You are interested in studying the evolution of Rnase-based gametophytic self-incompatibility. You go into a population and identify 4 S-alleles present. These alleles are designated S₁, S2, S3, and S4, and they occur in the sporophyte only as heterozygotes at equal frequency, in each of the 6 possible combinations: S1 S2 S1 S3 S1 S4 S2 S3 S2 S4 S3 S4 Assuming random dispersal of pollen, what fraction of S₁ pollen grains that land on stigmas will be able to fertilize ovules in this population? Why?
Explanation:
-Assuming random dispersal of pollen, the fraction of S1 pollen grains that will be able to fertilize ovules in this population is 1/4. This is because each of the S alleles is present in the sporophyte only as a heterozygote, and each combination of S alleles occurs with equal frequency. Thus, there is a 1 in 4 chance that an S1 pollen grain will land on a stigma that shares its S allele, and therefore be able to fertilize an ovule.
-The other 3/4 of the S1 pollen grains that land on stigmas will not be able to fertilize ovules in this population. This is because each of the S alleles is present in the sporophyte only as a heterozygote, and each combination of S alleles occurs with equal frequency. Thus, there is a 3 in 4 chance that an S1 pollen grain will land on a stigma that does not share its S allele, and therefore will not be able to fertilize an ovule.
-In a population with 4 S-alleles, the alleles would be designated as S1, S2, S3, and S4. If the alleles occur in the sporophyte only as heterozygotes at equal frequency, then the 6 possible combinations are S1 S2, S1S3, S1 S4, S2 S3, S2 S4, S3 S4. Assuming random dispersal of pollen, the fraction of S1 pollen grains that will be able to fertilize ovules in this population is 1/4. The other 3/4 of the S1 pollen grains that land on stigmas will not be able to fertilize ovules in this population for the same reason. In a population with 4 S-alleles, the alleles would be designated as S1, S2, S3, and S4. If the alleles occur in the sporophyte only as heterozygotes at equal frequency, then the 6 possible combinations are S1 S2, S1S3, S1 S4, S2 S3, S2 S4, S3 S4. Assuming random dispersal of pollen, the fraction of S1 pollen grains that will be able to fertilize ovules in this population is 1/4.
-The other 3/4 of the S1 pollen grains that land on stigmas will not be able to fertilize ovules in this population because they will land on a stigma that does not share its S allele. In a population with 4 S-alleles, the alleles would be designated as S1, S2, S3, and S4. If the alleles occur in the sporophyte only as heterozygotes at equal frequency, then the 6 possible combinations are S1 S2, S1S3, S1 S4, S2 S3, S2 S4, S3 S4. Assuming random dispersal of pollen, the fraction of S1 pollen grains that will be able to fertilize ovules in this population is 1/4.
-This 1/4 chance of S1 pollen being able to fertilize ovules in this population is due to the fact that each of the S alleles is present in the sporophyte only as a heterozygote. As a result, each combination of S alleles occurs with equal frequency. Thus, there is a 1 in 4 chance that an S1 pollen grain will land on a stigma that shares its S allele. When this occurs, the S1 pollen grain will be able to fertilize the ovule.
-The other 3/4 of the time, the S1 pollen grain will land on a stigma that does not share its S allele. As a result, the S1 pollen grain will not be able to fertilize the ovule. This is because each of the S alleles is present in the sporophyte only as a heterozygote, and each combination of S alleles occurs with equal frequency.
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