VMBO Year 3 · Inheritance and evolution · Section 3.4

Pedigrees

What can a pedigree tell you?

A pedigree shows how a characteristic is passed through a family. It can help infer possible genotypes and whether a trait fits dominant or recessive inheritance. The conclusions depend on the inheritance model and the information available.

Reading a pedigree

In a traditional pedigree, a square represents a male and a circle represents a female. Shading usually indicates the characteristic being studied. For a simple autosomal trait with complete dominance, two parents showing the dominant phenotype can have a child with the recessive phenotype only if both parents carry the recessive allele. Both are then Aa, and the child is aa.

An example using hair colour

A classroom model may describe brown hair as dominant and blond hair as recessive. In that model, two brown-haired parents with a blond-haired child must both carry the recessive allele. In reality, human hair colour is influenced by several genes, so this example is a simplified model rather than a reliable method for determining a real family's genotypes.

Analysing a pedigree in four steps

• Step 1: Identify each person's phenotype.
• Step 2: Look for offspring whose phenotype differs from their parents'.
• Step 3: Determine which inheritance pattern and genotypes could explain this.
• Step 4: Fill in other genotypes where the evidence permits. Leave uncertain alleles open rather than guessing.