IB Biology SL 2.5.2 Mendelian inheritance and Punnett grids Question Bank

Practise IB Biology SL 2.5.2 by predicting offspring genotypes and phenotypes from parental alleles.

Exam points

Use a Punnett grid to predict offspring genotypes.
Distinguish homozygous, heterozygous, dominant and recessive genotypes.

[Maximum number: 3]

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

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.

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.

Accept other upper-case and lowercase letters for the alleles.
No Punnett grid = [2 max].
Allow ECF if alleles are incorrect.
[3]

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D3.2.2—Genetic crosses in flowering plants question 1