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:

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

Three simplifying assumptions:

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

5 Key Clues

1. An unaffected individual cannot have any alleles of a dominant trait (because a single allele causes expression)

2. Individuals marrying into the family are assumed to have no disease alleles (trait is rare in population)

3. An unaffected individual can be a carrier of a recessive trait (two alleles required for expression)

4. When a trait is X-linked, a single recessive allele is sufficient for a male to be affected (males are hemizygous)

5. 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

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)

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Important Note

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