Family Pedigrees — Genetics & Inheritance | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 17% of your exam marks.

increasing

Very High

Increasing17%

Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.

A family pedigree is a diagram that traces how a particular characteristic (often a genetic disorder) passes through several generations of a family. Pedigree diagrams are useful for working out:

Whether the characteristic is or
The probability that a particular family member is affected, or could pass the condition on to their children

Pedigree symbols

Pedigree diagrams use standard symbols:

Squares represent males; circles represent females
Filled-in (shaded) shapes represent affected individuals (those showing the characteristic)
Empty shapes represent unaffected individuals
A horizontal line between a male and a female means they are a couple who have produced children
A vertical line going down from a couple connects them to their children, who are shown side by side

Reading a pedigree

To work out whether a trait is dominant or recessive:

If two unaffected parents have affected children, the trait must be recessive (because both parents are carriers but don't show the trait).
If every affected child has at least one affected parent, the trait is likely to be dominant.
If the trait appears mainly in males and skips generations, it might be X-linked recessive (covered in the next section).

Exam tip

Using a pedigree to determine genotypes and inheritance pattern

What comes up: given a pedigree diagram, the exam asks you to (1) state the genotypes of named individuals, and (2) draw a genetic diagram for a particular couple to predict offspring outcomes.

Write (two marks for the genetic diagram): 1. State the parental genotypes correctly (e.g. Ff and Ff for two carriers of a recessive condition). 2. Show the gametes each parent can produce (e.g. F or f for each parent), then list or grid the offspring genotypes (FF, Ff, ff) and their phenotypes with the correct ratio.

Watch out: the mark scheme requires both genotypes and phenotypes for the offspring — giving genotypes alone loses the final mark. When determining whether a trait is dominant or recessive from the pedigree, state the key evidence: if two unaffected parents produce an affected child, the trait must be recessive (both parents are carriers, genotype Ff, and the affected child is ff).