Family pedigrees and probabilities

Reproduction and inheritance · Inheritance · note 4 of 5

Spec 3.24, 3.25Download all Inheritance notes (PDF)Download all notes (PDF)6 pages

Family pedigrees and probabilitiesSpec 3.24, 3.25

In short

A family pedigree is a family tree showing who has a characteristic. If two parents without the characteristic have a child with it, it is recessive and both parents are carriers. Outcomes can be given as a ratio, probability or percentage: two carriers, Ff × Ff, have a 1 in 4, or 25%, probability of an affected child.

A family pedigree (family tree) shows who in a family has a characteristic. Squares are usually males and circles are females. Shaded shapes show people with the characteristic. A horizontal line joins the parents and a vertical line leads down to their children.

If two parents who do not have a characteristic have a child who does, the characteristic is recessive. Both parents must be carriers: heterozygous, with one copy of the recessive allele.
If two parents who both have a characteristic have a child who does not, the characteristic is dominant. Both parents must be heterozygous.

Three-generation family pedigree with squares for males and circles for females: an affected grandmother (rr) and her carrier son (Rr) and his unaffected carrier partner (Rr) have an affected daughter (rr) and two unaffected children (RR or Rr). — Two unaffected parents with an affected daughter: the characteristic is recessive and both parents are carriers (Rr).

Predicting probabilities

You can predict the outcome of a monohybrid cross as a ratio, a probability or a percentage.

Ways of giving the same outcome (Tt × Tt)
Outcome	Boxes out of 4	Probability	Percentage
Short plant (tt)	1	1/4 or 0.25	25%
Tall plant (TT or Tt)	3	3/4 or 0.75	75%

Probability for a recessive disorder

A disorder is caused by a recessive allele (f). Two parents do not have the disorder but both are carriers (Ff). What is the probability that their child has the disorder?

Gametes from each parent: F or f.
Possible offspring: FF, Ff, Ff and ff.
Only ff has the disorder. That is 1 out of 4 equally likely outcomes.

Answer: Probability = 1/4 = 0.25 = 25%.

Each child is an independent event. If the first child has the disorder, the probability for the next child is still 1 in 4. Probabilities describe chance. They do not guarantee the actual numbers in a family.

Maths skill:

A 3 : 1 ratio means 1 in 4 offspring show the recessive feature, not 1 in 3. Count the favourable boxes out of the total.

Exam tip:

In a pedigree question, find two parents with the same phenotype and a child with the other phenotype. That tells you which allele is dominant.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

What is a genotype and phenotype?

A genotype is the alleles an organism has for a gene, written as letters such as TT, Tt or tt. A phenotype is the characteristic that is expressed and can be observed, such as tall or short. A heterozygous Tt plant has the dominant phenotype, because the dominant allele is expressed with only one copy.

What is a Punnett square used for in genetics?

A Punnett square is a genetic diagram used to show the possible offspring of a cross. The gametes from each parent are written along the top and side, then combined to give the offspring genotypes, phenotypes and ratios. For example, Tt × Tt gives 1 TT : 2 Tt : 1 tt, or 3 tall : 1 short.

How do you know if a characteristic is dominant or recessive from a family pedigree?

Find two parents with the same phenotype who have a child with the other phenotype. If two parents without the characteristic have an affected child, it is recessive and both parents are heterozygous carriers. If two parents with the characteristic have an unaffected child, it is dominant and both parents are heterozygous.

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