Family pedigrees, probability and many-gene characteristics

Family trees (pedigrees) show how a characteristic is passed on. Most characteristics, though, are controlled by many genes, not just one.

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Reading a pedigree

Squares are males and circles are females. Shaded symbols are affected. A horizontal line joins parents; a vertical line leads to their children.
If two unaffected parents have an affected child, the condition must be recessive, and both parents must be carriers (heterozygous).
Example: cystic fibrosis is caused by a recessive allele.

Probabilities

Results from a Punnett square can be given as a ratio, a fraction, a percentage or a probability.
Ff × Ff gives FF, Ff, Ff, ff:
ratio of unaffected to affected = 3 : 1; fraction affected = 1/4; percentage = 25%; probability = 0.25.
Each child is a separate event: the chance is the same for every child, whatever happened before.

Many genes and variation

Most characteristics, such as height and skin colour, are controlled by many genes (polygenic), often with the environment too. They show continuous variation: a smooth range of values, not a few clear groups.
There is usually a lot of genetic variation within a population. New alleles arise through mutations.
Worked example

Two parents who are both carriers of cystic fibrosis (Ff) have a child. Give the probability that the child is a carrier, as a percentage.

  1. Ff × Ff gives FF, Ff, Ff, ff.
  2. Carriers are Ff: 2 out of 4.
  3. 2 ÷ 4 × 100 = 50%

Answer: 50%

Key idea

In a pedigree, two unaffected parents with an affected child means the allele is recessive and both parents are carriers. A Ff × Ff cross gives a 3 : 1 ratio, a 1/4 chance of ff. Most characteristics depend on many genes and show continuous variation.

Key words
Allele
A different form of the same gene.
Heterozygous
Having two different alleles of a gene, for example Bb.
Recessive allele
An allele that is only expressed when two copies are present.
Punnett square
A grid used to show the possible genotypes of the offspring of a genetic cross.
Population
All the organisms of one species living in a habitat.

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  1. Two carrier parents (Ff × Ff) have a child. What is the percentage chance the child does not have the recessive condition?

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    Answer: 75 %

    The Punnett square gives FF, Ff, Ff, ff. 3 out of 4 are FF or Ff, so 75%.

  2. Two carriers (Ff × Ff) have children. What fraction of the children are expected to have the condition (ff)? Give your answer as a fraction in its simplest form.

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    Answer: 1/4

    FF, Ff, Ff, ff: 1 out of 4 has the condition.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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