Interpret pedigrees to determine the mode of inheritance and predict genotypes

What this dot point is asking

SCSA wants you to read standard pedigree symbols, apply a logical set of rules to work out the mode of inheritance, and deduce genotypes including carriers. A strong answer states the reasoning, not just the conclusion.

Reading a pedigree

Standard symbols are used: squares are males, circles are females, shaded shapes are affected individuals, and unshaded shapes are unaffected. Horizontal lines join mating pairs, and vertical lines connect parents to offspring. Generations are usually numbered with Roman numerals.

Deciding dominant or recessive

Two questions usually settle this.

• If two affected parents have an unaffected child, the trait is dominant (the child must be homozygous recessive, so the unaffected phenotype is recessive).
• If two unaffected parents have an affected child, the trait is recessive (the parents must both be carriers).

Deciding autosomal or sex-linked

Once you know dominant or recessive, check the pattern across the sexes.

• For a recessive trait, if it is far more common in males and appears to pass from carrier mothers to sons, it is likely X-linked recessive.
• If affected and unaffected individuals appear in roughly equal numbers across both sexes, the trait is more likely autosomal.
• An affected father who never passes an X-linked recessive trait to his sons (because he gives them his Y) is a useful check.

Assigning genotypes

Work systematically:

1. Assign genotypes to affected individuals first, since their phenotype fixes their genotype (for example homozygous recessive for a recessive trait).
2. Use the affected children to deduce that unaffected parents must be carriers.
3. Fill in the remaining individuals, leaving a question mark where the genotype cannot be fully determined.

A logical sequence

A reliable approach is: first decide dominant or recessive using the two-parent rule, then decide autosomal or sex-linked using the pattern across sexes, and only then assign genotypes. Doing the steps in this order avoids guessing and produces an answer you can justify, which is what marks reward.

Why pedigrees matter

Pedigree analysis is the practical tool of genetic counselling. By working out the mode of inheritance, a counsellor can estimate the chance that a couple's child will inherit a condition such as cystic fibrosis or haemophilia. It connects the abstract rules of inheritance to real decisions made by Australian families and the genetic services that support them.