Predict codominant genotypes and phenotypic ratios from original marker crosses
The square method stays the same; the expression rule determines phenotype grouping.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall chromosomes, genes and alleles, meiosis, haploid gametes and fertilisation restoring two chromosome sets.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which gamete types can R1R2 contribute?
What you will learn
- Construct the heterozygote-by-heterozygote marker square
- Compare three-category and two-category phenotype ratios
Original teaching model. Pedigrees and records are fictional; allele and probability assumptions are stated.
| Feature | Meaning |
|---|---|
| Parents | R1R2 × R1R2 |
| Square | R1R1, R1R2, R1R2, R2R2 |
| Phenotype ratio | One only:both:two only = 1:2:1 |
| Alternative cross | R1R2 × R2R2 gives both:two only = 1:1 |
Construct the heterozygote-by-heterozygote marker square
In the R1/R2 codominant marker model, each R1R2 parent can contribute R1 or R2 at the gene. The equal-position square gives R1R1, R1R2, R1R2 and R2R2. The genotype ratio is 1:2:1. Because both marker contributions are detectable in the heterozygote, the three genotype groups also give three distinct phenotype categories in this stipulated model.
Label the phenotype ratio marker one only:both markers:marker two only = 1:2:1. Do not collapse the two heterozygote positions into a dominant category to produce 3:1; that belonged to the earlier complete-dominance rule. The gamete-combination arithmetic is the same, but the expression key changes how the internal cells are grouped.
Compare three-category and two-category phenotype ratios
A different cross, R1R2 × R2R2, gives R1R2 or R2R2 in equal positions. Its predicted phenotypes are both markers:marker two only = 1:1. R1R1 cannot occur because the R2R2 parent contributes no R1. For eighty expected offspring, this second model predicts forty in each category, rather than the twenty:forty:twenty expectation of the two-heterozygote cross.
These ratios are probabilities under equal transmission, independent combination and the supplied marker-expression rule. They do not require an exact observed pattern in every finite sample. Always state the parent genotypes and phenotype order. A ratio without its category labels can confuse a codominant 1:2:1 phenotype grouping with an earlier 1:2:1 genotype grouping under complete dominance.
Pause and explain
What phenotype ratio fits R1R2 × R1R2 under the supplied codominant rule?
Worked example
Compare R1R2 × R1R2 with R1R2 × R2R2 under the supplied marker key.
Show the worked solution
- For two heterozygotes, combine R1/R2 with R1/R2: R1R1, R1R2, R1R2, R2R2. Apply the key to count one-only, both and two-only as 1:2:1.
- For a heterozygote crossed with R2R2, combine R1/R2 with R2/R2: two R1R2 and two R2R2 positions give both:two-only = 1:1.
Answer R1R2 × R1R2 predicts marker one only:both:marker two only = 1:2:1; R1R2 × R2R2 predicts both:marker two only = 1:1.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Fill the R1/R2 rows and columns for R1R2 × R1R2 and give the phenotype ratio.
Give me a hint
Combine one allele from each gamete, then use the marker key.
Compare my reasoning
- List R1R1, R1R2, R1R2 and R2R2.
- Assign one only, both, both and two only.
Cells R1R1, R1R2, R1R2, R2R2; phenotype marker one only:both:marker two only = 1:2:1.
Look for these in your work
- Correct four genotypes
- Correct labelled three-category ratio
For 80 expected offspring from R1R2 × R2R2, give possible genotypes and both-marker versus marker-two-only counts.
Give me a hint
The second parent supplies only R2.
Compare my reasoning
- Combine R1 or R2 with R2.
- Assign probability one half to each category.
- Multiply eighty by one half.
Possible genotypes R1R2 and R2R2; expected both markers 40 and marker two only 40, a 1:1 ratio.
Look for these in your work
- Correct two possible genotypes
- Correct forty and forty expected counts
A student reports 3:1 after correctly filling R1R1, R1R2, R1R2, R2R2 under the codominant key. Explain the phenotype-grouping error.
Give me a hint
Use both-marker heterozygotes as a distinct category.
Compare my reasoning
- Recognise the two R1R2 cells show both contributions.
- Keep all three phenotype categories.
The student applied complete-dominance grouping to a codominant key. R1R2 forms the both-marker category, so the phenotype ratio is 1:2:1 with three labelled groups, not 3:1.
Look for these in your work
- Identifies wrong expression grouping
- Corrects to labelled 1:2:1
Common mistakes
- Using 3:1 for the three codominant phenotype categories
- Omitting phenotype labels from a ratio
- Predicting R1R1 with a parent supplying only R2
What can you explain now?
What phenotype ratio fits R1R2 × R1R2 under the supplied codominant rule?
Compare with the explanation
Marker one only:both:marker two only = 1:2:1
The heterozygote is its own observable both-marker category.
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Make it stick
Without your notes, construct the heterozygote-by-heterozygote marker square and compare three-category and two-category phenotype ratios. Use the supplied allele key and state what the evidence can establish.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.