D3.2.18 (HL)—Human gene loci
Human gene loci are fixed positions of genes on chromosomes, allowing alleles to be mapped and inherited from parents in inheritance problems.
- Syllabus
- First assessment 2025
- Objective
- D3.2.18
- Level
- HL
A Gene Database Connects a Symbol to a Locus and Polypeptide Product
- search an authoritative gene database with the gene name or symbol
- confirm the organism is Homo sapiens and select the correct record
- record chromosome, cytogenetic band or genomic coordinates from genomic context
- record the named RNA and polypeptide product and its supported function
- compare a second gene's chromosome and coordinates before calling the pair linked
| Database field | Question it answers |
|---|---|
| gene symbol and stable identifier | which gene record? |
| chromosome and genomic coordinates | where is the locus? |
| transcript or product annotation | what is produced? |
| curated functional summary | what evidence supports its role? |
Genes on different chromosomes are unlinked. Genes on the same chromosome are physically linked, but how often crossing over separates them depends on the distance between their loci.
A nearby genomic feature is not automatically the same gene or product. Record the database version, species, coordinates and identifier so the evidence can be checked again.
The Unlinked Model Runs from Meiosis to Dihybrid Ratios
homologs separate alleles → independently oriented bivalents combine alleles from unlinked genes → AaBb makes AB, Ab, aB and ab gametes equally → random fertilization produces 9:3:3:1 in an F2 or 1:1:1:1 in a test cross
| Prediction | Required conditions |
|---|---|
| 1:1 allele segregation | normal meiosis and equal gamete contribution |
| four equal AaBb gametes | loci unlinked or recombination effectively 50% |
| 9:3:3:1 F2 | complete dominance, random fertilization and equal survival |
| 1:1:1:1 test cross | double heterozygote × double recessive tester |
A database locates the genes; the cross tests their behaviour. If observed offspring depart systematically from the unlinked expectation, inspect chromosome location, parental phase, crossing over and viability before blaming random sampling alone.
Inheritance Models Are Built from Chromosomes and Tested with Offspring Counts
meiosis predicts gamete combinations → chromosome loci determine whether genes assort independently or travel together → crossing over creates reciprocal recombinants → test-cross counts reveal parental phase and recombinant frequency → chi-squared tests whether departures from a chosen ratio exceed chance expectation
| Offspring pattern | First model to examine |
|---|---|
| 9:3:3:1 F2 | two unlinked genes with complete dominance |
| 1:1:1:1 test cross | four equally frequent gametes from an unlinked double heterozygote |
| two large and two small reciprocal test-cross classes | linked loci with crossing over |
| significant χ² departure | assumptions, linkage, viability, fertilization or scoring require review |
Keep prediction and evidence separate: define the genetic model first, calculate expected counts from it, compare observations with the expectation, and change the model only when chromosome location or statistical evidence requires it.
Concept essentials
- A locus is a gene position on a chromosome.
- Different alleles of one gene occupy the same locus on homologous chromosomes.
- Loci on the same chromosome may be inherited together if they are linked.