How many possible combinations (unordered) of parental genotypes could produce this result?

Question

Ali was breeding two Pisum sativum plants for plant height and seed shape. He noticed the following in the offspring:

All plants were Tall

Seed shape ratio: 3 (Round) : 1 (Wrinkled)

How many possible combinations (unordered) of parental genotypes could produce this result?

1. A. 1
2. B. 2
3. C. 3 (Correct answer)
4. D. 4

Correct Answer

Option C — 3

Detailed Solution & Explanation

The correct answer is 3.

Key Points

• The two characters must be analysed separately, then the possibilities multiplied.
• For plant height — all offspring Tall. A cross produces only tall plants when at least one parent is homozygous dominant (TT), since a single dominant allele in every offspring guarantees the tall phenotype. The possible unordered parental pairs are:
  • TT × TT — all offspring TT
  • TT × Tt — offspring TT and Tt, all tall
  • TT × tt — all offspring Tt, all tall
  • That gives 3 possibilities. (Tt × Tt would produce dwarf plants, and Tt × tt likewise, so both are excluded.)
• For seed shape — a 3:1 ratio. A 3 Round : 1 Wrinkled ratio is the signature of a monohybrid cross between two heterozygotes:
  • Rr × Rr — the only possibility, giving 1 possibility.
• Total combinations:
  • $3 \times 1 = \mathbf{3}$

Additional Information

• Reading ratios backwards is a standard genetics skill:
  • 3:1 → both parents heterozygous (Rr × Rr)
  • 1:1 → test cross, heterozygous × homozygous recessive (Rr × rr)
  • All dominant → at least one parent homozygous dominant
  • All recessive → both parents homozygous recessive (rr × rr)
• The word "unordered" matters — TT × Tt and Tt × TT are the same cross and must be counted once. Counting them separately would give 5 instead of 3.
• Test cross: crossing an organism showing the dominant phenotype with a homozygous recessive individual reveals its genotype — a 1:1 ratio indicates a heterozygote, while all-dominant offspring indicate a homozygote.