Pedigrees

What is a pedigree and what can they be used for?

Pedigree:A diagram showing family relationships

• Shows how a trait is inherited over generations

• Used to trace the inheritance of one particular trait

• Analysing pedigrees helps determine the mode of inheritance: dominant/recessive, autosomal/X-linked

How can you identify certain indivduals in a pedigree?

• Roman numerals = generation number, left of each row

• Numbers across = individual's position within that row

• ID = generation-individual, e.g. II-4 (2nd generation, 4th individual)

What do pedigree symbols represent? How do you determine who is affected and what gender?

• Phenotype shown by symbol; genotype inferred from relationships

• Affected (coloured) vs unaffected (uncoloured) shows if trait is present

• Circles = females; squares = males

How are non-identical vs identical twins shown on a pedigree?

Non-identical twins: Separate eggs and sperm, sharing the womb

• Each linked separately to the mating line

Identical twins: Arise from a single fertilisation event (identical genetic material)

• Shown connected to each other by a short line from a single point on a pedigree

What are the key conventions used when drawing a pedigree?

• Birth order: Oldest on left, youngest on right

Mating line (or breeding/marriage line: Line connecting a couple

• Stillbirths/infant deaths included as they may be linked to genetic disorders

• Descendant line: Offspring below the mating line

What are the features of autosomal recessive inheritance?

Exam evidence:

• If both parents are affected, all offspring affected

• Two unaffected (carrier) parents can have an affected child

• Small samples can misrepresent the true pattern

• Large samples: males and females affected equally

• Carriers let the trait skip generations

• Two carriers → 25% chance of affected child

• Actual ratio in one family may differ, since probability resets each birth

What are the key pieces of evidence for autosomal recessive inheritance?

• If both parents are affected, all offspring affected

• Two unaffected (carrier) parents can have an affected child

What are the features of autosomal dominant inheritance?

Exam evidence:

• Every affected individual must have at least 1 affected parent

• Two affected (carrier) parents can have an unaffected child

• Males and females affected equally (approx.)

• Two unaffected parents can't have an affected child

• Trait appears in most/all generations

• Trait doesn't return once it leaves a branch

What are the key pieces of evidence for autosomal dominant inheritance?

• Every affected individual must have at least 1 affected parent

• Two affected (carrier) parents can have an unaffected child

Example left: Huntington’s disease

What are the features of X-linked recessive inheritance?

Eam evidence:

• More males than females affected over generations

• All daughters of an affected father are carriers

• As affected daughters need the recessive allele from both parents, so dad must carry/express it

• Affected sons often from unaffected carrier mothers, thus the trait can skip generations

• Trait never passed from father to son ( as males hemizygous for X; sons get Y (not X) from dad, so can't inherit it from him)

What are the key pieces of evidence for X-linked recessive inheritance?

• More males than females affected over generations

• All daughters of an affected father are carriers

Example left: Red-green colour blindness

What are the features of X-linked dominant inheritance?

Exam evidence:

• More females than males affected over generations

• If all daughters (no sons) of affected male show trait → most likely X-linked dominant

• Unaffected parents can't have an affected child (doesn't skip generations)

• Affected sons must have an affected mother (as she gives X)

• Affected daughters must have an affected mother or father

• Needs large samples across generations

• Compare affected parents to affected children as a key clue

What are the key pieces of evidence for X-linked dominant inheritance?

• More females than males affected over generations

• If all daughters (no sons) of affected male show trait → most likely X-linked dominant

Example left: Rett syndrome

What are the features of Y-linked inheritance?

Exam evidence:

• If all affected fathers have affected sons → most likely Y-linked

• Sons inherit Y chromosome from father

• Females never affected/carriers, since they lack a Y

• Trait appears in every generation with males

What is the key piece of evidence for Y-linked inheritance?

If all affected fathers have affected sons, then the trait is most likely Y-linked

Example left: Hairy ears

What are the 2 methods for determing the mode of inheritance of a pedigree?

1. Method 1: Trial and error/hypothesis

2. Method 2: Quick Questions

What is the Trial and error/hypothesis method for determining the mode of inheritance for a pedigree?

• Guess a mode of inheritance, then check for exceptions

• Assign genotypes to test each mode of inheritance:

  • X-linked dominant

  • X-linked recessive

  • Autosomal dominant

  • Autosomal recessive

What is the quick questions method for determining the mode of inheritance for a pedigree?

• Use once confident with Method 1; still check with genotypes

Q1: Anyone different to both parents? (dominant vs recessive)

• Unaffected parents, affected child → recessive

• Affected parents, unaffected child → dominant

Q2: Any recessive daughters differing from father/sons?

• Yes → X-linked

• No → autosomal

How do you use a decision-tree method to determine a pedigree's mode of inheritance?

Step 1: Is the trait in every generation of a family in which it occurs?

• YES →

  • Do only males have the trait? Do affected fathers pass the trait to their sons?

    • YES → Y-linked

    • NO → Do all the daughters of an affected father have the trait?

      • NO → Autosomal dominant

      • YES → X-linked dominant

• NO →

  • Are there relatively equal numbers of males and females affected? Do any affected daughters have unaffected fathers?

    • NO → X-linked recessive

    • YES → Autosomal recessive

What is the checklist for confirming a pedigree's mode of inheritance?

• Check whether both sexes are affected — if roughly equal, likely autosomal

• Assign genotypes to individuals using logic

• See if the genotypes fit naturally, without forcing them

• Base your final answer on the observed inheritance pattern as evidence

• Use large sample sizes for reliability, and combine with trial and error to confirm

Note: Always reference individuals by generation-number, e.g. II-4