Explain why the phenotypic ratio observed in the offspring of a dihybrid testcross may deviate from the expected 1:1:1:1 ratio when the two gene loci are located on the same chromosome.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Genes on the same chromosome are linked and therefore tend to be inherited together rather than independently.
During prophase I of meiosis I, crossing over can occur between homologous chromosomes and separate linked alleles.
Because crossing over is relatively rare, the parental (non‑recombinant) combinations are more frequent than the recombinant ones, so the two parental phenotypes appear more often than the 1:1:1:1 ratio.
The proportion of recombinant offspring reflects the map distance between the loci; the tighter the linkage, the greater the deviation from 1:1:1:1.
During prophase I of meiosis I, crossing over can occur between homologous chromosomes and separate linked alleles.
Because crossing over is relatively rare, the parental (non‑recombinant) combinations are more frequent than the recombinant ones, so the two parental phenotypes appear more often than the 1:1:1:1 ratio.
The proportion of recombinant offspring reflects the map distance between the loci; the tighter the linkage, the greater the deviation from 1:1:1:1.
Examiner tips
- Use the word ‘linked’ to show understanding of non‑independent assortment. Explain that crossing over can separate alleles but is infrequent. Show that parental phenotypes are more common. Mention that the deviation depends on map distance.
Common mistakes
- Confusing linkage with independent assortment. Forgetting to mention that parental phenotypes are more frequent. Saying crossing over always produces a 1:1:1:1 ratio.
Mark scheme (4 marks)
- Genes on the same chromosome are linked and tend to be inherited together rather than independently.
- Crossing over / recombination between homologous chromosomes during prophase I of meiosis I can separate linked alleles.
- Parental (non-recombinant) combinations are more frequent than recombinant combinations, so the two parental phenotypes appear more often than expected in a 1:1:1:1 ratio.
- The frequency of recombinant offspring reflects the map distance / recombination frequency between the two loci, with more tightly linked genes showing a greater deviation from 1:1:1:1.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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