Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
Explain the terms: chromosome, gene, allele, dominant, recessive,…
Full specification wording
Topic 3 – Genetics: 3.13 Explain the terms: chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, gamete and zygote
Genetics has its own vocabulary, and exam questions test whether you can apply each term to a real example rather than recite a definition.
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 11 of 20
Study notes
From the checked teaching notes for this specification point.
The idea
Genetics has its own vocabulary, and exam questions test whether you can apply each term to a real example rather than recite a definition. A chromosome is a long DNA molecule carrying many genes; a gene is a short section of DNA coding for a protein; an allele is one version of a gene. Dominant alleles are expressed even when only one copy is present, while recessive alleles are expressed only when two copies are present. Homozygous means two identical alleles for a gene; heterozygous means two different alleles. Genotype is the alleles an organism carries; phenotype is the observable characteristic. A gamete is a sex cell with half the chromosome number; a zygote is the cell formed when two gametes fuse.
What earns the marks
- Chromosome: a long, coiled DNA molecule carrying many genes; human body cells have 23 pairs.
- Gene: a section of DNA that codes for a protein, and so contributes to a characteristic.
- Allele: one of the different versions of a gene; a diploid organism carries two alleles per gene.
- Dominant allele: expressed in the phenotype even when only one copy is present.
- Recessive allele: expressed in the phenotype only when two copies are present.
- Homozygous: two identical alleles for a gene; heterozygous: two different alleles.
- Genotype: the combination of alleles present; phenotype: the observable characteristic produced.
- Gamete: a sex cell (sperm, egg, pollen) with half the normal chromosome number; zygote: the diploid cell formed by fusion of two gametes.
Common mistakes and how to put them right
Confusing gene and allele.
Put it right by saying 'allele = version of a gene'.
Saying a dominant allele is 'stronger' or 'more common'.
Put it right: Dominant means it is expressed when one copy is present; it need not be more frequent.
Treating genotype and phenotype as the same.
Put it right: Genotype is the alleles carried; phenotype is the visible or measurable characteristic.
Calling a gamete a zygote.
Put it right: Gametes fuse to form a zygote, which is diploid.
Exam tips
- Learn each term with a short example, such as Bb being heterozygous, so you can apply it to unfamiliar contexts.
- When a question gives a phenotype, work out the possible genotypes before choosing an answer.
- Check whether the question asks about alleles (letters) or the characteristic (words) before selecting an option.
Try the lesson
The start of the Topic 3 – Genetics: 3.13 Explain the terms: chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, gamete and zygote lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following describes a gene?
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