Biology · Grades 9–12
Monohybrid crosses with Punnett squares
Study guide for Grades 9 to 12: 2 worked examples and 2 practice questions. Write each parent’s two alleles along the sides of a 2 × 2 grid.
Rule and key idea
Write each parent’s two alleles along the sides of a 2 × 2 grid. Each box is one equally likely offspring genotype. A dominant allele hides a recessive one.
each parent gives one allele · 4 equally likely boxes
Worked examples
In pea plants, the allele for a tall stem (T) is dominant over the allele for a short stem (t). Two plants, TT × Tt, are crossed. What percentage of the offspring are expected to show a short stem?
TT × Tt → TT, Tt, TT, Tt → 0/4 = 0%
0 %
In pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). Two plants, RR × RR, are crossed. What percentage of the offspring are expected to have the genotype rr?
RR × RR → RR, RR, RR, RR → 0/4 = 0%
0 %
Try two questions
In pea plants, the allele for a tall stem (T) is dominant over the allele for a short stem (t). Two plants, TT × TT, are crossed. What percentage of the offspring are expected to show a short stem?
In pea plants, the allele for yellow seeds (Y) is dominant over the allele for green seeds (y). Two plants, Yy × Yy, are crossed. What percentage of the offspring are expected to have the genotype yy?
Answers and reasoning
Answers and marking notes
- 0 %
TT × TT → TT, TT, TT, TT → 0/4 = 0%
- 25 %
Yy × Yy → YY, Yy, Yy, yy → 1/4 = 25%
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