AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Genetic inheritance: recessive
A recessive allele is one whose characteristic is only expressed in the phenotype when two copies are present, because its effect is masked by a dominant allele in a heterozygote.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 7 of 20
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A recessive allele is one whose characteristic is only expressed in the phenotype when two copies are present, because its effect is masked by a dominant allele in a heterozygote. In a monohybrid cross, a recessive phenotype appears only in offspring that inherit the recessive allele from both parents, so both parents must carry at least one copy even if neither shows the trait. For example, if B codes for brown eyes and b for blue eyes, a bb individual has blue eyes, while Bb has brown eyes because B is dominant. Recessive alleles can therefore remain hidden for generations in carriers, which explains why two brown-eyed parents can have a blue-eyed child.
What earns the marks
- A recessive allele is only expressed in the phenotype when two copies are present, one inherited from each parent.
- In a heterozygote, the dominant allele masks the recessive allele, so the recessive characteristic is not seen.
- A recessive phenotype can appear in offspring of parents who do not show it, provided both parents carry the recessive allele.
- Genetic diagrams and Punnett squares use lower-case letters for recessive alleles, for example b, and show the 3:1 ratio when two heterozygotes are crossed.
- Carriers are heterozygous individuals who possess a recessive allele but do not express the recessive phenotype.
Common mistakes and how to put them right
Thinking a recessive allele is always rare or always harmful.
Put it right: Recessive describes masking in a heterozygote, and recessive alleles can be common and may be beneficial.
Assuming a recessive characteristic must appear in every generation.
Put it right: It can skip generations because carriers hide it.
Writing a recessive genotype with a capital letter.
Put it right: Use lower-case letters for recessive alleles, for example bb, and capital letters for dominant alleles.
Exam tips
- Define recessive in terms of expression only when two copies are present, then apply it to the cross shown.
- When completing a Punnett square, label gametes clearly and state the phenotype of each genotype before giving the ratio.
- If asked why a child shows a recessive trait, state that both parents must have contributed a recessive allele, even if neither parent shows the trait.
Try the lesson
The start of the Genetic inheritance: recessive lesson, made from checked teaching notes for this specification point.
Have a go first
What is the meaning of the term recessive when describing an allele?
Listen
A recessive allele is one whose characteristic is only expressed in the phenotype when two copies are present, because its effect is masked by a dominant allele in a heterozygote. In a monohybrid cross, a recessive phenotype appears only in offspring that inherit the recessive allele from both parents, so both parents must carry at least one copy even if neither shows the trait. For example, if B codes for brown eyes and b for blue eyes, a bb individual has blue eyes, while Bb has brown eyes because B is dominant. Recessive alleles can therefore remain hidden for generations in carriers, which explains why two brown-eyed parents can have a blue-eyed child. A strong answer earns marks for these points. A recessive allele is only expressed in the phenotype when two copies are present, one inherited from each parent. In a heterozygote, the dominant allele masks the recessive allele, so the recessive characteristic is not seen. A recessive phenotype can appear in offspring of parents who do not show it, provided both parents carry the recessive allele. Genetic diagrams and Punnett squares use lower-case letters for recessive alleles, for example b, and show the 3:1 ratio when two heterozygotes are crossed. A common slip: Thinking a recessive allele is always rare or always harmful. Instead, recessive describes masking in a heterozygote, and recessive alleles can be common and may be beneficial. Exam tip: Define recessive in terms of expression only when two copies are present, then apply it to the cross shown.
Key words
Say what each word means before you turn the card.
recessive
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carriers
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See every definition
- recessive
- Describes an allele whose characteristic is only expressed in the phenotype when two copies are present, because a dominant allele masks it in a heterozygote.
- carriers
- Heterozygous individuals who possess a recessive allele but do not express the recessive phenotype.
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