D3.2.8—SNPs and multiple alleles
SNPs are single-base DNA differences, and multiple alleles are more than two allele forms of one gene in a population.
- Syllabus
- First assessment 2025
- Objective
- D3.2.8
- Level
- SL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceNovember 2022
Most common paperPaper1
Typical marks1
Common command terms
- Identify
Recent exam appearances
November 2022Paper1 ["SL"] · TZ013[ 1 ]D3.2.8—SNPs and multiple alleles
May 2022Paper1 ["SL"] · TZ213[ 1 ]D3.2.8—SNPs and multiple alleles
Practice this objective
Coverage 2022–2022 · Updated 16 Jul 2026
SNPs and Multiple Alleles Create Variation
A SNP is a common one-base DNA difference; multiple alleles are variants of one locus present in a population.
A base change may affect coding, regulation or nothing observable; a diploid person carries at most two alleles even when a population has many. Trace the allele combination through the stated biological mechanism before predicting the result.
Separate population allele variety from the two alleles in one individual.; compare the stated alleles and outcome
The ABO locus has three common alleles, while one person may carry only IA and IB. This gives a concrete prediction from the stated parental information.
A SNP is not automatically harmful or visible. Interpret the result within the stated inheritance model and its sample or environmental limits.
SNPs and multiple alleles
Assessment in practice
1 marksHow it is assessed
This objective is assessed through multiple choice, commonly using Identify.
Command terms
Identify
What earns marks
Build the answer around this relationship: An SNP is variation at a single nucleotide position.
Representative question
Question 1
Which statement defines alleles?
A
They are the different forms of a gene that have the same effect on the phenotype.
B
They are the similar forms of a gene in different positions of a chromosome.
C
They are the various forms of a gene with slight differences in their base sequences.
D
They are the different forms of a gene coding for identical polypeptide chains.
Retrieve the Core Inheritance Route
Core D3.2 is secure when the student can move from allele rules into predictions and evidence: gametes form genotypes, genotypes can produce phenotypes, different dominance patterns need different notation, and pedigrees or plots require evidence-based interpretation.
- haploid gametes carry one allele and fertilization restores a diploid genotype
- dominance, codominance, incomplete dominance, environment, and plasticity affect the observed trait
- PKU, ABO, sex determination, and haemophilia use different inheritance rules and notation
- pedigrees infer inheritance patterns and box plots summarize continuous variation
Solve Core Inheritance Questions
Core inheritance exam questions reward disciplined reasoning. First identify the inheritance rule, then write the correct notation or evidence, then state the phenotype, ratio, or conclusion. This prevents the common mistake of writing definitions without solving the genetic problem.
- Use allele and genotype notation correctly for monohybrid, ABO, PKU, haemophilia, and sex-determination contexts.
- Connect genotype, dominance pattern, environment, or plasticity to phenotype.
- Use pedigree or box-plot evidence to justify an inheritance or variation conclusion.
Concept essentials
- An SNP is variation at a single nucleotide position.
- A population can have more than two alleles for one gene.
- A diploid individual still carries only two alleles for a gene.