Q32
1 markSection D

(a) What can you infer about the genotype of parents in crosses I and II? [1]

OR

(a) Which genetic cross is represented by these crosses? [1]

Bar graph of Tall and Dwarf plant phenotypes across crosses I, II and III
Principles of Inheritance and Variation
Genotype of Parents in Monohybrid Crosses
Official Answer

Read each cross from its progeny ratio (Tall dominant T, dwarf recessive t).


Cross I — all Tall (500 : 0)


  • No dwarf appears, so at least one parent is homozygous tall (TT).
  • Most consistent genotype: TT × TT (all offspring TT). (TT × Tt also gives all tall.)

Cross II — Tall : Dwarf = 375 : 125 = 3 : 1


  • A 3 : 1 F2 ratio comes only from two heterozygous parents.
  • Parental genotype: Tt × Tt.

OR alternative: Both are monohybrid crosses (involving a single pair of contrasting traits, tall vs dwarf).

homozygousheterozygousTTTt3:1 ratiomonohybrid crossparental genotypedominance

Marking Scheme

  • 10.5 mark: Cross I parents homozygous tall — TT × TT (TT × Tt accepted, as both give all-tall progeny).
  • 20.5 mark: Cross II parents both heterozygous — Tt × Tt (deduced from the 3:1 ratio). OR: both are monohybrid crosses.

Hint

All-tall progeny needs a TT parent; a 3:1 ratio needs Tt × Tt.

Quick Oral Answer

Cross I gives only tall plants, so a parent must be homozygous TT (TT × TT); Cross II gives a 3:1 ratio, so both parents are heterozygous Tt × Tt.

Analysis & Explanation

Concept


Parental genotypes are deduced by working backwards from the phenotypic ratio of the offspring — the essence of Mendelian analysis.


Working the data


  • Cross I gives only tall plants → a dwarf (tt) cannot form, so the dominant allele must be supplied homozygously by at least one parent → TT × TT (safe answer) or TT × Tt.
  • Cross II gives a clean 3 : 1 ratio (375 : 125) → this is the classic F2 of a monohybrid cross → Tt × Tt.

Exam trap


  • Do not write Tt × Tt for Cross I — that would produce a 3 : 1 ratio, not all-tall. Match the genotype strictly to the observed ratio.

Application


  • This backward reasoning is exactly how breeders confirm whether a tall plant is pure-breeding (TT) or a hybrid (Tt) before using it as a parent stock.

Common Mistakes

  1. 1Assigning Tt × Tt to Cross I — that gives a 3:1 ratio, but Cross I is all tall, requiring a TT parent.
  2. 2Writing phenotypes (Tall × Tall) instead of genotypes (TT, Tt) when a genotype is explicitly asked.
  3. 3Ignoring that a 3:1 progeny ratio is the fingerprint of a heterozygous × heterozygous cross.

Interesting Facts

Mendel's classic tall × dwarf pea monohybrid cross gave 787 tall : 277 dwarf in the F2 — a ratio of 2.84:1, remarkably close to the ideal 3:1.

Tall vs dwarf height in garden pea (Pisum sativum) is controlled by a single gene affecting gibberellin synthesis, which is why it behaves as a clean Mendelian trait.

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Frequently Asked Questions

Why must a parent in Cross I be homozygous tall (TT)?

Cross I produces only tall offspring with no dwarfs. A dwarf plant (tt) can appear only if both parents carry a t allele. Since none appear, at least one parent supplies T homozygously — TT — guaranteeing every offspring receives a T allele and is tall.

How do you know Cross II parents are both heterozygous?

Cross II gives a 3:1 tall-to-dwarf ratio (375:125). This exact ratio is produced only when two heterozygotes (Tt × Tt) are crossed, giving genotypes 1 TT : 2 Tt : 1 tt, i.e. 3 tall : 1 dwarf.