Polygenic inheritanceSpec 3.22
In short
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.
Some characteristics, like flower colour in Mendel's pea plants, are controlled by a single gene. These are the ones used in simple monohybrid crosses.
Most phenotypic features are the result of polygenic inheritance rather than single genes. This means that several genes work together to produce the feature.
Features controlled by many genes, such as height, show a wide range of values rather than a few clear categories.
Exam tip:If a question asks why a feature does not follow a simple 3 : 1 ratio, one answer is that it is controlled by more than one gene.
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.
All 4 questions on Inheritance
