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D3.2.2—Genetic crosses in flowering plants

Genetic crosses in flowering plants use parental genotypes, gametes and offspring ratios to infer inheritance patterns in inheritance problems in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.2
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceMay 2015
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Recent exam appearances

May 2015Paper1 ["HL"] · TZ113[ 1 ]D3.2.2—Genetic crosses in flowering plants
May 2014Paper1 ["HL"] · TZ210[ 1 ]D3.2.2—Genetic crosses in flowering plants

Coverage 2014–2015 · Updated 16 Jul 2026

A Controlled Plant Cross Assigns Which Pollen Fertilizes Which Ovules

  • choose parents with known, contrasting phenotypes and preferably true-breeding genotypes
  • remove immature anthers from the recipient flower before they release pollen
  • transfer pollen from the chosen donor to the receptive stigma
  • prevent stray pollen from reaching the flower, then label the cross
  • collect seeds and score enough offspring to compare with a predicted ratio

Emasculation and isolation prevent self-pollination, so the experimenter knows the parental sources of both gametes. Reciprocal crosses can also test whether switching the pollen and ovule parents changes the result.

A controlled flowering-plant cross removes self-pollinating anthers, transfers pollen from a chosen parent, grows F1 offspring and crosses the F1 to obtain an F2 generation.

Mendel's P, F1 and F2 Generations Reveal Segregating Alleles

1

P generation: true-breeding tall TT × true-breeding dwarf tt → every gamete carries T or t, respectively.

2

F1 generation: every offspring is Tt and tall. The t allele has not blended away; its phenotypic effect is masked in the heterozygote.

3

F1 self-cross: Tt × Tt → F2 genotypes 1 TT : 2 Tt : 1 tt → phenotypes 3 tall : 1 dwarf in a large sample.

Reappearance of dwarf offspring shows that hereditary factors remain discrete across generations. The observed ratio approaches the probability prediction only when many offspring are counted.

A Punnett Grid Combines Gamete Probabilities, Not Whole Parental Genotypes

  1. define allele symbols and the dominance relationship
  2. write both parental genotypes
  3. derive the allele carried by each possible gamete
  4. place one parent's gametes across the grid and the other's down the side
  5. combine alleles in each cell, then convert genotypes to phenotypes
Tt × Tt T gamete t gamete
T gamete TT, tall Tt, tall
t gamete Tt, tall tt, dwarf

Each cell has probability 1/4 because each parent produces T and t gametes with probability 1/2. The grid predicts probabilities for each fertilization, not a guarantee that every four offspring will contain one of each cell.

Inheritance Predictions Move from Chromosomes to Alleles to Phenotypes

homologous chromosomes carry two alleles in a diploid parent → meiosis separates one allele into each gamete → fertilization restores a two-allele genotype → allele relationship and environment shape phenotype → offspring data test the prediction

For any genetic cross:

  • define allele symbols and their relationship
  • write parental genotypes, not only phenotypes
  • derive gametes before drawing the grid
  • combine gametes, then translate genotype to phenotype
  • state probabilities and the assumptions behind them
If the simple 3:1 expectation fails First question to ask
heterozygote has a third phenotype incomplete dominance?
both products appear codominance?
phenotype changes with conditions environmental or plastic response?
more than two alleles occur in the population multiple-allele locus such as ABO?

Genetic crosses in flowering plants

[ 3 ]

L. purpureus can have purple or white flowers. Two pure-breeding varieties were crossed: HA 4 with white flowers and GL 424 with purple flowers. All of the F1F_{1} plants had purple flowers. The F1F_{1} plants were self-pollinated to produce an F2F_{2} generation. There were 97 plants with purple flowers and 38 plants with white flowers in the F2F_{2} generation.

Using a Punnett grid, explain the results of this cross.

Concept essentials

  • Parental genotypes determine the gametes available in a cross.
  • Offspring ratios can reveal whether a parent is homozygous or heterozygous.
  • Controlled pollination in plants allows inheritance predictions to be tested.