Concepts in Genetics

Early-Onset.
Multi-organ manifestations. For example, birth defects, sensory deficits, developmental problems, facial dysmorphism.
Absence of clear environmental causes. Cautionary Note: Many patients with a genetic condition do have other medical problems that can be thought to cause their disease, like diabetes, heart conditions, or hypertension.
Positive Family History. Other family members have a similar disease. Cautionary Note: Many patients with a genetic condition do not have affected family members.

For definitions, please visit the Genetics Glossary on Genome.gov.

Inheritance mode	Variant zygosity	Family pattern	Family counseling
Dominant Disorders (AD and XLD)	Heterozygote or Hemizygote variant	One of the parent has the disorder or the patient is the first to have the disorder in the family (de novo).	50% risk for each of the patient’s kid to have the disorder.
Autosomal Recessive Disorders (AR)	Biallelic variants	Parents are not affected carriers. Siblings may be affected (25% risk for each).	Unaffected siblings have a 66% risk to be carriers. Risk for the next generation depends on the spouse’s carrier status (higher risk in case of consanguinity).
X-linked Recessive Disorders (XLR)	Hemizygote variants	Females are not affected. Male siblings may be affected (50% risk for each). Additional male family members may be affected on the mother’s side of the family.	Sisters have a 50% chance of being carriers
Mitochondrial Disorders (Mito)	Variants in the mitochondrial genome or depletion of the mitochondrial genome	Variable expressivity, depending on the proportion of affected mitochondria. Siblings may be affected, as well as additional family members on the mother’s side of the family.	Maternal inheritance. Sisters of affected individual at risk of transmitting the disorder.