Question 28

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

When two heterozygous plants with genotypes Rr are crossed, the offspring will exhibit a phenotypic ratio of 3:1, with three red-flowered plants (RR or Rr) for every one white-flowered plant (rr).

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

This option is correct because the cross Rr × Rr follows Mendelian inheritance principles. The Punnett square for this cross shows that the genotypes of the offspring will result in three combinations that express the red flower phenotype (RR and Rr) and one combination that expresses the white flower phenotype (rr).

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

This option is incorrect as it contradicts the basic principles of Mendelian genetics. In the cross Rr × Rr, the recessive phenotype (white flowers) will only appear once among the four possible offspring, not three times.

C) All white-flowered plants

This option is incorrect because it implies that all offspring are homozygous recessive (rr). In fact, the Rr × Rr cross will produce a mix of homozygous dominant, heterozygous, and homozygous recessive genotypes, leading to a predominance of the red-flowered phenotype.

D) All red-flowered plants

This option is also incorrect because it suggests that every offspring will have the dominant phenotype. The cross Rr × Rr will yield a 3:1 ratio, including offspring that will express the recessive white flower phenotype.

Conclusion

The correct answer, 3 red-flowered plants to 1 white-flowered plant, accurately reflects the expected phenotypic ratio from the cross Rr × Rr according to Mendelian genetics. All other options fail to recognize the fundamental principles of inheritance, leading to incorrect interpretations of the offspring's phenotypes.