Use inheritance terminology precisely
Define genes, loci, alleles, genotype, phenotype, dominance, linkage and cross-generation notation.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- Genes are DNA sequences that can be inherited; meiosis and fertilisation link parental genomes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is an allele?
What you will learn
- Explain the required terminology for inheritance
- Distinguish genotype from phenotype and dominance from frequency
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| TT | One model genotype; pigment present under the rule |
| Tt | Different genotype; same pigment phenotype |
| tt | Genotype giving pigment absence under the rule |
| Observed phenotype | Does not always identify a unique genotype |
Build the genetic explanation
A gene is a DNA sequence at a locus; an allele is an alternative version of that gene. Genotype is the alleles present, while phenotype is the observable characteristic arising from genotype and environmental effects. Homozygous means two identical alleles; heterozygous means two different alleles. A dominant allele affects phenotype in a heterozygote; recessive expression usually requires two recessive alleles. Codominant alleles both contribute to a heterozygote phenotype. F1 and F2 denote first and second filial generations; a test cross uses a homozygous recessive individual to investigate an unknown genotype. Linked genes are on the same chromosome and tend to be inherited together.
Check the evidence and limits
Dominant does not mean common, stronger or better. Genotype notation must name alleles at loci; phenotype describes the resulting characteristic. Linkage is a chromosome relationship, not the same as dominance.
Pause and explain
A linked pair of genes is described as “dominant together.” Correct the terminology.
Worked example
An organism has genotype Aa and shows the A-associated phenotype. Is it homozygous or heterozygous?
Show the worked solution
- The two alleles differ.
- Different alleles mean heterozygous.
Answer Heterozygous.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Distinguish genotype and phenotype using a heterozygous organism.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Genotype names its alleles.
- Phenotype describes the expressed characteristic.
Genotype is the allele combination, such as Aa. Phenotype is the observable characteristic, which depends on the genotype and may also be influenced by the environment.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A recessive allele is rarer in a population than the dominant allele. Does rarity define recessiveness? Explain.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- Dominance describes heterozygote expression.
- Population frequency is a separate property.
No. Recessive describes how an allele is expressed in a heterozygote, not how common it is in the population.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A linked pair of genes is described as “dominant together.” Correct the terminology.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- Linkage describes loci on the same chromosome.
- Dominance describes allele expression in a heterozygote.
Linked genes are on the same chromosome and tend to be inherited together. Dominance describes how alleles at a locus affect a heterozygote phenotype; linkage and dominance are different concepts.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Defining dominant as the most common allele
- Using phenotype as a synonym for genotype
What can you explain now?
What is an allele?
Compare with the explanation
An alternative version of a gene
Different alleles occur at the same locus.
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Make it stick
Which terms describe allele combinations, visible characteristics and inheritance between loci?
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