Mendelian inheritance: autosomal dominant, recessive and X-linked !

Topic

When a disease depends on a single gene, it passes through families in predictable patterns that let us estimate each child's risk.

Updated 2026-11-01

What it is

Single-gene disorders follow Mendel's laws: the gene lies on an autosome or on the X chromosome, and either one altered copy is enough (dominant) or two are needed (recessive). Recognising the pattern in a pedigree guides diagnosis, genetic counselling and testing of relatives.

Key facts

New (de novo) mutation: a child with a dominant disease may have healthy parents; this is the case in most achondroplasia and is linked to older paternal age.
Pleiotropy: one gene affects several organs (Marfan: eye, aorta and skeleton).
Locus heterogeneity: different genes cause the same picture; allelic heterogeneity: different mutations in the same gene.
Codominance: both alleles are expressed, as in blood group AB.
In females, random inactivation of one X explains why some carriers have mild symptoms.

Single-gene inheritance patterns

Pattern	Who is affected	How it is passed on	Examples
Autosomal dominant	Both sexes equally; seen in every generation	One affected parent; each child has a one-in-two chance	Marfan, familial hypercholesterolaemia, Huntington, adult polycystic kidney disease, achondroplasia
Autosomal recessive	Both sexes; siblings in one generation	Two healthy carrier parents; one in four children affected	Cystic fibrosis, sickle cell disease, phenylketonuria, Tay-Sachs
X-linked recessive	Almost only males	Carrier mother: half of her sons affected; no male-to-male transmission	Duchenne muscular dystrophy, haemophilia A and B, G6PD deficiency
X-linked dominant	Both sexes; females milder	Affected father: all of his daughters and none of his sons	X-linked hypophosphataemic rickets
Mitochondrial	Both sexes	Only through the mother; very variable expression (heteroplasmy)	MELAS, Leber hereditary optic neuropathy

Clinical notes

In a pedigree, a disease that ‘skips’ generations suggests recessive inheritance or incomplete penetrance.
Genetic counselling uses the pattern to estimate risk; molecular testing of the gene confirms it.

Test yourself

Think of the answer, then tap the question to see it.

Which pedigree feature rules out X-linked inheritance?

Father-to-son transmission: a father passes his Y chromosome, not his X, to his sons.

Two parents who carry cystic fibrosis: what is their risk in each pregnancy?

One in four of an affected child, one in two of a carrier child and one in four of a non-carrier.

Why can a child with achondroplasia have parents of normal height?

Because most cases come from a new mutation in the FGFR3 gene rather than from an affected parent.

Which kind of protein is usually defective in autosomal recessive diseases?

Enzymes: half of the normal activity is enough to work, which is why carriers are healthy.

Add to my cards (4)

Elsewhere in DurgiPedia

Types of mutations

← All of general pathology

Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: What are the different ways a genetic condition can be inherited?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Marfan syndrome: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Duchenne and Becker muscular dystrophy: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.