Explain how a dihybrid cross between two heterozygous parents for seed shape (round, R, dominant over wrinkled, r) and seed colour (yellow, Y, dominant over green, y) produces offspring in a 9:3:3:1 phenotypic ratio, assuming the two genes are on different chromosomes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Each parent is RrYy, so the four possible gametes are RY, Ry, rY and ry in equal amounts.
Because the genes are on different chromosomes they assort independently (Mendel’s Law of Independent Assortment). Thus the 4 male gametes can combine with the 4 female gametes to give 16 equally likely genotypic combinations.
Counting the phenotypes:
• 9 have at least one dominant allele at both loci (RRYY, RRYy, RrYY, RrYy, RrYy, RrYy, RrYy, RrYy, RrYy) → round, yellow.
• 3 have dominant R only (round, green) – genotypes RRYy, RrYY, RrYy.
• 3 have dominant Y only (wrinkled, yellow) – genotypes RrYy, RrYy, RrYy.
• 1 is homozygous recessive at both loci (wrinkled, green) – rryy.
Hence the phenotypic ratio is 9:3:3:1. This ratio comes from the product of the two independent 3:1 ratios for each trait (3×3 : 3×1 : 1×3 : 1×1).
Because the genes are on different chromosomes they assort independently (Mendel’s Law of Independent Assortment). Thus the 4 male gametes can combine with the 4 female gametes to give 16 equally likely genotypic combinations.
Counting the phenotypes:
• 9 have at least one dominant allele at both loci (RRYY, RRYy, RrYY, RrYy, RrYy, RrYy, RrYy, RrYy, RrYy) → round, yellow.
• 3 have dominant R only (round, green) – genotypes RRYy, RrYY, RrYy.
• 3 have dominant Y only (wrinkled, yellow) – genotypes RrYy, RrYy, RrYy.
• 1 is homozygous recessive at both loci (wrinkled, green) – rryy.
Hence the phenotypic ratio is 9:3:3:1. This ratio comes from the product of the two independent 3:1 ratios for each trait (3×3 : 3×1 : 1×3 : 1×1).
Examiner tips
- Show the four gametes and state independent assortment explicitly.
- Explain how 16 combinations give the 9:3:3:1 ratio, linking to the 3:1 ratios for each trait.
Common mistakes
- Confusing the number of gametes (e.g. 3 instead of 4).
- Failing to mention independent assortment or the 3×3 multiplication rule.
Mark scheme (4 marks)
- Each parent is heterozygous for both genes (RrYy), producing four types of gametes (RY, Ry, rY, ry) in equal proportions.
- Because the genes are on different (non-homologous) chromosomes, they assort independently during meiosis (independent assortment / Mendel's Law of Independent Assortment).
- Random fusion of the four male and four female gametes produces 16 equally probable genotypic combinations, giving 9 offspring with at least one dominant allele at both loci (round and yellow), 3 with dominant allele only at the R locus (round and green), 3 with dominant allele only at the Y locus (wrinkled and yellow), and 1 homozygous recessive at both loci (wrinkled and green).
- The 9:3:3:1 ratio arises because each trait individually gives a 3:1 ratio (due to dominance), and the two independent 3:1 ratios combine multiplicatively (3×3 : 3×1 : 1×3 : 1×1).
Key terms in this question
dihybrid cross · heterozygous · phenotypic ratio
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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