pedigree charts and patterns of inheritance, including autosomal dominant, autosomal recessive and X-linked inheritance

What this dot point is asking

VCAA wants you to read pedigree charts (family trees), recognise the three main inheritance patterns (autosomal dominant, autosomal recessive, X-linked recessive), and deduce genotypes and probabilities from the chart.

The answer

Pedigree symbols

A standard pedigree uses these conventions:

• Circle: female.
• Square: male.
• Filled symbol: affected by the trait.
• Open symbol: unaffected.
• Half-filled symbol (or dot inside): carrier (heterozygous for a recessive trait, when known).
• Horizontal line between two symbols: mating.
• Vertical line down from a couple: their offspring.
• Roman numerals (I, II, III) label generations from oldest to youngest.
• Arabic numerals label individuals within a generation, left to right.
• A double horizontal line between mates indicates a consanguineous (related-couple) mating.

How to identify the inheritance pattern

Work through these questions in order. The pattern that fits all the clues wins.

1. Does the trait skip generations?

• Yes (parents both unaffected but child affected): the trait is most likely recessive.
• No (every affected child has at least one affected parent): the trait is likely dominant.

2. Is the trait sex-biased?

• Affects mostly males: likely X-linked recessive (males express it with one allele; females need two).
• Affects both sexes about equally: likely autosomal.

3. Inheritance specifics for each pattern:

Autosomal dominant:

• Trait appears in every generation.
• Affected parents have about 50% affected children (Aa times aa).
• Both sexes affected equally.
• Two unaffected parents cannot have affected children.
• Examples: Huntington's disease, achondroplasia, polydactyly, Marfan syndrome.

Autosomal recessive:

• Trait skips generations.
• Two unaffected parents can have affected children (Aa times Aa gives 1/4 affected).
• Both sexes affected equally.
• More common in consanguineous matings (related parents more likely to share recessive alleles).
• Examples: cystic fibrosis, sickle cell anaemia, phenylketonuria, Tay-Sachs.

X-linked recessive:

• Mostly males affected.
• Affected males inherit the allele from their mother (carrier).
• Daughters of an affected father become at least carriers (they get his only X).
• Trait can skip generations through female carriers.
• Affected females are rare and require an affected father and a carrier mother.
• Examples: haemophilia, Duchenne muscular dystrophy, red-green colour blindness.

X-linked dominant (rare in VCE):

• Both sexes affected.
• Affected father passes trait to all daughters but no sons (sons get Y from father).
• Carrier mother passes trait to about 50% of all children regardless of sex.
• Example: vitamin D-resistant rickets.

Y-linked (extremely rare):

• Only males affected.
• Always passed father to all sons.
• Never passed to daughters.

Deducing genotypes

Once you know the inheritance pattern, work through the pedigree filling in what you can.

For autosomal dominant:

• Affected individual is at minimum heterozygous (Aa). If a parent is unaffected, the affected child must be Aa.
• Unaffected individuals are aa.
• Use offspring ratios to check.

For autosomal recessive:

• Affected individuals are aa.
• Unaffected parents of an affected child must both be carriers (Aa).
• Unaffected children of carriers could be AA (1/3 chance) or Aa (2/3 chance) given they are unaffected (because the AA:Aa:aa ratio is 1:2:1, and we have ruled out aa).

For X-linked recessive:

• Affected male is X-h Y.
• Carrier female is X-H X-h.
• Affected female is X-h X-h (rare; needs an affected father).
• Unaffected male is X-H Y.

Calculating probabilities

Apply a Punnett square to the cross. Probabilities multiply across independent events.

Example: Both parents are Aa carriers for cystic fibrosis. Probability the next child is affected: 1/4. Probability the next two children are both affected: 1/4 × 1/4 = 1/16. Probability of at least one affected in two children: 1 - (3/4 × 3/4) = 7/16.

For conditional probabilities, use:

P(genotype | phenotype) = P(genotype and phenotype) / P(phenotype).

If an unaffected child has Aa carrier parents, the probability they are AA versus Aa: prior was 1 AA : 2 Aa : 1 aa. Knowing they are unaffected eliminates aa, so 1 AA : 2 Aa, meaning P(AA | unaffected) = 1/3 and P(Aa | unaffected) = 2/3.

Worked walk-through

Pedigree: Generation I has an unaffected couple (man and woman). They have two daughters, one affected and one unaffected. The affected daughter marries an unaffected man; they have three children, two affected sons and one unaffected daughter.

• The affected daughter has unaffected parents, ruling out autosomal dominant.
• Both sexes are affected (daughter in generation II, sons in generation III), but the only affected female is in generation II.
• The trait appears in both sexes, but we should check for X-linked recessive: an X-linked recessive daughter needs an affected father, but her father is unaffected, ruling out X-linked recessive (X-h X-h is impossible if father is X-H Y).
• So the most likely pattern is autosomal recessive.
• Both parents in generation I must be Aa. The affected daughter is aa.
• She married an unaffected man; if he is aa, all children would be aa. But only two of three are affected, so the husband must be Aa (carrier). aa times Aa gives 1/2 aa : 1/2 Aa, consistent with two affected and one unaffected.

Common traps

Saying "skipped generations means X-linked". Skipped generations means recessive (autosomal or X-linked). Use male bias to distinguish X-linked.

Forgetting that X-linked recessive males get the allele from their mother. Their father gave them the Y; mother gave the X.

Saying "affected female means X-linked dominant". Females can be affected for autosomal traits too. Look at the pattern in males before deciding.

Forgetting that small pedigrees can fit multiple patterns. With only one generation visible, be prepared to give both possibilities.

Ignoring consanguinity. A double horizontal line means the parents are related, which makes recessive traits more likely.

Treating probability as certainty. Pedigree ratios are expected; small families often deviate by chance.

In one sentence

A pedigree is a family tree using circles for females, squares for males and filled symbols for affected individuals; you identify autosomal dominant inheritance (every generation, both sexes, affected child needs affected parent), autosomal recessive (skips generations, two unaffected carriers can have affected children, both sexes), or X-linked recessive (male-biased, affected males inherit from carrier mothers, no father-to-son transmission), and then use Punnett squares to assign genotypes and compute the probability of each phenotype in future offspring.