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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
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2014
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Recent exam appearances

May 2014Paper1 ["HL"] · TZ211[ 1 ]D3.2.8—SNPs and multiple alleles

Coverage 2014–2014 · Updated 16 Jul 2026

Gene, Allele, Locus and Genotype Describe Different Levels

Term Meaning Example
gene DNA sequence with a functional product or role a gene affecting pea height
locus the gene's position on a chromosome the height-gene position
allele one sequence version at that locus T or t
genotype allele combination carried TT, Tt or tt
  • homozygous: two matching alleles, such as TT or tt
  • heterozygous: two different alleles, such as Tt

Use genotype for the allele combination, not for the visible trait. A genotype can be written for one locus, several loci or the whole genome, so state the scope when it matters.

A Population Can Hold Many Alleles While One Diploid Individual Holds Two

A single-nucleotide polymorphism (SNP) is a common difference at one nucleotide position among genomes. A SNP within or near a gene can create or mark a different allele.

Scale What may exist at one locus
one haploid gamete one allele
one diploid individual at most two alleles
a population gene pool two, three or many alternative alleles

Multiple alleles means several versions of one gene in the population. It does not mean that one ordinary diploid person carries every version.

Inheritance Predictions Move from Chromosomes to Alleles to Phenotypes

homologous chromosomes carry two alleles in a diploid parent → meiosis separates one allele into each gamete → fertilization restores a two-allele genotype → allele relationship and environment shape phenotype → offspring data test the prediction

For any genetic cross:

  • define allele symbols and their relationship
  • write parental genotypes, not only phenotypes
  • derive gametes before drawing the grid
  • combine gametes, then translate genotype to phenotype
  • state probabilities and the assumptions behind them
If the simple 3:1 expectation fails First question to ask
heterozygote has a third phenotype incomplete dominance?
both products appear codominance?
phenotype changes with conditions environmental or plastic response?
more than two alleles occur in the population multiple-allele locus such as ABO?

SNPs and multiple alleles

[ 1 ]

Which statement defines alleles?

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.