Inheritance — Edexcel International GCSE Biology
Alleles, dominant and recessive, codominance, genetic diagrams, family pedigrees, probability and how sex is inherited.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Alleles are different versions, or alternative forms, of the same gene. A dominant allele is expressed when only one copy is present, and a recessive allele only when two copies are present. Homozygous means two identical alleles, such as TT, and heterozygous means two different alleles, Tt. Genotype is the alleles, and phenotype is the observed characteristic.
Codominance
Biology onlyCodominance is when both alleles of a heterozygous organism are expressed in the phenotype. In some cattle, the alleles for red hair (R) and white hair (W) are codominant, so an RW animal has both red and white hairs and is called roan. Crossing two roan cattle gives a 1 red : 2 roan : 1 white ratio.
Polygenic inheritance is when several genes work together to produce a phenotypic feature, rather than a single gene. Most phenotypic features are the result of polygenic inheritance. Features controlled by many genes, such as height, show a wide range of values rather than a few clear categories, so they do not follow a simple 3 : 1 ratio.
Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. It is shown with a genetic diagram such as a Punnett square, giving the parent genotypes, gametes, and offspring genotypes and phenotypes. Crossing two heterozygous tall pea plants, Tt × Tt, gives 1 TT : 2 Tt : 1 tt, so 3 tall : 1 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.
Sex determination in humans depends on the sex chromosomes: females are XX and males are XY. Every egg carries an X chromosome, while half of the sperm carry an X and half carry a Y. So the sperm decides the sex of the baby at fertilisation, and there is a 50% probability of a boy or a girl.
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Quick check questions
What is an allele?
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A different version (alternative form) of the same gene.
What is the difference between genotype and phenotype?
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Genotype is the alleles an organism has; phenotype is the characteristic that is expressed.
What does homozygous mean?
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Having two identical alleles of a gene.
Two parents without a disorder have an affected child. Is the disorder dominant or recessive?
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Recessive.
Biology only What is codominance?
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Both alleles are expressed in the phenotype of a heterozygous organism.
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.
Why is there a 50% chance of having a boy or a girl?
Females are XX and males are XY. Every egg carries an X chromosome, but half of the sperm carry an X and half carry a Y. A genetic diagram gives two XX and two XY outcomes, a 1 : 1 ratio, so at each fertilisation there is a 50% probability of a boy or a girl.
How is codominance different from a dominant and recessive allele?
Biology only In codominance, neither allele is dominant, so both are expressed in the phenotype of a heterozygous organism. A roan cow, RW, has both red and white hairs, not pink hairs. Because the heterozygote has its own phenotype, crossing two roan cattle gives 1 red : 2 roan : 1 white, not 3 : 1.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026