Question 11

The expected phenotypic ratio of the offspring from the cross Rr * Rr is 3 red-flowered plants to 1 white-flowered plant.

In a cross of Rr (heterozygous red) with Rr, the expected offspring will exhibit a phenotypic ratio of 3:1, where three out of four plants will display the dominant red flower color, and one will show the recessive white flower color.

A) 3 red-flowered plants: 1 white-flowered plant

This option accurately represents the expected phenotypic ratio from the Rr * Rr cross. According to Mendelian genetics, the offspring genotypes will be 1 RR (homozygous red), 2 Rr (heterozygous red), and 1 rr (homozygous white), resulting in three red and one white flowered plant.

B) 3 white-flowered plants: 1 red-flowered plant

This option is incorrect as it reverses the expected ratio of red to white flowers. Since red flower color is dominant, a cross between two red-flowered plants (Rr) cannot yield more white-flowered plants than red ones.

C) All white-flowered plants

This option is incorrect because it suggests that the offspring would be entirely white-flowered. Given the presence of the dominant allele R in both parents, it is impossible for all offspring to exhibit the recessive phenotype.

D) All red-flowered plants

This option is also incorrect because it implies that the cross would yield only red-flowered plants. While the dominant trait is expressed more frequently, the presence of the recessive genotype (rr) in the offspring cannot be ignored, resulting in some white-flowered plants.

Conclusion

The correct answer, A, reflects the fundamental principles of Mendelian inheritance, specifically the 3:1 phenotypic ratio observed in a monohybrid cross. All other options misrepresent the genetic outcomes possible from the cross Rr * Rr, failing to consider the effects of dominant and recessive alleles accurately.