Solve an S-Allele Compatibility Cross
Self-incompatibility is a genetic recognition system that stops pollen carrying an allele already present in the recipient plant from succeeding.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Test each pollen type against the stigma
A plant with genotype S₁S₂ produces two pollen types in equal proportions: S₁ and S₂. A recipient with genotype S₂S₃ rejects pollen that shares either stigma allele. S₂ pollen is rejected because S₂ is shared; S₁ pollen shares neither allele and is compatible. Therefore one of the two pollen types germinates: 1/2, or 50%.
A reliable calculation routine
Write the donor's haploid pollen alleles. Write both alleles in the recipient stigma genotype. Reject each pollen type that matches either recipient allele. Count the remaining types and divide by the total, assuming equal pollen production and no other viability difference.
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- ✓Pollen carries one allele from the diploid donor.
- ✓A shared S allele triggers rejection under the rule stated in the question.
- ✓Count compatible pollen types, then convert to a fraction or percentage.
Do not compare the two plant genotypes as whole pairs. Test each haploid pollen allele separately against both stigma alleles.
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