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Genetic Alliance; District of Columbia Department of Health. Understanding Genetics: A District of Columbia Guide for Patients and Health Professionals. Washington (DC): Genetic Alliance; 2010 Feb 17.

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Understanding Genetics: A District of Columbia Guide for Patients and Health Professionals.

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Appendix BClassic Mendelian Genetics (Patterns of Inheritance)

The basic laws of inheritance are important in understanding patterns of disease transmission. The inheritance patterns of single gene diseases are often referred to as Mendelian since Gregor Mendel first observed the different patterns of gene segregation for selected traits in garden peas and was able to determine probabilities of recurrence of a trait for subsequent generations. If a family is affected by a disease, an accurate family history will be important to establish a pattern of transmission. In addition, a family history can even help to exclude genetic diseases, particularly for common diseases where behavior and environment play strong roles.

Most genes have one or more versions due to mutations or polymorphisms referred to as alleles. Individuals may carry a ‘normal’ allele and/or a ‘disease’ or ‘rare’ allele depending on the impact of the mutation/polymorphism (e.g., disease or neutral) and the population frequency of the allele. Single-gene diseases are usually inherited in one of several patterns depending on the location of the gene and whether one or two normal copies of the gene are needed for the disease phenotype to manifest.

The expression of the mutated allele with respect to the normal allele can be characterized as dominant, co-dominant, or recessive. There are five basic modes of inheritance for single-gene diseases: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial.

Genetic heterogeneity is a common phenomenon with both single-gene diseases and complex multi-factorial diseases. It should not be surprising that multiple affected family members may experience different levels of disease severity and outcomes. This effect may be due to other genes influencing the disease phenotype or different mutations in the same gene resulting in similar, but not identical phenotypes. Some excellent resources for information about single-gene diseases is the Online Mendelian Inheritance in Man (OMIM; and GeneTests/GeneClinics (

Patterns of Inheritance

Autosomal Dominant

  • Each affected person has an affected parent
  • Occurs in every generation

Autosomal Recessive

  • Both parents of an affected person are carriers
  • Not typically seen in every generation

X-linked Dominant

  • Females more frequently affected
  • Can have affected males and females in same generation

X-linked Recessive

  • Males more frequently affected
  • Affected males often present in each generation


  • Can affect both males and females, but only passed on by females
  • Can appear in every generation
Inheritance PatternDisease Examples
Autosomal DominantHuntington’s disease, neurofibromatosis, achondroplasia, familial hypercholesterolemia
Autosomal RecessiveTay-sachs disease, sickle cell anemia, cystic fibrosis, phenylketonuria (PKU)
X-linked DominantHypophatemic rickets (vitamin D-resistant rickets), ornithine transcarbamylase deficiency
X-linked RecessiveHemophilia A, Duchenne muscular dystrophy
MitochondrialLeber’s hereditary optic neuropathy, Kearns-Sayre syndrome
Copyright © 2008, Genetic Alliance.

All Genetic Alliance content, except where otherwise noted, is licensed under a Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Bookshelf ID: NBK132145


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