Diseases caused by mutations in a single gene are usually inherited in a simple pattern, depending on the location of the gene and whether one or two normal copies of the gene are needed. This is often referred to as Mendelian inheritance because Gregor Mendel first observed these patterns in garden pea plants. Most single gene disorders are rare; but, in total, they affect millions of people in the United States.
Several basic modes of inheritance exist for single-gene disorders: autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive. However, not all genetic conditions will follow these patterns, and other rare forms of inheritance such as mitochondrial inheritance exist. (See table at the end of this section.)
Dominant mutations are expressed when only one copy of that mutation is present. Therefore, anyone who inherits one dominant disease mutation such as the mutation for Huntington’s disease will have that disease. Dominantly inherited genetic diseases tend to occur in every generation of a family. Each affected person usually has one affected parent. However, dominant mutations can also happen in an individual for the first time, with no family history of the condition (spontaneous mutation).
Recessive mutations require two mutated copies for disease to develop. Recessive genetic diseases are typically not seen in every generation of an affected family. The parents of an affected person are generally carriers: unaffected people who have a copy of a mutated gene. If both parents are carriers of the same mutated gene and both pass it to the child, the child will be affected.
Inheritance patterns differ for genes on sex chromosomes (chromosomes X and Y) compared to genes located on autosomes, non-sex chromosomes (chromosomes numbers 1-22). This is due to the fact that, in general, females carry two X chromosomes (XX), while males carry one X and one Y chromosome (XY). Therefore, females carry two copies of each X-linked gene, but males carry only one copy each of X-linked and Y-linked genes. Females carry no copies of Y-linked genes.
Diseases caused by mutated genes located on the X chromosome can be inherited in either a dominant or recessive manner. Since males only have one X chromosome, any mutated gene on the X chromosome, dominant or recessive, will result in disease. Because females have two copies of X-linked genes, they will not be affected by inheriting of a single recessive mutation on an X-linked gene. For X-linked recessive diseases to occur in females, both copies of the gene must be mutated. Families with an X-linked recessive disorder often have affected males, but rarely affected females, in each generation.
For X-linked dominant diseases, however, a mutation in one copy of an X-linked gene will result in disease for both males and females. Families with an X-linked dominant disorder often have both affected males and affected females in each generation.
A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons; fathers only pass X chromosomes to their daughters and Y chromosomes to their sons. In contrast, mothers pass X-linked genes to both sons and daughters.