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
Investigate Human Gene Loci and Polypeptide Products
HL onlyA locus is a gene's specific chromosomal position. A human-gene database can connect a gene symbol to its genomic location, summary and polypeptide product.
| Database step | Evidence to record |
|---|---|
| Search a gene and select Homo sapiens | Correct gene record and symbol |
| Read genomic context | Chromosome and locus coordinates |
| Read the summary/product fields | Named polypeptide and its function |
| Compare a second record | Pair on different chromosomes, or zoom in to find a nearby pair on the same chromosome |
The mapped local textbook suggests TDF and CFTR as candidate searches. Record what the database reports for each gene rather than guessing its locus or product from the name.
Genes on different chromosomes can assort independently, whereas genes in close proximity on the same chromosome are likely to be linked and inherited together more often.
A database record is evidence only for its stated genome assembly and species. Do not treat two alleles at one locus as two nearby genes, and do not invent coordinates when the database has not been checked.
Retrieve the HL Inheritance Route
HL onlyHL D3.2 is secure when chromosome behaviour explains the ratios: segregation and independent assortment produce unlinked dihybrid expectations, gene loci explain linkage, recombinants reveal crossing over, and chi-squared decides whether observed counts fit the expected model.
- homologous chromosomes separate and random bivalent orientation assort unlinked genes
- unlinked autosomal genes can produce 9:3:3:1 or 1:1:1:1 ratios
- linked genes give more parental types and fewer recombinants after crossing over
- observed counts are compared with expected ratios using df and p = 0.05
Solve HL Linkage and Chi-Squared Questions
HL onlyHL inheritance transfer is about deciding whether the expected ratio should be Mendelian or linked, then testing the evidence. Start from meiosis and gene location, predict gametes or ratios, identify parental and recombinant classes, and use chi-squared when observed counts need a statistical conclusion.
- Explain segregation and independent assortment from meiosis before using dihybrid ratios.
- Use gene loci, linkage, crossing over, and recombinant frequency to interpret offspring classes.
- Apply chi-squared with observed/expected values, degrees of freedom, p = 0.05, and a null-hypothesis conclusion.
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.