AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
A dominant allele is always expressed, even if only one copy is present.
Full specification wording
Genetic inheritance: A dominant allele is always expressed, even if only one copy is present.
An allele is dominant when its effect is seen in the phenotype even if only one copy is inherited.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 4 of 11
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An allele is dominant when its effect is seen in the phenotype even if only one copy is inherited. In a heterozygous organism, one dominant allele and one recessive allele are present, but the dominant allele determines the characteristic. This happens because the dominant allele codes for a functional protein, or because one working copy is enough to produce the characteristic. For example, in pea plants the allele for tall stems is dominant over the allele for short stems, so a plant with one tall allele and one short allele has tall stems. Genetic diagrams use a capital letter for the dominant allele, such as T for tall, and the same letter in lower case for the recessive allele, such as t for short.
What earns the marks
- Define a dominant allele as one whose effect is expressed in the phenotype even when only one copy is present.
- State that a heterozygous organism carrying one dominant and one recessive allele shows the dominant characteristic.
- Use a genetic diagram with a capital letter for the dominant allele and the same lower-case letter for the recessive allele.
- Explain that the dominant allele produces a functional protein or that one working copy is sufficient for the characteristic to appear.
- Apply the idea to a named example, such as tall versus short stems in pea plants.
Common mistakes and how to put them right
Believing a dominant allele is always the most common allele in a population.
Put it right by stating that dominance describes expression in the phenotype, not frequency.
Thinking a dominant allele must be present in two copies to be expressed.
Put it right by stating that one copy is enough.
Writing a genetic diagram with two different letters for one gene.
Put it right by using one letter, capital for the dominant allele and lower case for the recessive allele.
Exam tips
- Define dominance in terms of expression in the phenotype, not in terms of how common the allele is.
- In genetic diagrams, keep the same letter for both alleles of a gene and use capital and lower case consistently.
- When explaining a heterozygous cross, state the parental genotypes, the gametes and the possible offspring genotypes before giving the phenotypes.
Try the lesson
The start of the Genetic inheritance: A dominant allele is always expressed, even if only one copy is present. lesson, made from checked teaching notes for this specification point.
Have a go first
An organism has one dominant allele and one recessive allele for a characteristic. Which allele is expressed?
Listen
An allele is dominant when its effect is seen in the phenotype even if only one copy is inherited. In a heterozygous organism, one dominant allele and one recessive allele are present, but the dominant allele determines the characteristic. This happens because the dominant allele codes for a functional protein, or because one working copy is enough to produce the characteristic. For example, in pea plants the allele for tall stems is dominant over the allele for short stems, so a plant with one tall allele and one short allele has tall stems. Genetic diagrams use a capital letter for the dominant allele, such as T for tall, and the same letter in lower case for the recessive allele, such as t for short. A strong answer earns marks for these points. Define a dominant allele as one whose effect is expressed in the phenotype even when only one copy is present. State that a heterozygous organism carrying one dominant and one recessive allele shows the dominant characteristic. Use a genetic diagram with a capital letter for the dominant allele and the same lower-case letter for the recessive allele. Explain that the dominant allele produces a functional protein or that one working copy is sufficient for the characteristic to appear. A common slip: Believing a dominant allele is always the most common allele in a population. Instead, stating that dominance describes expression in the phenotype, not frequency. Exam tip: Define dominance in terms of expression in the phenotype, not in terms of how common the allele is.
See it
Key words
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Dominant allele
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Heterozygous
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Allele notation
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See every definition
- Dominant allele
- An allele whose effect is expressed in the phenotype even when only one copy is present.
- Heterozygous
- Describes an organism with one dominant allele and one recessive allele present, where the dominant allele determines the characteristic.
- Allele notation
- Genetic diagrams use a capital letter for the dominant allele and the same letter in lower case for the recessive allele.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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