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OCR GCSE Biology B · J257
OCR J257 Check the specification (PDF) (opens in a new tab)
A phenotypic feature is a characteristic of an organism. It results from genetic information interacting with the environment. are sections of DNA, and different versions of a gene are called or genetic variants.
Some simplified inheritance problems concern a feature determined mainly by one gene. In such a model, the combination of two alleles—perhaps one dominant and one recessive allele—can be used to predict the phenotype with a Punnett square.
Most phenotypic features do not follow this simple one-gene pattern. Instead, genetic variants in multiple parts of the genome contribute to the same feature. This is called polygenic inheritance, and the feature is described as polygenic.
Think of the difference between one switch and a bank of controls. A single-gene model treats one gene as the main genetic control being considered. For a polygenic feature, information from several genes contributes to the outcome. Different combinations of variants across those genes can therefore produce many possible phenotypes.
A one-gene model follows one pair of alleles, whereas most phenotypic features receive contributions from variants in several genes; environmental conditions may also modify the phenotype.
Human height is an example of a polygenic feature. Many genes contribute to differences in height, helping to produce a continuous range rather than only two sharply separated categories. Environmental factors also matter: for example, conditions affecting growth can modify the phenotype produced from a person's genetic information.
A monohybrid Punnett square follows the inheritance of alleles for one gene. It is useful for genuinely single-gene models, but it does not include the many possible combinations of variants involved in a typical polygenic feature. This is why most characteristics cannot be reduced to a single pair of letters or a simple dominant-to-recessive ratio.
The essential conclusion is:
Most phenotypic features result from multiple genes rather than single-gene inheritance.
At this level, you do not need to describe epistasis or its effects. You only need to recognise that variants in several genes can contribute to one feature and that the environment may also modify the phenotype.
Single-gene model: one gene is followed; dominant and recessive alleles and a simple Punnett square may be used.
Multiple-gene model: variants in several parts of the genome contribute to one feature; a one-gene Punnett square is insufficient.
Human height is a polygenic example and shows a continuous range.
Recall the multiple-gene principle. Do not describe epistasis or its effects.
Do not assume that every characteristic follows a simple dominant–recessive pattern controlled by one gene.
For this specification point, you need to recall that most phenotypic features involve multiple genes; you are not expected to describe epistasis or its effects.
If a question contrasts single-gene and multiple-gene inheritance, explain that a one-gene Punnett square does not represent all the genetic contributions to a multiple-gene feature.
Keep genetic and environmental influences distinct: multiple genes contribute inherited information, while environmental conditions can modify the resulting phenotype.
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Phenotype
A characteristic of an organism produced by its genotype interacting with its environment.
Gene
A section of DNA that contains genetic information, often providing instructions for making a protein.
Allele
A different version of a gene.
Polygenic inheritance
Inheritance in which variants in two or more genes contribute to a single phenotypic feature.
Polygenic characteristic
A characteristic influenced by multiple genes rather than by only one gene.
Put your knowledge into practice — try past paper questions for Biology B
Phenotype
A characteristic of an organism produced by its genotype interacting with its environment.
Gene
A section of DNA that contains genetic information, often providing instructions for making a protein.
Allele
A different version of a gene.
Polygenic inheritance
Inheritance in which variants in two or more genes contribute to a single phenotypic feature.
Polygenic characteristic
A characteristic influenced by multiple genes rather than by only one gene.