Monohybrid inheritance and pedigree diagrams

Membranes, Proteins, DNA and Gene Expression · Inheritance, cystic fibrosis and genetic screening · note 2 of 6

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Monohybrid inheritance and pedigree diagramsSpec 2.15

In short

Monohybrid inheritance is the inheritance of one gene with two alleles. A genetic cross between two heterozygotes gives a 3 : 1 ratio of dominant to recessive phenotypes. A pedigree diagram shows the phenotypes of family members over generations, and can be used to deduce whether an allele is dominant or recessive.

A monohybrid cross looks at the inheritance of one gene. Each parent makes gametes containing one allele of the gene (as a result of meiosis), and the alleles combine at random when gametes fuse at fertilisation.

A cross between two heterozygotes

Cystic fibrosis is caused by a recessive allele, f. Two parents are both heterozygous (Ff). Draw a genetic diagram, and find the probability that a child has cystic fibrosis.

Parent genotypes: Ff × Ff.
Gametes: F or f from each parent.
Possible offspring: FF, Ff, Ff, ff (Punnett square).
Phenotypes: FF and Ff do not have the condition (3), ff has cystic fibrosis (1).

Answer: Ratio 3 : 1 (unaffected : affected). Probability of a child with cystic fibrosis is 1 in 4 (25 %).

Pedigree diagrams

A pedigree diagram shows the phenotypes of family members across generations. Males are usually drawn as squares and females as circles; shaded symbols show people with the condition; horizontal lines join parents and vertical lines lead to their children.

Clues for working out the type of inheritance
Clue in the pedigree	Conclusion
Two unaffected parents have an affected child	The condition is caused by a recessive allele, and both parents are heterozygous carriers.
Two affected parents have an unaffected child	The condition is caused by a dominant allele, and both parents are heterozygous.
Every affected person has an affected parent	Suggests a dominant allele.
Condition skips generations	Suggests a recessive allele.

Exam tip:

To prove that an allele is recessive, find a pair of unaffected parents with an affected child, and say so, naming the individuals. To prove dominant, find affected parents with an unaffected child. Give the genotypes of the individuals concerned.

A three-generation pedigree where unaffected parents have an affected daughter showing the allele is recessive, and a Punnett square for Ff × Ff giving a 3 : 1 phenotype ratio. — Unaffected parents with an affected child show that the allele is recessive. Ff × Ff gives **3 : 1**.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between an allele and a gene?

A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide. An allele is one of the different forms of that gene. A person has two alleles of each gene, one from each parent, and these may be the same or different.

How do you tell from a pedigree diagram whether an allele is dominant or recessive?

If two unaffected parents have an affected child, the allele for the condition is recessive and both parents are carriers. If two affected parents have an unaffected child, the allele is dominant and both parents are heterozygous. Always name the individuals and give their genotypes.

Why does cystic fibrosis cause problems in the lungs, digestive system and reproductive system?

The faulty CFTR chloride channel means less water moves out of cells by osmosis, so mucus is thick and sticky. It blocks the airways, causing infections, blocks the pancreatic duct so enzymes cannot reach the intestine, and blocks the tubes that carry sperm.

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