Pedigree Analysis Cheat Sheet

Inheritance Patterns Overview

Autosomal Dominant

Key Features:

Appears in every generation (vertical transmission)
Both males and females are affected equally
Affected individuals have at least one affected parent
Approximately 50% of offspring of an affected individual are also affected

Look for: No affected parent having unaffected children AND affected child having an affected parent.

Autosomal Recessive

Key Features:

May skip generations (horizontal transmission)
Both males and females are affected equally
Affected individuals can be born to unaffected (carrier) parents
Approximately 25% of offspring of two carrier parents are affected

Look for: Affected offspring coming from unaffected parents.

X-linked Dominant

Key Features:

Appears in every generation
Both males and females are affected, but females may be more frequently affected (due to two X chromosomes)
Affected fathers will pass the trait to all daughters but not to sons
Affected mothers can pass the trait to both sons and daughters

Look for: ALL daughters of affected father having the disease AND sons getting the disease from ONLY affected mother.

X-linked Recessive

Key Features:

More males than females are affected
Can skip generations

Criss-Cross Inheritance X-linked recessive traits often show a "criss-cross" pattern: an affected father passes the gene to his daughter (who becomes a carrier), and she then passes it to her son (who becomes affected).

Affected males are usually born to carrier mothers
Affected fathers cannot pass the trait to their sons (no male-to-male transmission)
All daughters of affected fathers are carriers (if the mother is not a carrier)

Look for: Some daughters affected by affected father, skipping generations, sons getting disease from mother. LOOK FOR CRISS CROSS INHERITANCE.

Additional Tips by Pattern

Autosomal Dominant:

Look for affected individuals in each generation
Check for both male and female transmission equally

Autosomal Recessive:

Look for the trait skipping generations
Check for both males and females being affected equally
Look for consanguinity (mating between relatives), which is common in autosomal recessive pedigrees

X-linked Dominant:

Look for all daughters of affected fathers being affected
Check if affected mothers pass the trait to both sons and daughters

X-linked Recessive:

Look for more males affected than females
Check if affected males are often born to carrier mothers
Look for no male-to-male transmission

General Assumptions

In pedigree analysis problems, you'll be reasoning about genetic traits controlled by one gene with two alleles (dominant and recessive).

Step-by-step approach:

First start looking from the top
Choose any affected individual and look at their parent
Look for the sex ratio
Assume the genotype (handle one small inheritance at a time - one father, mother, and their children)
While assuming genotype, handle only one small inheritance at a time
Simplify the pedigree by eliminating outbreeding or marriage between non-related family

Three simplifying assumptions:

Complete Penetrance: An individual will be affected when carrying at least one dominant allele (dominant trait) or two recessive alleles (recessive trait)
Rare-in-Population: The trait is rare in the general population. Individuals marrying into the pedigree in second and third generations are not carriers
Not-Y-Linked: Causative genes may be autosomal or X-linked, but not Y-linked

5 Key Clues

An unaffected individual cannot have any alleles of a dominant trait (because a single allele causes expression)
Individuals marrying into the family are assumed to have no disease alleles (trait is rare in population)
An unaffected individual can be a carrier of a recessive trait (two alleles required for expression)
When a trait is X-linked, a single recessive allele is sufficient for a male to be affected (males are hemizygous)
Father transmits X-linked alleles to daughters only, not sons. Mother transmits to both daughters and sons

Case Classification System

Case 1: Autosomal Recessive

Both sexes affected (males and females)
Trait can skip generations

Case 2: Autosomal Dominant

Both sexes affected (males and females)
Trait does NOT skip generations

Case 3: X-linked Recessive

More affected males than females
Males never transmit to sons
Daughters of affected males always inherit mutation (carriers)
Trait can skip generations when females are heterozygotes

Case 4: X-linked Dominant

Both sexes affected (males and females)
Females transmit to daughters and sons
Males ALWAYS transmit to daughters, but NOT to sons
Trait does NOT skip generations

Case 5: Y-linked

Only males are affected
Males ALWAYS transmit to sons

Case 6: Mitochondrial Inheritance

Both sexes are affected
Females transmit to ALL of their progeny
Males do NOT transmit to any of their progeny

Pattern Recognition Guide

Generational Appearance:

Every Generation: Likely Autosomal Dominant or X-linked Dominant
Skips Generations: Likely Autosomal Recessive or X-linked Recessive

Sex Ratio:

Both Sexes Equally: Likely Autosomal
More Males Affected: Likely X-linked Recessive
More Females Affected: Likely X-linked Dominant

Parent-to-Child Transmission:

Affected Father to All Daughters but No Sons: X-linked Dominant
No Male-to-Male Transmission: X-linked (either dominant or recessive)

