Dihybrid crosses: Class 12 practice
With two independently assorting genes, RrYy × RrYy gives four phenotypes in the ratio 9 : 3 : 3 : 1. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Monohybrid crosses
- Probability
The idea behind dihybrid crosses
With two independently assorting genes, RrYy × RrYy gives four phenotypes in the ratio 9 : 3 : 3 : 1. Multiply single-gene probabilities to find any particular class.
Step-by-step method
- Find each gene's probability separately.
- Multiply for the combination.
- Scale by the number of offspring.
Worked example 1 (Basic)
RrYy × RrYy (R round, Y yellow dominant). What fraction of offspring are wrinkled green?
- Ratio 9 : 3 : 3 : 1
- wrinkled green = 1/16
- Answer: 1/16
Worked example 2 (Standard)
RrYy × RrYy gives 160 seeds. How many are expected to be wrinkled yellow?
- 3/16 × 160
- Answer: 30
Worked example 3 (Challenge)
RrYy × rryy: what fraction of 640 offspring are expected to be RrYy, and how many?
- Rr from Rr × rr = 1/2; Yy from Yy × yy = 1/2
- 1/2 × 1/2 = 1/4 → 160
- Answer: 1/4, 160
Common mistakes and misconceptions
- Adding probabilities instead of multiplying
- Assuming linkage
Practice questions
- (Basic) RrYy × RrYy (R round, Y yellow dominant). What fraction of offspring are round green?
- (Basic) RrYy × RrYy (R round, Y yellow dominant). What fraction of offspring are round yellow?
- (Standard) RrYy × RrYy gives 1600 seeds. How many are expected to be wrinkled green?
- (Standard) RrYy × RrYy gives 640 seeds. How many are expected to be round yellow?
- (Challenge) RrYy × rryy: what fraction of 480 offspring are expected to be RrYy, and how many?
- (Challenge) RrYy × rryy: what fraction of 320 offspring are expected to be RrYy, and how many?
Answer key with full working
- Q1: Ratio 9 : 3 : 3 : 1 → round green = 3/16 → Answer: 3/16
- Q2: Ratio 9 : 3 : 3 : 1 → round yellow = 9/16 → Answer: 9/16
- Q3: 1/16 × 1600 → Answer: 100
- Q4: 9/16 × 640 → Answer: 360
- Q5: Rr from Rr × rr = 1/2; Yy from Yy × yy = 1/2 → 1/2 × 1/2 = 1/4 → 120 → Answer: 1/4, 120
- Q6: Rr from Rr × rr = 1/2; Yy from Yy × yy = 1/2 → 1/2 × 1/2 = 1/4 → 80 → Answer: 1/4, 80
Syllabus note
This skill is listed under "Principles of Inheritance" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is dihybrid crosses in?
"Principles of Inheritance", Class 12 Biology (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.