Pedigree Charts & Patterns of Inheritance

Pedigree Chart Basics

• Diagram tracing a phenotype through successive generations (family tree format).

• Standard symbols:

  • Square = male, Circle = female, Filled = expresses trait, Empty = unaffected.

  • Horizontal line = mating; double line = consanguineous mating.

  • Vertical line to offspring; siblings joined by a horizontal sibship line.

  • Roman numerals/name left ➔ generation indicator; individuals numbered left → right.

Modes of Inheritance (four hypotheses)

• \text{Autosomal dominant}

• \text{Autosomal recessive}

• \text{X\text{-}linked dominant}

• \text{X\text{-}linked recessive}

Systematic ("long") Analysis Strategy

• Treat each mode as a hypothesis; test against pedigree evidence.

• Key evidence cues:

  • Trait appears in every generation ➔ likely dominant.

  • Affected offspring from unaffected parents ➔ likely recessive.

  • Male-biased appearance and transmission from mothers to sons ➔ likely X-linked recessive.

  • No male-to-male transmission, affected fathers pass trait to all daughters ➔ likely X-linked dominant.

  • Equal sex incidence ➔ likely autosomal.

Douchy’s Faster Shortcut

• Step 1: Scan for any individual phenotypically different from both parents.

  • If found ➔ trait recessive.

  • If none ➔ trait probably dominant.

• Step 2: Highlight every female with recessive phenotype.

  • If any highlighted female has a father or son with opposite phenotype ➔ trait X-linked.

  • If all fathers/sons match phenotype ➔ trait autosomal.

• Combine Steps 1 and 2 to assign one of the four patterns quickly.

Multiple-Choice Pedigree Questions – Core Checks

• Confirm sex of each queried individual (square vs circle).

• Trace exact relationships (parent, child, cousin) through connecting lines, not proximity.

• Check generation numbers to avoid confusion in written answers.

Key Takeaways

• Always be able to work through the full genetic logic before using shortcuts.

• Dominant vs recessive is revealed by appearance in offspring vs parents; autosomal vs X-linked by sex patterns.

• Practise converting pedigree observations into the four standard inheritance hypotheses for exam speed.