IB Biology SL D3.2.2 Genetic Crosses in Flowering Plants Questions

Use Punnett grids to track parental, F1 and F2 flowering-plant crosses, then interpret offspring genotypes, phenotypes and ratios in IB Biology SL D3.2.2.

Exam points

Construct and interpret a flowering-plant Punnett grid across parental, F1 and F2 generations, showing gametes, offspring genotypes, phenotypes and expected ratios.
Identify parental genotype combinations used in a plant test cross and use them to predict the resulting offspring pattern.

[Maximum number: 4]

The dolichos bean (Lablab purpureus) is cultivated as a food crop in tropical countries. Leaf cells in L. purpureus have 24 chromosomes.

(a)

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 $F_{1}$ plants had purple flowers. The $F_{1}$ plants were self-pollinated to produce an $F_{2}$ generation. There were 97 plants with purple flowers and 38 plants with white flowers in the $F_{2}$ generation.

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

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Male and female gamete genotypes/alleles shown as P and p, or other letters following convention with a suitable key, in a Punnett grid.
$\mathrm{F}_{2}$ genotypes shown as $\mathrm{PP}$ , $\mathrm{Pp}$ , $\mathrm{pP}$ and $\mathrm{pp}$ .
$\mathrm{F}_{2}$ phenotypes indicated for each genotype on the Punnett grid / 3 purple to 1 white ratio indicated.

Official Punnett-grid answer image from the markscheme.

(b)

Suggest a reason for the $F_{2}$ generation not corresponding exactly to the expected ratio of purple-flowered and white-flowered plants.

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IB Biology SL D3.2.2 Genetic Crosses in Flowering Plants Questions question 1

Question (a)

Question (b)