Genotype Assumption Rules

Inheritance Pattern	Golden Rules	How to Assume Genotype	Key Things to Remember
Autosomal Dominant	- Trait appears in every generation - Both sexes affected equally - Affected individuals have ≥1 affected parent	- Heterozygous (Aa): Most affected individuals - Homozygous (AA): Rare, only if both parents affected	- One affected parent (Aa) can produce affected (Aa) and unaffected (aa) offspring - Homozygous affected (AA) are less common
Autosomal Recessive	- Trait may skip generations - Both sexes affected equally - Unaffected parents can have affected children	- Heterozygous (Aa): Unaffected carriers - Homozygous (aa): Affected individuals	- Two unaffected parents with affected child → both parents are carriers (Aa) - Affected individuals (aa) have two carrier parents
X-linked Dominant	- Trait appears in every generation - More females may be affected - Affected fathers pass to all daughters, not sons - Affected mothers pass to both sons and daughters	- Heterozygous (X^A X^a): Most affected females - Homozygous (X^A X^A): Rare - Hemizygous (X^A Y): Affected males	- Affected males (X^A Y) inherit from affected mothers - All daughters of affected father will be affected
X-linked Recessive	- More males than females affected - Can skip generations - Affected males born to carrier mothers - No father-to-son transmission - All daughters of affected fathers are carriers	- Carrier Females (X^A X^a): Unaffected carriers - Affected Females (X^a X^a): Rare - Hemizygous (X^a Y): Affected males	- Carrier females have 50% chance of passing to sons - Affected males inherit from carrier mothers - No male-to-male transmission

Quick Application Guide

Autosomal Dominant:

Affected individual with unaffected parent → likely heterozygous (Aa)
Both parents affected → could be homozygous (AA), but less common

Autosomal Recessive:

Two unaffected parents with affected child → both parents are carriers (Aa)
Affected individuals (aa) must have two carrier parents

X-linked Dominant:

Affected female with one unaffected parent → likely heterozygous (X^A X^a)
All daughters of affected father and unaffected mother → heterozygous (X^A X^a)

X-linked Recessive:

Affected male → mother must be carrier (X^A X^a) or affected (X^a X^a)
Affected female → father must be affected (X^a Y) and mother carrier/affected

Easy Memory Aids

Autosomal Dominant: Affected individuals usually heterozygous (Aa) if one affected parent
Autosomal Recessive: Affected individuals homozygous (aa) with both parents as carriers (Aa)
X-linked Dominant: Affected males hemizygous (X^A Y), affected females usually heterozygous (X^A X^a)
X-linked Recessive: Affected males hemizygous (X^a Y), carrier females heterozygous (X^A X^a)

Important Note

Pedigrees can frequently rule out, but not necessarily prove, a certain mode of inheritance.

Pedigree Analysis Cheat Sheet

Inheritance Patterns Overview

Autosomal Dominant

Key Features:

Appears in every generation (vertical transmission)
Both males and females are affected equally
Affected individuals have at least one affected parent
Approximately 50% of offspring of an affected individual are also affected

Look for: No affected parent having unaffected children AND affected child having an affected parent.

Autosomal Recessive

Key Features:

May skip generations (horizontal transmission)
Both males and females are affected equally
Affected individuals can be born to unaffected (carrier) parents
Approximately 25% of offspring of two carrier parents are affected

Look for: Affected offspring coming from unaffected parents.

X-linked Dominant

Key Features:

Appears in every generation
Both males and females are affected, but females may be more frequently affected (due to two X chromosomes)
Affected fathers will pass the trait to all daughters but not to sons
Affected mothers can pass the trait to both sons and daughters

Look for: ALL daughters of affected father having the disease AND sons getting the disease from ONLY affected mother.

X-linked Recessive

Key Features:

More males than females are affected
Can skip generations

Affected males are usually born to carrier mothers
Affected fathers cannot pass the trait to their sons (no male-to-male transmission)
All daughters of affected fathers are carriers (if the mother is not a carrier)

Look for: Some daughters affected by affected father, skipping generations, sons getting disease from mother. LOOK FOR CRISS CROSS INHERITANCE.

Additional Tips by Pattern

Autosomal Dominant:

Look for affected individuals in each generation
Check for both male and female transmission equally

Autosomal Recessive:

Look for the trait skipping generations
Check for both males and females being affected equally
Look for consanguinity (mating between relatives), which is common in autosomal recessive pedigrees

X-linked Dominant:

Look for all daughters of affected fathers being affected
Check if affected mothers pass the trait to both sons and daughters

X-linked Recessive:

Look for more males affected than females
Check if affected males are often born to carrier mothers
Look for no male-to-male transmission

General Assumptions

In pedigree analysis problems, you'll be reasoning about genetic traits controlled by one gene with two alleles (dominant and recessive).

