Key takeaways

An inheritance pattern describes how a genetic variant can pass through a family. It does not measure the severity of a condition or predict every child’s health. The same family may also have health concerns that follow entirely different patterns.

Illustrated overview of patterns across a family, highlighting dominant, recessive, context.
Patterns across a family: dominant, recessive, context. Read the article for evidence and limitations. Original conceptual illustration.

Dominant and recessive inheritance

For many autosomal dominant conditions, a disease-causing variant in one copy of a gene can be sufficient to increase risk or cause disease. When a parent has one altered and one usual copy, each child generally has a one-in-two chance of inheriting the altered copy. The chance of developing particular symptoms may be a separate question.

For an autosomal recessive condition, disease-causing variants usually affect both copies of the relevant gene. Parents may each carry one variant without having the condition themselves. If both are carriers for the same recessive condition, each pregnancy generally has a one-in-four chance of an affected child, a one-in-two chance of a carrier child, and a one-in-four chance of a child who inherits neither parental variant.

Each pregnancy is a new event

These probabilities describe repeated possibilities, not a schedule. Four children in a carrier couple’s family do not have to include exactly one affected child. Having one unaffected child does not use up an unaffected outcome for the next pregnancy. This is why family size cannot reliably confirm or exclude an inheritance pattern.

Editorial illustration: A multigenerational family arranging four different coloured wooden counters at a kitchen table to discuss family probabilities, relaxed attentive human interaction.
Conceptual illustration accompanying this article. People and situations depicted are fictional; this is not a diagnostic image.

Other patterns and important exceptions

X-linked conditions involve genes on the X chromosome. Their effects and transmission depend on the gene and the person’s chromosome complement; a father does not pass his X chromosome to a son. Mitochondrial DNA follows a different route through egg cells. Chromosome changes and conditions influenced by many genes may not fit these simple models.

A condition can also appear without an earlier family history because a variant arose for the first time. Small families, incomplete records and relatives with unrecognized features can obscure a pattern. A family tree is a starting point for interpretation, not proof of a particular diagnosis.

What to discuss with a genetics professional

  • Is the inheritance pattern confirmed for this particular diagnosis?
  • Does the quoted probability concern inheriting a variant or developing disease?
  • Which relatives might benefit from learning about the result?

Bring the actual laboratory report when possible. Similar-sounding condition names can have different genetic causes and different implications for relatives.