Patterns of Mendelian and non-Mendelian inheritance

Mode of inheritance	Sex distribution	Probability sibling of affected individual will be affected	Comments
Mendelian patterns of inheritance
Autosomal dominant	Males and females are equally affected	50%	Autosomal dominant genetic variants may arise de novo in an affected individual, in which case siblings of that individual are very unlikely to be affected. In highly penetrant conditions, affected individuals are generally observed in all generations.
Autosomal recessive	Males and females are equally affected	25%	Affected individuals often have consanguineous parents.
Autosomal codominant	Males and females are equally affected	50%	Each allele contributes equally, such that individuals with one variant allele generally have a milder phenotype than individuals with two variant alleles.
X-linked recessive*	Males are predominantly affected; females can be affected	Brothers: 50% Sisters: 0% (50% carriers)	Transmission is only from mothers to sons; there is no father-to-son transmission. The phenotype appears more rarely in females because manifestation in females typically requires skewed X-inactivation, an X-chromosome abnormality, or a variant on both X-chromosomes (one from each parent, including an affected father).
X-linked dominant*	Males are more severely affected	50%	Transmission is only from mothers to sons and fathers to daughters; there is no transmission from fathers to sons. Males and females are affected at similar rates, but the phenotype is typically more severe in males.
Y-linked	Males only are affected	Brothers: 100%	Theoretical only; no disorders with Y-linked inheritance have been described. Transmission would be from fathers to sons exclusively.
Deviations from strict Mendelian inheritance
Incomplete penetrance	If expression is sex-limited	For AD: 50% × penetrance function For AR: 25% × penetrance function	The penetrance function is the probability of disease given the risk genotype. It can be influenced by multiple factors, including: Environmental risk factors Modifier genes Gender Parent-of-origin effects (effects that depend on which parent transmits the trait)
Sex-limited expression	Yes	Sex-dependent	Expression is either exclusive to males only or females only, or penetrance is substantially higher in one sex.
Imprinting	None	Similar to AD, AR traits	Penetrance is dependent on the parent-of-origin, but allele transmission is similar from both parents.
Multigenic inheritance	None	Variable	Often seen in common diseases; deleterious variants only modestly increase disease risk.
Anticipation	None	Variable	Manifestations in parents and grandparents of affected offspring are typically diagnosed retrospectively. The disease is more severe in children than in parents.
Mitochondrial inheritance	None	Very high, can approach 100%	Exclusive transmission from mothers to children. Father-daughter transmission is not observed.

Refer to UpToDate topics on inheritance patterns and specific conditions for additional information.

AD: autosomal dominant; AR: autosomal recessive.

* Some experts believe X-linked transmission should not be divided into recessive and dominant because it is possible for females with skewed X-inactivation (skewed lyonization) to be affected in both cases.

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Patterns of Mendelian and non-Mendelian inheritance