Step-by-step approach:

First start looking from the top
Choose any affected individual and look at their parent
Look for the sex ratio
Assume the genotype (handle one small inheritance at a time - one father, mother, and their children)
While assuming genotype, handle only one small inheritance at a time
Simplify the pedigree by eliminating outbreeding or marriage between non-related family

Three simplifying assumptions:

Complete Penetrance: An individual will be affected when carrying at least one dominant allele (dominant trait) or two recessive alleles (recessive trait)
Rare-in-Population: The trait is rare in the general population. Individuals marrying into the pedigree in second and third generations are not carriers
Not-Y-Linked: Causative genes may be autosomal or X-linked, but not Y-linked

5 Key Clues

An unaffected individual cannot have any alleles of a dominant trait (because a single allele causes expression)
Individuals marrying into the family are assumed to have no disease alleles (trait is rare in population)
An unaffected individual can be a carrier of a recessive trait (two alleles required for expression)
When a trait is X-linked, a single recessive allele is sufficient for a male to be affected (males are hemizygous)
Father transmits X-linked alleles to daughters only, not sons. Mother transmits to both daughters and sons

Case Classification System

Case 1: Autosomal Recessive

Both sexes affected (males and females)
Trait can skip generations

Case 2: Autosomal Dominant

Both sexes affected (males and females)
Trait does NOT skip generations

Case 3: X-linked Recessive

More affected males than females
Males never transmit to sons
Daughters of affected males always inherit mutation (carriers)
Trait can skip generations when females are heterozygotes

Case 4: X-linked Dominant

Both sexes affected (males and females)
Females transmit to daughters and sons
Males ALWAYS transmit to daughters, but NOT to sons
Trait does NOT skip generations

Case 5: Y-linked

Only males are affected
Males ALWAYS transmit to sons

Case 6: Mitochondrial Inheritance

Both sexes are affected
Females transmit to ALL of their progeny
Males do NOT transmit to any of their progeny

Pattern Recognition Guide

Generational Appearance:

Every Generation: Likely Autosomal Dominant or X-linked Dominant
Skips Generations: Likely Autosomal Recessive or X-linked Recessive

Sex Ratio:

Both Sexes Equally: Likely Autosomal
More Males Affected: Likely X-linked Recessive
More Females Affected: Likely X-linked Dominant

Parent-to-Child Transmission:

Affected Father to All Daughters but No Sons: X-linked Dominant
No Male-to-Male Transmission: X-linked (either dominant or recessive)

Genotype Assumption Rules

Inheritance Pattern	Golden Rules	How to Assume Genotype	Key Things to Remember
Autosomal Dominant	- Trait appears in every generation - Both sexes affected equally - Affected individuals have ≥1 affected parent	- Heterozygous (Aa): Most affected individuals - Homozygous (AA): Rare, only if both parents affected	- One affected parent (Aa) can produce affected (Aa) and unaffected (aa) offspring - Homozygous affected (AA) are less common
Autosomal Recessive	- Trait may skip generations - Both sexes affected equally - Unaffected parents can have affected children	- Heterozygous (Aa): Unaffected carriers - Homozygous (aa): Affected individuals	- Two unaffected parents with affected child → both parents are carriers (Aa) - Affected individuals (aa) have two carrier parents
X-linked Dominant	- Trait appears in every generation - More females may be affected - Affected fathers pass to all daughters, not sons - Affected mothers pass to both sons and daughters	- Heterozygous (X^A X^a): Most affected females - Homozygous (X^A X^A): Rare - Hemizygous (X^A Y): Affected males	- Affected males (X^A Y) inherit from affected mothers - All daughters of affected father will be affected
X-linked Recessive	- More males than females affected - Can skip generations - Affected males born to carrier mothers - No father-to-son transmission - All daughters of affected fathers are carriers	- Carrier Females (X^A X^a): Unaffected carriers - Affected Females (X^a X^a): Rare - Hemizygous (X^a Y): Affected males	- Carrier females have 50% chance of passing to sons - Affected males inherit from carrier mothers - No male-to-male transmission

Quick Application Guide

Autosomal Dominant:

Affected individual with unaffected parent → likely heterozygous (Aa)
Both parents affected → could be homozygous (AA), but less common

Autosomal Recessive:

Two unaffected parents with affected child → both parents are carriers (Aa)
Affected individuals (aa) must have two carrier parents

X-linked Dominant:

Affected female with one unaffected parent → likely heterozygous (X^A X^a)
All daughters of affected father and unaffected mother → heterozygous (X^A X^a)

X-linked Recessive:

Affected male → mother must be carrier (X^A X^a) or affected (X^a X^a)
Affected female → father must be affected (X^a Y) and mother carrier/affected

Easy Memory Aids

Autosomal Dominant: Affected individuals usually heterozygous (Aa) if one affected parent
Autosomal Recessive: Affected individuals homozygous (aa) with both parents as carriers (Aa)
X-linked Dominant: Affected males hemizygous (X^A Y), affected females usually heterozygous (X^A X^a)
X-linked Recessive: Affected males hemizygous (X^a Y), carrier females heterozygous (X^A X^a)

Important Note

Pedigrees can frequently rule out, but not necessarily prove, a certain mode of inheritance.

Pedigree Cheat Sheet

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Pedigree Cheat Sheet

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