The roles of genes in determining phenotype
Cambridge A-Level Biology (9700) · Unit 16: Inheritance · 15 flashcards
The roles of genes in determining phenotype is topic 16.2 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 16 — Inheritance , alongside Passage of information and Gene control. In one line: A gene is a length of DNA (a sequence of nucleotides) that codes for a specific polypeptide. The locus is the fixed position of a gene on a chromosome. An allele is an alternative form of a gene found at the same locus, differing in its nucleotide sequence.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 15 flashcards — 7 definitions, 7 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 7 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The terms 'gene', 'locus', and 'allele'
A gene is a length of DNA (a sequence of nucleotides) that codes for a specific polypeptide. The locus is the fixed position of a gene on a chromosome. An allele is an alternative form of a gene found at the same locus, differing in its nucleotide sequence.
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Explain these terms: allele, gene, locus, recessive, dominant, codominant, genotype, phenotype, heterozygous, homozygous, linkage, test cross, F1 and F2.
- Construct and interpret genetic diagrams (Punnett squares among them) that explain and predict the outcomes of monohybrid and dihybrid crosses involving sex linkage, multiple alleles, dominance and codominance.
- Construct and interpret genetic diagrams (Punnett squares among them) that explain and predict the outcomes of dihybrid crosses involving epistasis and autosomal linkage (you do not need to know the expected ratios for the various types of epistasis).
- Construct and interpret genetic diagrams, Punnett squares included, that explain and predict what test crosses produce.
- Use a chi-squared test to decide whether the gap between expected and observed results is significant (you will be given the formula, as set out in the Mathematical requirements).
- Explain how genes, proteins and phenotype are related in: albinism (TYR gene, tyrosinase); sickle cell anaemia (HBB gene, haemoglobin); haemophilia (F8 gene, factor VIII); Huntington's disease (HTT gene, huntingtin).
- Explain how gibberellin is involved in elongation of stems, including the roles of two alleles: dominant Le, which codes for a working enzyme of the pathway that makes gibberellin, and recessive le, which codes for a non-working version.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in The roles of genes in determining phenotype
- › A 9:3:3:1 ratio from a dihybrid cross shows the two genes are unlinked and assort independently; use a chi-squared test to check whether results fit it.
Define the terms 'gene', 'locus', and 'allele'.
A gene is a length of DNA (a sequence of nucleotides) that codes for a specific polypeptide. The locus is the fixed position of a gene on a chromosome. An allele is an alternative form of a gene found at the same locus, differing in its nucleotide sequence.
Explain the difference between dominant, recessive, and codominant alleles.
A dominant allele is expressed in the phenotype even when a recessive allele is present (in the heterozygote). A recessive allele is expressed only when homozygous. Codominant alleles are both expressed in the phenotype of a heterozygote, e.g. blood group AB, where both A and B antigens are present.
Define 'genotype' and 'phenotype'.
Genotype refers to the genetic makeup of an organism for a specific trait (
What is the purpose of a test cross, and how is it performed?
A test cross is used to determine the genotype of an individual showing the dominant phenotype. It involves crossing the individual with a homozygous recessive individual. The offspring phenotypes reveal the genotype of the unknown parent (
Explain the concept of sex linkage.
Sex linkage refers to genes located on sex chromosomes (X or Y in humans). Traits controlled by these genes show different inheritance patterns in males and females because males have only one X chromosome.
Define autosomal linkage.
Autosomal linkage refers to genes located on the same autosome (non-sex chromosome) and are therefore inherited together unless separated by crossing over during meiosis. Linked genes do not assort independently and will deviate from the expected Mendelian ratios in dihybrid crosses.
What is epistasis?
Epistasis is the interaction of genes where one gene masks or suppresses the expression of another gene at a different locus. This alters the expected phenotypic ratios in a dihybrid cross.
Explain how to calculate the chi-squared (χ²) value and what it indicates.
The chi-squared (χ²) value is calculated using the formula: χ² = Σ [(O - E)² / E], where O is the observed frequency and E is the expected frequency. It measures the difference between observed and expected results; a high value suggests a statistically significant difference, while a low value suggests the differences are due to chance.
What is the relationship between a gene, a protein, and a phenotype?
A gene contains the DNA sequence that codes for a specific protein. The protein produced then influences the observable characteristics (phenotype) of an organism. A mutation in a gene can alter the protein, leading to a different phenotype.
Explain the genetic basis of albinism, referencing the TYR gene and tyrosinase.
Albinism is often caused by a mutation in the TYR gene, which codes for the enzyme tyrosinase. Tyrosinase is essential for melanin production. A non-functional tyrosinase enzyme, due to a mutation in the TYR gene, results in reduced or absent melanin, leading to albinism.
Describe the molecular basis of sickle cell anaemia, linking the HBB gene to haemoglobin.
Sickle cell anaemia is caused by a mutation in the HBB gene, which codes for a subunit of haemoglobin. This mutation leads to the production of abnormal haemoglobin, causing red blood cells to become sickle-shaped. These cells can block blood vessels and reduce oxygen delivery.
Explain how a mutation in the F8 gene can lead to haemophilia.
Haemophilia is often caused by a mutation in the F8 gene, which codes for factor VIII, a protein essential for blood clotting. A mutation in the F8 gene can result in a non-functional or deficient factor VIII, impairing the blood clotting process.
Describe the genetic cause of Huntington's disease, referring to the HTT gene and huntingtin protein.
Huntington's disease is caused by an expanded CAG repeat within the HTT gene. This mutated HTT gene produces an abnormal huntingtin protein with an elongated polyglutamine region, which causes the protein to misfold and aggregate, leading to neuronal damage.
What is the role of gibberellin in stem elongation?
Gibberellin is a plant hormone that promotes stem elongation. It does this by stimulating cell division and cell elongation in the stem. The concentration of gibberellin affects the rate of stem growth.
Explain the roles of the Le and le alleles in the gibberellin synthesis pathway and their effect on stem length.
The dominant Le allele codes for a functional enzyme in the gibberellin synthesis pathway, leading to normal gibberellin production and stem elongation. The recessive le allele codes for a non-functional enzyme, resulting in reduced gibberellin synthesis and shorter stems.
Review the material
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Read NotesMore topics in Unit 16 — Inheritance
The roles of genes in determining phenotype sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
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