pedigrees

What is a pedigree?

A visual diagram that tracks the occurrence of a specific trait through several generations of a family.

Symbol for a male

A square.

Symbol for a female

A circle.

Meaning of a shaded symbol

The individual is 'affected' or expresses the trait being studied.

Meaning of an unshaded symbol

The individual is 'unaffected' or does not express the trait.

Horizontal line between a square and circle

Represents a mating or marriage line.

Vertical line extending from a mating line

Connects parents to their offspring.

How are generations labeled?

Using Roman numerals (e.g., I, II, III) on the left side of the pedigree.

How are individuals within a generation numbered?

Using Arabic numerals (e.g., 1, 2, 3) from left to right.

Key characteristic of Autosomal Dominant traits

The trait usually appears in every generation; every affected person has at least one affected parent.

Key characteristic of Autosomal Recessive traits

The trait can 'skip' generations; affected individuals may have two unaffected parents.

X-linked Recessive gender bias

The trait is significantly more common in males than in females.

X-linked Recessive: Father to Son inheritance

An affected father can never pass an X-linked trait to his son.

X-linked Dominant: Father to Daughter rule

An affected father will pass the trait to 100\% of his daughters and none of his sons.

Y-linked inheritance pattern

The trait is passed exclusively from fathers to all of their sons.

Mitochondrial inheritance pattern

The trait is passed from an affected mother to all of her children; affected fathers never pass it on.

Definition of a genetic carrier

An individual who has one recessive allele for a trait (Aa) but does not express the phenotype.

Symbol for a known carrier (Autosomal)

A half-shaded square or circle.

Symbol for a carrier of an X-linked trait

A circle with a dot in the center.

Consanguinity symbol

A double horizontal line, indicating mating between close relatives (e.g., first cousins).

What is a Proband?

The specific individual being studied who brought the family to the attention of geneticists, usually indicated by an arrow.

Monozygotic (Identical) Twins symbol

Two symbols branching from the same point with a horizontal bar connecting the diagonal lines.

Dizygotic (Fraternal) Twins symbol

Two symbols branching from a single point but without a horizontal bar connecting them.

Deceased individual symbol

A square or circle with a diagonal slash through it.

Genotype of an unaffected person in an Autosomal Dominant pedigree

They must be homozygous recessive (aa).