Q32 - Gametophytic Self-Incompatibility Genetics in Clover

Theoretical B Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Population Genetics & Hardy-Weinberg Equilibrium

In diploid clover species (Trifolium), fertilization is determined by (gametophytic) self-incompatibility alleles (S-alleles). Pollen with a given allele, e.g. S1, cannot germinate on the stigma of another plant, if this plant has the same allele (e.g. if the mother has the genotype S1 S2 or S1 S3), and therefore no fertilization takes place (Fig.). In species with S-systems, one often finds many alleles: S1 … etc. in a population.

Self-(in)compatibility reactions in three pistils; pollen and pollen tubes are coloured blue. S1 to S4 are S-alleles, and S1S2 etc. are genotypes of mother plants.

Figure 1. Figure 1.

Using the information and data, determine which of the statements are true or which are false.

A. The genotypes in the S gene are in Hardy-Weinberg equilibrium

B. In a population with three S alleles and equal frequencies of all possible genotypes, 1/3 of all crosses will be incompatible

C. The smallest possible number of S alleles in a viable population is four

D. In another incompatibility system with only two alleles (S1 and S2, and S1 being dominant over S2), 1/3 of all crossing types are compatible

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Question reproduced from IBO 2015, Theoretical Exam B, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)