Pedigree Chart Essentials for Exam

Pedigree Chart: Overview

  • Diagram showing occurrence and appearance of phenotypes for a gene across generations; used to trace inherited conditions, identify carriers, and predict traits in offspring.

Symbols and Structure

  • Male: ■ square; Female: ○ circle
  • Shaded: Affected
  • Half-shaded: Carrier (especially for recessive or sex-linked traits)
  • Unshaded: Unaffected
  • Horizontal line: Marriage/mating
  • Vertical line: Children
  • Generations labeled with Roman numerals (I, II, III)
  • Individuals within a generation numbered left to right (I-1, II-2, II-6, etc.)
  • Labels: Couple, Offspring count, Carrier/Affected/Unaffected indicators as shown in examples

Generations and Numbering Rules

  • Generations: I, II, III, …
  • Individuals in a generation: numbered from left to right (e.g., II-2, II-6)

Traits Traced in Pedigrees

  • Autosomal Dominant
  • Autosomal Recessive
  • Sex-linked (X-linked) Traits

Autosomal Traits (Autosomes 1–22)

  • Inherited on non-sex chromosomes; affect males and females equally
  • Each person has two copies of each autosome (one from each parent)
  • Genes on autosomes typically use notation D (dominant) and d (recessive)

Autosomal Dominant Traits

  • Dominant allele expressed if at least one copy present
  • Genotypes: AA, Aa
    ightarrow ext{affected}; aa
    ightarrow ext{unaffected}
  • Pedigree patterns:
    • Every generation affected
    • An affected person has at least one affected parent
    • If one parent is affected and heterozygous (Aa), about half the children are affected
    • Males and females affected equally

Examples of Autosomal Dominant Conditions

  • Huntington’s disease
  • Polydactyly
  • Marfan syndrome
  • Other typical cues: Widows Peak, Dimples, Mid-Digital Hair, etc.

Worked Example (Autosomal Dominant Pattern)

  • Scenario: Generation I mother affected; father unaffected; two children (one daughter, one son): daughter unaffected, son affected
  • Deductions:
    • Mother genotype: Dd (affected but not necessarily homozygous)
    • Father genotype: dd (unaffected)
    • Daughter genotype: dd (unaffected)
    • Son genotype: Dd (affected)
  • Later, affected son (Gen II, Dd) marries an unaffected woman (Gen II, dd); they have a daughter diagnosed with Huntington’s (affected)
  • Who passed the Huntington’s allele to the third-generation child?
    • The son (Gen II) passed the dominant allele D to the child
  • Probability that an affected son (Gen II, Dd) and an unaffected wife (Gen II, dd) would have a child with Huntington’s disease:
    • P( ext{child affected}) = frac{1}{2} = 50 ext{ %}

Quick Reference: Pedigree Analysis Checklist

  • Identify whether the trait is autosomal or sex-linked from patterns across generations
  • Check if affected individuals have affected parents (dominant) or if unaffected parents can have affected offspring (recessive)
  • Note whether males and females are affected equally (autosomal) or if one sex is more affected (sex-linked)
  • Use genotype notation to reason about probabilities: AA, Aa, aa; consider parental genotypes to deduce offspring possibilities